Greybush, S J., H E Gillespie, and R John Wilson, January 2019: Transient eddies in the TES/MCS Ensemble Mars Atmosphere Reanalysis System (EMARS). Icarus, 317, DOI:10.1016/j.icarus.2018.07.001. Abstract
Transient eddies are important features of Mars atmosphere weather, and are linked to the genesis of dust storms. Many previous studies of transient eddies, also known as traveling waves, generally used either spacecraft observations or model simulations alone. Reanalyses, which optimally combine observations with a forecast model, provide an unprecedented opportunity to examine these traveling weather systems: their temperature, wind, pressure signatures and structure; the evolution between various wave regimes; and their seasonality and interannual variability. Using the GFDL Mars Global Climate Model (MGCM) with the Local Ensemble Transform Kalman Filter (LETKF), we have created a six year reanalysis of both Thermal Emission Spectrometer (TES) and Mars Climate Sounder (MCS) temperature retrievals, which we name the Ensemble Mars Atmosphere Reanalysis System (EMARS). We demonstrate that the transient eddies in analyses with different assumptions in the model and assimilation system, including between EMARS and the Mars Analysis Correction Data Assimilation reanalysis (MACDA), are generally robust; EMARS and MACDA eddies are more similar to each other than their respective freely running control simulations. We also reveal lower atmosphere transient eddies derived from MCS data for the first time, and compare to those derived from TES data. MCS, as a limb sounder, demonstrates some challenges in constraining the shallow eddies in EMARS compared to reanalyses using TES nadir measurements. Ensemble reanalyses are valuable in that they provide an assessment of convergence upon a unique synoptic state. We examine the six year climatology and interannual variability of transient eddies, synoptic maps, and transitions between dominant wavenumber regimes. Finally, we compare reanalysis products to other products derived from observational data, including radio science and the Viking lander surface pressure records.
Mooring, Todd A., Isaac M Held, and R John Wilson, August 2019: Effects of the Mean Flow on Martian Transient Eddy Activity: Studies with an Idealized General Circulation Model. Journal of the Atmospheric Sciences, 76(8), DOI:10.1175/JAS-D-18-0247.1. Abstract
The extent to which the eddy statistics of the Martian atmosphere can be inferred from the mean state and highly simplified assumptions about diabatic and frictional processes is investigated using an idealized general circulation model (GCM) with Newtonian relaxation thermal forcing. An iterative technique, adapted from previous terrestrial studies, is used to generate radiative equilibrium temperatures such that the three-dimensional time mean temperature fields of the idealized model match means computed from the Mars Analysis Correction Data Assimilation (MACDA) reanalysis. Focusing on a period of strong northern hemisphere eddy activity prior to winter solstice, it is found that the idealized model reproduces some key features of the spatial patterns of the MACDA eddy temperature variance and kinetic energy fields. The idealized model can also simulate aspects of MACDA’s seasonal cycle of spatial patterns of low-level eddy meridional wind and temperature variances. The most notable weakness of the model is its eddy amplitudes—both their absolute values and seasonal variations are quite unrealistic, for reasons unclear. The idealized model was also run with a mean flow based on output from the Geophysical Fluid Dynamics Laboratory (GFDL) full-physics Mars GCM. The idealized model captures the difference in mean flows between MACDA and the GFDL Mars GCM and reproduces a bias in the more complex model’s eddy zonal wavenumber distribution. This implies that the mean flow is an important influence on transient eddy wavenumbers and that improving the GFDL Mars GCM’s mean flow would make its eddy scales more realistic.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
Atmospheric tides are the primary source of daily air pressure variation at the surface of Mars. These tides are forced by solar heating of the atmosphere and modulated by the presence of atmospheric dust, topography, and surface albedo and thermal inertia. This results in a complex mix of sun-synchronous and non-sun-synchronous tides propagating both eastward and westward around the planet in periods that are integer fractions of a solar day. The Rover Environmental Monitoring Station on board the Mars Science Laboratory has observed air pressure at a regular cadence for over 1 Mars year and here we analyze and diagnose atmospheric tides in this pressure record. The diurnal tide amplitude varies from 26 to 63 Pa with an average phase of 0424 local true solar time, while the semidiurnal tide amplitude varies from 5 to 20 Pa with an average phase of 0929. We find that both the diurnal and semidiurnal tides in Gale Crater are highly correlated to atmospheric opacity variations at a value of 0.9 and to each other at a value of 0.77, with some key exceptions occurring during regional and local dust storms. We supplement our analysis with MarsWRF general circulation modeling to examine how a local dust storm impacts the diurnal tide in its vicinity. We find that both the diurnal tide amplitude enhancement and regional coverage of notable amplitude enhancement linearly scales with the size of the local dust storm. Our results provide the first long-term record of surface pressure tides near the martian equator.
Lewis, S R., D P Mullholland, P L Read, L Montabone, R John Wilson, and M D Smith, January 2016: The solsticial pause on Mars: 1. A planetary wave reanalysis. Icarus, 264, DOI:10.1016/j.icarus.2015.08.039. Abstract
Large-scale planetary waves are diagnosed from an analysis of profiles retrieved from the Thermal Emission Spectrometer aboard the Mars Global Surveyor spacecraft during its scientific mapping phase. The analysis is conducted by assimilating thermal profiles and total dust opacity retrievals into a Mars global circulation model. Transient waves are largest throughout the northern hemisphere autumn, winter and spring period and almost absent during the summer. The southern hemisphere exhibits generally weaker transient wave behaviour. A striking feature of the low-altitude transient waves in the analysis is that they show a broad subsidiary minimum in amplitude centred on the winter solstice, a period when the thermal contrast between the summer hemisphere and the winter pole is strongest and baroclinic wave activity might be expected to be strong. This behaviour, here called the ‘solsticial pause,’ is present in every year of the analysis. This strong pause is under-represented in many independent model experiments, which tend to produce relatively uniform baroclinic wave activity throughout the winter. This paper documents and diagnoses the transient wave solsticial pause found in the analysis; a companion paper investigates the origin of the phenomenon in a series of model experiments.
Waugh, D W., A D Toigo, S D Guzewich, S J Greybush, R John Wilson, and L Montabone, September 2016: Martian Polar Vortices: Comparison of Reanalyses. Journal of Geophysical Research: Planets, 121(9), DOI:10.1002/2016JE005093. Abstract
The structure and evolution of the Martian polar vortices is examined using two recently available reanalysis systems: version 1.0 of the Mars Analysis Correction Data Assimilation (MACDA) and a preliminary version of the Ensemble Mars Atmosphere Reanalysis System (EMARS). There is quantitative agreement between the reanalyses in the lower atmosphere, where Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data is assimilated, but there are differences at higher altitudes reflecting differences in the free-running general circulation model simulations used in the two reanalyses. The reanalyses show similar potential vorticity (PV) structure of the vortices: There is near-uniform small PV equatorward of the core of the westerly jet, steep meridional PV gradients on the polar side of the jet core, and a maximum of PV located off of the pole. In maps of 30 sol-mean PV, there is a near-continuous elliptical ring of high PV with roughly constant shape and longitudinal orientation from fall to spring. However, the shape and orientation of the vortex varies on daily time scales, and there is not a continuous ring of PV but rather a series of smaller scale coherent regions of high PV. The PV structure of the Martian polar vortices is, as has been reported before, very different from that of Earth's stratospheric polar vortices, but there are similarities with Earth's tropospheric vortices which also occur at the edge of the Hadley Cell, have near-uniform small PV equatorward of the jet, and a large increase of PV poleward of the jet due to increased stratification.
Kahre, M A., J L Hollingsworth, R M Haberle, and R John Wilson, November 2015: Coupling the Mars dust and water cycles: The importance of radiative-dynamic feedbacks during northern hemisphere summer. Icarus, 260, DOI:10.1016/j.icarus.2014.07.017. Abstract
Mars Global Climate Model (MGCM) simulations are carried out with and without cloud radiative forcing to investigate feedbacks between the dust and water cycles that contribute to the middle-atmosphere polar warming during northern hemisphere summer. Compared to the simulation without clouds, the simulation with clouds produces stronger polar warming, which is in better agreement with observations. The enhanced polar warming in the presence of cloud formation is caused by a radiative-dynamic feedback between a strengthened circulation due to cloud radiative effects, vertical dust transport, and further circulation intensification.
We have produced a multiannual climatology of airborne dust from martian year 24–31 using multiple datasets of retrieved or estimated column optical depths. The datasets are based on observations of the martian atmosphere from April 1999 to July 2013 made by different orbiting instruments: the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey, and the Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). The procedure we have adopted consists of gridding the available retrievals of column dust optical depth (CDOD) from TES and THEMIS nadir observations, as well as the estimates of this quantity from MCS limb observations. Our gridding method calculates averages and uncertainties on a regularly spaced spatio-temporal grid, using an iterative procedure that is weighted in space, time, and retrieval quality. The lack of observations at certain times and locations introduces missing grid points in the maps, which therefore may result in irregularly gridded (i.e. incomplete) fields. In order to evaluate the strengths and weaknesses of the resulting gridded maps, we compare with independent observations of CDOD by PanCam cameras and Mini-TES spectrometers aboard the Mars Exploration Rovers “Spirit” and “Opportunity”, by the Surface Stereo Imager aboard the Phoenix lander, and by the Compact Reconnaissance Imaging Spectrometer for Mars aboard MRO. We have statistically analyzed the irregularly gridded maps to provide an overview of the dust climatology on Mars over eight years, specifically in relation to its interseasonal and interannual variability, in addition to provide a basis for instrument intercomparison. Finally, we have produced regularly gridded maps of CDOD by spatially interpolating the irregularly gridded maps using a kriging method. These complete maps are used as dust scenarios in the Mars Climate Database (MCD) version 5, and are useful in many modeling applications. The two datasets for the eight available martian years are publicly available and distributed with open access on the MCD website.
Mooring, Todd A., and R John Wilson, October 2015: Transient eddies in the MACDA Mars reanalysis. Journal of Geophysical Research: Planets, 120(10), DOI:10.1002/2015JE004824. Abstract
We present a survey of the transient eddy activity in the Mars Analysis Correction Data Assimilation reanalysis. The spatial structure and propagation characteristics of the eddies are emphasized. Bandpass-filtered variance and covariance fields are found to be zonally modulated, indicating a longitude dependence of the typical amplitudes of Martian transient eddies. Considerable repeatability of the eddy field spatial structures is found across Mars years, including a roughly wavenumber 3 pattern of low-level eddy meridional temperature transport inline image in the northern hemisphere that is evident before and after winter solstice and a possible tendency for northern hemisphere eddy kinetic energy maxima to be located above low-lying areas. Southern hemisphere eddy fields tend to feature two local maxima, one roughly south of Tharsis and the other associated with Hellas. Eddies are weakened near winter solstice in both hemispheres and were generally weakened in the northern hemisphere during the 2001 (Mars year 25) global dust storm, albeit with little change in spatial patterns. Because the transient eddies propagate in space, we also used a teleconnection map-based technique to estimate their phase velocities. Eddy propagation at the surface is found to follow topography, a phenomenon less evident at higher altitude. Possible physical mechanisms underlying the documented eddy phenomena are discussed.
Zhao, Yongjing, S J Greybush, R John Wilson, R N Hoffman, and E Kalnay, May 2015: Impact of assimilation window length on diurnal features in a Mars atmospheric analysis. Tellus A, 67, DOI:10.3402/tellusa.v67.26042. Abstract
ective simulation of diurnal variability is an important aspect of many geophysical data assimilation systems. For the Martian atmosphere, thermal tides are particularly prominent and contribute much to the Martian atmospheric circulation, dynamics and dust transport. To study the Mars diurnal variability and Mars thermal tides, the Geophysical Fluid Dynamics Laboratory Mars Global Climate Model with the 4D-local ensemble transform Kalman filter (4D-LETKF) is used to perform an analysis assimilating spacecraft temperature retrievals. We find that the use of a ‘traditional’ 6-hr assimilation cycle induces spurious forcing of a resonantly enhanced semi-diurnal Kelvin waves represented in both surface pressure and mid-level temperature by forming a wave 4 pattern in the diurnal averaged analysis increment that acts as a ‘topographic’ stationary forcing. Different assimilation window lengths in the 4D-LETKF are introduced to remove the artificially induced resonance. It is found that short assimilation window lengths not only remove the spurious resonance, but also push the migrating semi-diurnal temperature variation at 50 Pa closer to the estimated ‘true’ tides even in the absence of a radiatively active water ice cloud parameterisation. In order to compare the performance of different assimilation window lengths, short-term to mid-range forecasts based on the hour 00 and 12 assimilation are evaluated and compared. Results show that during Northern Hemisphere summer, it is not the assimilation window length, but the radiatively active water ice clouds that influence the model prediction. A ‘diurnal bias correction’ that includes bias correction fields dependent on the local time is shown to effectively reduce the forecast root mean square differences between forecasts and observations, compensate for the absence of water ice cloud parameterisation and enhance Martian atmosphere prediction. The implications of these results for data assimilation in the Earth's atmosphere are discussed.
Guzewich, S D., and R John Wilson, et al., March 2014: Thermal Tides During the 2001 Martian Global-Scale Dust Storm. Journal of Geophysical Research: Planets, 119(3), DOI:10.1002/2013JE004502. Abstract
The 2001 (Mars Year 25) global dust storm radically altered the dynamics of the Martian atmosphere. Using observations from the Thermal Emission Spectrometer onboard the Mars Global Surveyor spacecraft and MarsWRF general circulation model simulations, we examine the changes to thermal tides and planetary waves caused by the storm. We find that the extratropical diurnal migrating tide is dramatically enhanced during the storm, particularly in the southern hemisphere, reaching amplitudes of more than 20 K. The tropical diurnal migrating tide is weakened to almost undetectable levels. The diurnal Kelvin waves are also significantly weakened, particularly during the period of global expansion at Ls = 200°-210°. In contrast, the westward-propagating diurnal wavenumber 2 tide strengthens to 4-8 K at altitudes above 30 km. The wavenumber 1 stationary wave reaches amplitudes of 10-12 K at 50°-70°N, far larger than is typically seen during this time of year. The phase of this stationary wave and the enhancement of the diurnal wavenumber 2 tide appear to be responses to the high altitude westward-propagating equatorial wavenumber 1 structure in dust mixing ratio observed during the storm in previous works. This work provides a global picture of dust storm wave dynamics that reveals the coupling between the tropics and high latitude wave responses. We conclude that the zonal distribution of thermotidal forcing from atmospheric aerosol concentration is as important to understanding the atmospheric wave response as the total global mean aerosol optical depth.
Heavens, N G., M S Johnson, W A Abdou, D M Kass, A Kleinbohl, D J McCleese, J H Shirley, and R John Wilson, August 2014: Seasonal and Diurnal Variability of Detached Dust Layers in the Tropical Martian Atmosphere. Journal of Geophysical Research: Planets, 119(8), DOI:10.1002/2014JE004619. Abstract
Evidence for widespread non-uniform vertical mixing of dust in Mars's tropical atmosphere (in the form of features called “detached dust layers” or DDLs) is a challenge for atmospheric modeling. We characterize the seasonal, diurnal, and geographic variability of DDL activity in retrievals from observations by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter (MRO). We find that dust injection above the boundary layer, which forms DDLs, is a spatially ubiquitous phenomenon in the tropics during the daytime, implying that it has a significant non-topographic component. DDL formation is more intense in northern spring and summer than in southern spring and summer, but is still common when the zonal average dust distribution appears uniformly mixed. DDLs do not appear to follow the upwelling associated with Mars's Hadley circulation or the extant climatology of local dust storm activity in the tropics. Geographic variability in the nightside vertical dust distribution does not always correlate with the dayside vertical dust distribution, implying that there is spatial and seasonal variability in the efficiency of dust deposition/removal processes. Nighttime dust removal is especially efficient over the Tharsis Montes during northern spring and summer, which suggests some association between water ice clouds and removal. Intense injection combined with efficient removal results in a high amplitude of diurnal variability in the dust distribution at 15-30 km above the surface of the tropics during much of the Martian year.
Wilson, R J., and S D Guzewich, May 2014: Influence of Water Ice Clouds on Nighttime Tropical Temperature Structure as Seen by the Mars Climate Sounder. Geophysical Research Letters, 41(10), DOI:10.1002/2014GL060086. Abstract
An analysis of nighttime temperature and water ice cloud extinction profiles fromthe Mars Climate
Sounder aboard the Mars Reconnaissance Orbiter provides evidence for the close relationship between
tropical temperature structure and water ice clouds. The tropical temperature structure that evolves over the
spring and summer seasons is closely coupled to the waxing and waning of tropical cloud activity. The
presence of strong elevated nighttime temperature inversions in the Tharsis region is a robust feature of
the equatorial atmosphere during the Ls=0–135° season, with little interannual variation seen in the three
Mars years examined. Mars global circulation model simulations imply that cloud radiative forcing plays a
dominant role in the seasonal modulation of the observed longitude distribution of warm and cold anomalies
in surface and low-altitude air temperatures, respectively.
Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO3, reducing ozone destruction by the NOx catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.
Greybush, S J., E Kalnay, M J Hoffman, and R John Wilson, April 2013: Identifying Martian atmospheric instabilities and their physical origins using bred vectors. Quarterly Journal of the Royal Meteorological Society, 139(672), DOI:10.1002/qj.1990. Abstract
The bred vector (BV) technique applied to the Geophysical Fluid Dynamics Laboratory (GFDL) Mars General Circulation Model (MGCM) identifies regions and seasons of instability of the Martian atmosphere, and a kinetic energy equation applied to the control and perturbed states elucidates their physical origins. Instabilities prominent in the late autumn to early spring seasons of each hemisphere along the polar temperature front result from baroclinic conversions from BV potential to BV kinetic energy near the surface, whereas both baroclinic and barotropic conversions play a role for the westerly jets aloft. The low-level tropics and the northern hemisphere summer are relatively stable, with negative bred vector growth rates. Bred vector growth precedes initiation of travelling wave activity in the midlatitudes during the transition seasons, and their structure relates to the eddy field. Topography plays a role in determining favoured locations for near-surface instabilities. Bred vectors are also linked to forecast ensemble spread in data assimilation and help explain the growth of forecast errors. We finally note that the ability to use breeding to identify instabilities as well as their physical origin depends on the fact that both the control and the perturbed solutions that give rise to bred vectors satisfy exactly the model's governing equations. As a result, this approach can be used with any dynamical system represented by a model.
Kavulich, Jr, M J., I Szunyogh, G Gyarmati, and R John Wilson, November 2013: Local Dynamics of Baroclinic Waves in the Martian Atmosphere. Journal of the Atmospheric Sciences, 70(11), DOI:10.1175/JAS-D-12-0262.1. Abstract
The paper investigates the processes that drive the spatiotemporal evolution of baroclinic transient waves in the Martian atmosphere by a simulation experiment with the Geophysical Fluid Dynamics Laboratory (GFDL) Mars General Circulation Model (GCM). The main diagnostic tool of the study is the (local) eddy kinetic energy equation. Results are shown for a pre-winter season of the northern hemisphere, in which a deep baroclinic wave of zonal wavenumber two circles around the planet at an eastward phase speed of about 70 degree per Sol (Martian day). The regular structure of the wave gives the impression that the classical models of baroclinic instability, which describe the underlying process by a temporally unstable global wave (e.g., Eady-model and Charney model), may have a direct relevance for the description of the Martian baroclinic waves. The results of the diagnostic calculations show, however, that while the Martian waves remain zonally global features at all times, there are large spatiotemporal changes in their amplitude. The most intense episodes of baroclinic energy conversion, which take place in the two great plain regions (Acidalia Planitia and Utopia Planitia), are strongly localized in both space and time. In addition, similar to the situation for terrestrial baroclinic waves, geopotential flux convergence plays an important role in the dynamics of the downstream propagating unstable waves.
Kleinbohl, A, and R John Wilson, et al., May 2013: The semi-diurnal tide in the middle atmosphere of Mars. Geophysical Research Letters, 40(10), DOI:10.1002/grl.50497. Abstract
Atmospheric thermal tides are global oscillations in atmospheric fields that are sub-harmonics of a solar day. While atmospheric tides on Earth are mainly relevant in the upper atmosphere, on Mars they dominate temperature variations and winds throughout the atmosphere. Observations and model simulations to date have suggested that the migrating diurnal tide is the predominant mode in the martian atmosphere, and that the semi-diurnal tide is only relevant in the tropical middle atmosphere during conditions of high dust loading. New comprehensive observations by the Mars Climate Sounder in a geometry that allows coverage of multiple local times show that the semi-diurnal tide is a dominant response of the martian atmosphere throughout the martian year. The maximum semi-diurnal amplitude of ∼ 16 K is found at southern winter high latitudes, which makes it the largest tidal amplitude observed in the martian middle atmosphere outside of dust storm conditions. The semi-diurnal tide can be successfully modeled due to recent advances of Mars General Circulation Models (MGCMs) that include the radiatively active treatment of water ice clouds. Tidal forcing occurs through absorption of radiation by aerosols and points to the vertical structure of dust and clouds and their radiative effects as being essential for our understanding of the thermal structure and the general circulation of the martian atmosphere. As with terrestrial GCMs trying to quantify mechanisms affecting climate, future Mars modeling efforts will require microphysical schemes to control aerosol distributions, and vertically and temporally resolved measurements of temperature and aerosols will be essential for their validation.
Greybush, S J., and R John Wilson, et al., November 2012: Ensemble Kalman Filter data assimilation of Thermal Emission Spectrometer (TES) temperature retrievals into a Mars GCM. Journal of Geophysical Research: Planets, 117, E11008, DOI:10.1029/2012JE004097. Abstract
Thermal Emission Spectrometer (TES) retrieved temperature profiles are assimilated into the GFDL Mars Global Climate Model (MGCM) using the Local Ensemble Transform Kalman Filter (LETKF) to produce synoptic maps of temperature, winds, and surface pressure and their uncertainties over the course of a Martian year. Short term (0.25 sol) forecasts compared to independent observations show reduced root mean square error (to 3-4 K global RMSE for a 30-sol evaluation period during the northern hemisphere autumn) and bias compared to a free running model. Several enhanced techniques result in further performance gains. A 4D-LETKF considers observations at their correct hour of occurrence rather than every 6 hours. Spatially-varying adaptive inflation and varying the dust distribution among ensemble members refine estimates of analysis uncertainty through the ensemble spread. Enhancing dust and water ice aerosol schemes and the application of empirical bias correction using time mean analysis increments help account for model biases. Full-year experiments using prescribed dust opacities and observed TES dust opacities show that while realistic dust distributions are essential to match observed temperatures with a free run simulation, analyses from data assimilation are more robust with respect to imperfections in aerosol distribution. The data assimilation system described here is being used to generate a new reanalysis of Mars weather and climate, which will have many scientific and engineering applications.
Many prior studies clearly document episodic Asian pollution in the western U.S.
free troposphere. Here, we examine the mechanisms involved in the transport of Asian
pollution plumes into western U.S. surface air through an integrated analysis of in situ
and satellite measurements in May–June 2010 with a new global high-resolution
(50 50 km2) chemistry-climate model (GFDL AM3). We find that AM3 with
full stratosphere-troposphere chemistry nudged to reanalysis winds successfully
reproduces observed sharp ozone gradients above California, including the interleaving
and mixing of Asian pollution and stratospheric air associated with complex interactions of
midlatitude cyclone air streams. Asian pollution descends isentropically behind cold fronts;
at 800 hPa a maximum enhancement to ozone occurs over the southwestern U.S.,
including the densely populated Los Angeles Basin. During strong episodes, Asian
emissions can contribute 8–15 ppbv ozone in the model on days when observed daily
maximum 8-h average ozone (MDA8 O3) exceeds 60 ppbv. We find that in the absence
of Asian anthropogenic emissions, 20% of MDA8 O3 exceedances of 60 ppbv in the model
would not have occurred in the southwestern USA. For a 75 ppbv threshold, that
statistic increases to 53%. Our analysis indicates the potential for Asian emissions to
contribute to high-O3 episodes over the high-elevation western USA, with implications
for attaining more stringent ozone standards in this region. We further demonstrate a
proof-of-concept approach using satellite CO column measurements as a qualitative early
warning indicator to forecast Asian ozone pollution events in the western U.S. with
lead times of 1–3 days.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.
Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.
Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.
The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Austin, John, and R John Wilson, September 2010: Sensitivity of polar ozone to sea surface temperatures and halogen amounts. Journal of Geophysical Research: Atmospheres, 115, D18303, DOI:10.1029/2009JD013292. Abstract
Coupled chemistry-climate model simulations are presented for the period 1951 to 2099, and for shorter periods. All the simulations include variations in the concentrations of the well-mixed greenhouse gases and chlorofluorocarbons. Run 1 also includes observed levels of bromine, and sea surface temperatures (SSTs) from a coupled ocean-atmosphere model. Run 2 includes observed levels of both bromine and SSTs. In Run 3 the bromine amount is reduced by 25\% but is otherwise as Run 2. The analysis concentrates on the period 1980 onwards when the Antarctic ozone hole developed. The results show that the ozone hole is sensitive to SSTs and bromine amounts. For the period 1990 to 2007, when the ozone hole was fully developed, the area of the ozone hole was simulated to be largest in Run 1 (7\% smaller than observed), compared with underpredictions of 13\% and 24\% for Runs 2 and 3 respectively. The sensitivity of the ozone hole to tropical SSTs is a manifestation of stratosphere-troposphere coupling. The effect of SSTs is shown to arise from changes in the strength of the Brewer-Dobson circulation which is weaker for the simulation with model SSTs. The sensitivity of the model results
to bromine indicates the need to include realistic bromine amounts and may explain in part the substantial model underpredictions of the ozone hole area in previous publications. Linear regression analysis of the results confirms that the ozone hole is sensitive to chlorine, bromine, SSTs and meridional heat flux, but insensitive to the solar cycle. The results also suggest that the ozone hole may not disappear entirely this century, and that a small, residual ozone hole may still be present after 2060.
Austin, John, R John Wilson, and Y Yamashita, et al., November 2010: The decline and recovery of total column ozone using a multi-model time series analysis. Journal of Geophysical Research: Atmospheres, 115, D00M10, DOI:10.1029/2010JD013857. Abstract
Simulations of 15 coupled chemistry climate models, for the period 1960–2100, are
presented. The models include a detailed stratosphere, as well as including a realistic
representation of the tropospheric climate. The simulations assume a consistent set of
changing greenhouse gas concentrations, as well as temporally varying chlorofluorocarbon
concentrations in accordance with observations for the past and expectations for the future.
The ozone results are analyzed using a nonparametric additive statistical model.
Comparisons are made with observations for the recent past, and the recovery of ozone,
indicated by a return to 1960 and 1980 values, is investigated as a function of latitude.
Although chlorine amounts are simulated to return to 1980 values by about 2050, with
only weak latitudinal variations, column ozone amounts recover at different rates due to
the influence of greenhouse gas changes. In the tropics, simulated peak ozone amounts
occur by about 2050 and thereafter total ozone column declines. Consequently,
simulated ozone does not recover to values which existed prior to the early 1980s. The
results also show a distinct hemispheric asymmetry, with recovery to 1980 values in the
Northern Hemisphere extratropics ahead of the chlorine return by about 20 years. In the
Southern Hemisphere midlatitudes, ozone is simulated to return to 1980 levels only
10 years ahead of chlorine. In the Antarctic, annually averaged ozone recovers at
about the same rate as chlorine in high latitudes and hence does not return to 1960s values
until the last decade of the simulations.
Hoffman, M J., S J Greybush, and R John Wilson, et al., October 2010: An ensemble Kalman filter data assimilation system for the martian atmosphere: Implementation and simulation experiments. Icarus, 209(2), DOI:10.1016/j.icarus.2010.03.034. Abstract
The local ensemble transform Kalman filter (LETKF) is applied to the GFDL Mars general circulation model (MGCM) to demonstrate the potential benefit of an advanced data assimilation method. In perfect model (aka identical twin) experiments, simulated observations are used to assess the performance of the LETKF–MGCM system and to determine the dependence of the assimilation on observational data coverage. Temperature retrievals are simulated at locations that mirror the spatial distribution of the Thermal Emission Spectrometer (TES) retrievals from the Mars Global Surveyor (MGS). The LETKF converges quickly and substantially reduces the analysis and subsequent forecast errors in both temperature and velocity fields, even though only temperature observations are assimilated. The LETKF is also found to accurately estimate the magnitude of forecast uncertainties, notably those associated with the phase and amplitude of baroclinic waves along the boundary of the polar ice cap during Northern Hemisphere winter.
Zalucha, A M., R A Plumb, and R John Wilson, March 2010: An analysis of the effect of topography on the Martian Hadley cells. Journal of the Atmospheric Sciences, 67(3), DOI:10.1175/2009JAS3130.1. Abstract
Previous work with Mars General Circulation Models (MGCMs) has shown that the northsouth
slope in Martian topography causes asymmetries in the Hadley cells at equinox and in
the annual average. To quantitatively solve for the latitude of the dividing streamline and
poleward boundaries of the cells, the Hadley cell model of Lindzen and Hou was modified
to include topography. The model was thermally forced by Newtonian relaxation to an
equilibrium temperature profile calculated with daily averaged solar forcing at constant
season. Two sets of equilibrium temperatures were considered that either contained the
effects of convection or did not. When convective effects were allowed, the presence of the
slope component shifted the dividing streamline upslope, qualitatively similar to a change
in season in Lindzen and Hou’s original (flat) model. The modified model also confirmed
that the geometrical effects of the slope are much smaller than the thermal effects of the
slope on the radiative-convective equilibrium temperature aloft. The results are compared
to a simple MGCM forced by Newtonian relaxation to the same equilibrium temperature
profiles, and the two models agree except at the winter pole near solstice. The simple MGCM
results for radiative-convective forcing also show an asymmetry between the strengths of the
Hadley cells at northern summer and northern winter solstices. The Hadley cell weakens with
increasing slope steepness at northern summer solstice, but has little effect on the strength
at northern winter solstice.
Austin, John, and R John Wilson, et al., July 2009: Coupled chemistry climate model simulations of stratospheric temperatures and their trends for the recent past. Geophysical Research Letters, 36, L13809, DOI:10.1029/2009GL038462. Abstract
Temperature results from multi-decadal simulations of coupled chemistry climate models for the recent past are analyzed using multi-linear regression including a trend, solar cycle, lower stratospheric tropical wind, and volcanic aerosol terms. The climatology of the models for recent years is in good agreement with observations for the troposphere but the model results diverge from each other and from observations in the stratosphere. Overall, the models agree better with observations than in previous assessments, primarily because of corrections in the observed temperatures. The annually averaged global and polar temperature trends simulated by the models are generally in agreement with revised satellite observations and radiosonde data over much of their altitude range. In the global average, the model trends underpredict the radiosonde data slightly at the top of the observed range. Over the Antarctic some models underpredict the temperature trend in the lower stratosphere, while others overpredict the trends.
Hinson, D P., M. Pätzold, R John Wilson, B Häusler, S Tellmann, and G L Tyler, 2008: Radio occultation measurements and MGCM simulations of Kelvin waves on Mars. Icarus, 193(1), DOI:10.1016/j.icarus.2007.09.009. Abstract
We have derived new results concerning thermal tides on Mars from a combination of radio occultation measurements and numerical simulations by a Mars General Circulation Model (MGCM). This investigation exploits a set of concurrent observations by Mars Express (MEX) and Mars Global Surveyor (MGS) in mid-2004, when the season on Mars was midspring in the northern hemisphere. The MEX occultations sampled the atmosphere near the evening terminator at latitudes ranging from 54° N to 15° S. The MGS occultations provided complementary coverage near the morning terminator at latitudes of 35° N and 71° S. The geopotential field derived from these measurements contains distinctive modulation caused by solar-asynchronous thermal tides. Through careful analysis of the combined observations, we characterized two prominent wave modes, obtaining direct solutions for some properties, such as the amplitude and phase, as well as constraints on others, such as the period, zonal wave number, and meridional structure. We supplemented these observations with MGCM simulations. After evaluating the performance of the MGCM against the measurements, we used the validated simulation to deduce the identity of the two tidal modes and to explore their behavior. One mode is a semidiurnal Kelvin wave with a zonal wave number of 2 (SK2), while the other is a diurnal Kelvin wave with a zonal wave number of 1 (DK1). Both modes are known to be close to resonance in the martian atmosphere. Our observations of the SK2 are more complete and less ambiguous than any previous measurement. The well-known DK1 is the dominant solar-asynchronous tide in the martian atmosphere, and our results confirm and extend previous observations by diverse instruments.
Li, Feng, John Austin, and R John Wilson, January 2008: The strength of the Brewer-Dobson Circulation in a changing climate: Coupled chemistry-climate model simulations. Journal of Climate, 21(1), DOI:10.1175/2007JCLI1663.1. Abstract
The strength of the Brewer–Dobson circulation (BDC) in a changing climate is studied using multidecadal simulations covering the 1960–2100 period with a coupled chemistry–climate model, to examine the seasonality of the change of the BDC. The model simulates an intensification of the BDC in both the past (1960–2004) and future (2005–2100) climate, but the seasonal cycle is different. In the past climate simulation, nearly half of the tropical upward mass flux increase occurs in December–February, whereas in the future climate simulation the enhancement of the BDC is uniformly distributed in each of the four seasons. A downward control analysis implies that this different seasonality is caused mainly by the behavior of the Southern Hemisphere planetary wave forcing, which exhibits a very different long-term trend during solstice seasons in the past and future. The Southern Hemisphere summer planetary wave activity is investigated in detail, and its evolution is found to be closely related to ozone depletion and recovery. In the model results for the past, about 60% of the lower-stratospheric mass flux increase is caused by ozone depletion, but because of model ozone trend biases, the atmospheric effect was likely smaller than this. The remaining fraction of the mass flux increase is attributed primarily to greenhouse gas increase. The downward control analysis also reveals that orographic gravity waves contribute significantly to the increase of downward mass flux in the Northern Hemisphere winter lower stratosphere.
Wilson, R J., S R Lewis, L Montabone, and M D Smith, April 2008: Influence of water ice clouds on Martian tropical atmospheric temperatures. Geophysical Research Letters, 35, L07202, DOI:10.1029/2007GL032405. Abstract
The Reanalysis derived from the UK Mars general circulation model assimilation of Thermal Emission Spectrometer temperature and dust opacity retrievals at present provides the best estimate of the evolving state of the Martian atmosphere over the course of the Mars Global Surveyor mapping mission. A Control simulation has also been carried out using the same evolving dust distribution as the Reanalysis, but without the temperature assimilation. Differences in zonal mean temperatures between these two simulations reflect possible biases in the representation of dynamical and radiative forcing in the assimilating model. We have identified a cold bias in the Control simulation of tropical temperature which develops in the northern hemisphere summer solstice season. We attribute this bias to the absence of radiatively active water ice clouds in the model and show that clouds likely play a prominent role in shaping the vertical thermal structure of the tropical atmosphere during this season.
Austin, John, R John Wilson, Feng Li, and H Vömel, 2007: Evolution of water vapor concentrations and stratospheric age of air in coupled chemistry-climate model simulations. Journal of the Atmospheric Sciences, 64(3), DOI:10.1175/JAS3866.1. Abstract
Stratospheric water vapor concentrations and age of air are investigated in an ensemble of coupled chemistry-climate model simulations covering the period from 1960 to 2005. Observed greenhouse gas concentrations, halogen concentrations, aerosol amounts, and sea surface temperatures are all specified in the model as time-varying fields. The results are compared with two experiments (time-slice runs) with constant forcings for the years 1960 and 2000, in which the sea surface temperatures are set to the same climatological values, aerosol concentrations are fixed at background levels, while greenhouse gas and halogen concentrations are set to the values for the relevant years.
The time-slice runs indicate an increase in stratospheric water vapor from 1960 to 2000 due primarily to methane oxidation. The age of air is found to be significantly less in the year 2000 run than the 1960 run. The transient runs from 1960 to 2005 indicate broadly similar results: an increase in water vapor and a decrease in age of air. However, the results do not change gradually. The age of air decreases significantly only after about 1975, corresponding to the period of ozone reduction. The age of air is related to tropical upwelling, which determines the transport of methane into the stratosphere. Oxidation of increased methane from enhanced tropical upwelling results in higher water vapor amounts. In the model simulations, the rate of increase of stratospheric water vapor during the period of enhanced upwelling is up to twice the long-term mean. The concentration of stratospheric water vapor also increases following volcanic eruptions during the simulations.
Wilson, R J., G A Neumann, and M D Smith, 2007: Diurnal variation and radiative influence of Martian water ice clouds. Geophysical Research Letters, 34, L02710, DOI:10.1029/2006GL027976. Abstract
We have identified regions in the Martian tropics with anomalously warm nighttime surface temperatures. The seasonal evolution of these anomalies is strongly correlated with the waxing and waning of the tropical cloud belt that is most prominent during Northern Hemisphere summer. We attribute the anomalies to enhanced downward infrared radiation from water ice clouds. The close agreement with spatial maps of atmospheric extinction derived from Mars Orbiter Laser Altimeter radiometry strongly supports this interpretation. We show that a Mars general circulation model simulation with radiatively active water ice clouds is able to reasonably match the observed spatial pattern and amplitude of the surface temperature anomaly. The nighttime clouds are most prominent in the Tharsis and Arabia regions and are thicker (optical depth ¡ 1) and more extensive than daytime clouds. Our cloud retrievals are the first to spatially map the nighttime clouds and provide an estimate of their thermal influence.
Austin, John, and R John Wilson, 2006: Ensemble simulations of the decline and recovery of stratospheric ozone. Journal of Geophysical Research, 111, D16314, DOI:10.1029/2005JD006907. Abstract
An ensemble of simulations of a coupled chemistry-climate model is completed for 1960–2100. The simulations are divided into two periods, 1960–2005 and 1990–2100. The modeled total ozone amount decrease throughout the atmosphere from the 1960s until about 2000–2005, depending on latitude. The Antarctic ozone hole develops rapidly in the model from about the late 1970s, in agreement with observations, but it does not disappear until about 2065, about 15 years later than previous estimates. Spring averaged ozone takes even longer to recover to 1980 values. Ozone amounts in the Antarctic are determined largely by halogen amounts. In contrast, in the Arctic, ozone recovers to 1980 values about 25–35 years earlier, depending on the recovery criterion adopted. By the end of the 21st century, the climate change associated with greenhouse gas changes gives rise to a significant superrecovery of ozone in the Arctic but a less marked recovery in the Antarctic. For both polar regions, ensemble and interannual variability is greater in the future than in the past, and hence the timing of the full recovery of polar ozone is very sensitive to the definition of recovery. It is suggested that the range of recovery rates between the hemispheres simulated in the model is related to the overall increase in the strength of the Brewer-Dobson circulation, driven by increases in greenhouse gas concentrations
Basu, S, R John Wilson, M Richardson, and A Ingersoll, 2006: Simulation of spontaneous and variable global dust storms with the GFDL Mars GCM. Journal of Geophysical Research, 111, E09004, DOI:10.1029/2005JE002660. Abstract
We report on the successful simulation of global dust storms in a general circulation model. The simulated storms develop spontaneously in multiyear simulations and exhibit significant interannual variability. The simulated storms produce dramatic increases in atmospheric dustiness, global-mean air temperatures, and atmospheric circulation intensity, in accord with observations. As with observed global storms, spontaneous initiation of storms in the model occurs in southern spring and summer, and there is significant interannual variability in storm development: years with no storms are interspersed with years with storms of various sizes and specific seasonal date of initiation. Our results support the idea that variable and spontaneous global dust storm behavior can emerge from a periodically forced system (the only forcing being the diurnal and seasonal cycles) when the dust injection mechanism involves an activation threshold. In our simulations, surface wind stresses associated with resolved, large-scale (>300 km) wind systems initiate the storms. These winds are generally associated with the seasonally migrating CO2 cap boundary and sloping topography of the Hellas basin, thermal tides, and traveling waves. A very limited number of large storms begin with lifting along the frontal zones associated with traveling waves in the northern hemisphere. Explosive growth to global scales results from the intensification of the Hadley circulation and the activation of secondary dust-lifting centers
Dentener, Frank, S Kinne, T Bond, Olivier Boucher, J Cofala, S Generoso, Paul Ginoux, S Gong, J J Hoelzemann, Akihiko Ito, L Marelli, Joyce Penner, J-P Putaud, C Textor, M Schulz, Guido R van der Werf, and R John Wilson, 2006: Emissions of primary aerosol and precursor gases in the years 2000 and 1750, prescribed data-sets for AeroCom. Atmospheric Chemistry and Physics, 6, 4321-4344. Abstract
Inventories for global aerosol and aerosol precursor emissions, and auxiliary information, have been collected, assessed and prepared for the year 2000 (present-day conditions) and for the year 1750 (pre-industrial conditions). These global datasets establish a reference for input in global modeling, when simulating the aerosol impact on climate with state-of-the-art aerosol component modules. These modules stratify aerosol by type, distinguishing among dust, seasalt, sulfate, organic matter and soot. The datasets are also intended to serve as systematic constraints in sensitivity studies of the AeroCom initiative, which aims to evaluate uncertainties in aerosol global modeling. The datasets comprise daily size-resolved emissions of sea-salt and dust and monthly-to-yearly emissions for all other currently known emissions of natural and anthropogenic aerosol (precursors). The emissions are a reference dataset for aerosol modeling in the coming years and benchmark the emissions according to our knowledge in the year 2004.
Fisher, J A., M Richardson, C E Newman, M A Szwast, C Graf, S Basu, S P Ewald, A D Toigo, and R John Wilson, 2005: A survey of Martian dust devil activity using Mars Global Surveyor Mars Orbiter Camera images. Journal of Geophysical Research, 110, E03004, DOI:10.1029/2003JE002165. Abstract
A survey of dust devils using the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) wide- and narrow-angle (WA and NA) images has been undertaken. The survey comprises two parts: (1) sampling of nine broad regions from September 1997 to July 2001 and (2) a focused seasonal monitoring of variability in the Amazonis region, an active dust devil site, from March 2001 to April 2004. For part 1, dust devils were identified in NA and WA images, and dust devil tracks were identified in NA images. Great spatial variability in dust devil occurrence is highlighted, with Amazonis Planitia being the most active region examined. Other active regions included Cimmerium, Sinai, and Solis. Numerous dust devil tracks, but very few dust devils, were observed in Casius. This may suggest dust devils here occur at local times other than that of the MGS orbit (¡2 pm). Alternatively, variations in surface properties may affect the ability of dust devils to leave visible tracks. The seasonal campaign within Amazonis shows a relatively smooth variation of dust devil activity with season, peaking in mid northern summer and falling to zero in southern spring and summer. This pattern of activity correlates well with the boundary layer maximum depth and hence the vigor of convection. Global maps of boundary layer depth and surface temperature do not predict that Amazonis should be especially active, potentially suggesting a role for mesoscale circulations. Measurement of observed dust devils yields heights of up to 8 km and widths in excess of 0.5 km.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Banfield, D, B J Conrath, P J Gierasch, R John Wilson, and M D Smith, August 2004: Traveling waves in the martian atmosphere from MGS TES Nadir data. Icarus, 170(2), 365-403. Abstract
We have characterized the annual behavior of martian atmospheric traveling waves in the MGS TES data set from the first two martian years of mapping. There is a high degree of repeatability between the two years. They are dominated by strong low zonal wavenumber waves with high amplitudes near the polar jets, strongest in late northern fall and early northern winter. The m=1 waves have amplitudes up to about 20 K, are vertically extended, and occasionally extend even into the tropics. Periods for m=1 range from 2.5 to 30 sols. Much weaker waves were identified in the south, with amplitudes less than about 3.5 K. Traveling waves with m=2 and m=3 are also seen, but their amplitudes are typically limited to less than 4 K, and are generally more confined near the surface. In the north, they are more evident in fall and spring rather than winter solstice, which is clearly dominated by m=1 waves. Some evidence of storm tracks has been identified in the data, with accentuated weather-related temperature perturbations near longitudes 200° to 320° E for both the southern and northern hemispheres near latitude ±65° at the surface. Some evidence was also found for a sharpening of longitudinal gradients into what may be frontal systems. EP flux divergences show the waves extracting energy from the zonal mean winds. When the m=1 waves were strongest, decelerations of the zonal jet of order 30 m/(s sol) were measured. Above 1 scale height, the waves extract energy from the jet predominately through barotropic processes, but their character is overall mixed barotropic/baroclinic. Inertial instabilities may exist at altitude on the equatorward flanks of the polar jets, and marginal stability extends through to the tropics. This may explain the coordination of the tropical behavior of the waves with that centered along the polar jet, consistent with the ideas expressed in Wilson et al. (2002, Geophys. Res. Lett. 29, #1684) and similar to those in Barnes et al. (1993, J. Geophys. Res. 98, 3125–3148). Throughout the year, there exist large regions with the meridional gradient of PV less than zero, but they are strongest near winter solstice. Poleward of the winter jet, the regions of instability reach the surface, equatorward they do not. These regions, satisfying a necessary criterion for instability, likely explain the genesis of the waves, and perhaps also their bimodal character between surface (faster waves) and altitude (slow m=1 waves).
Basu, S, M Richardson, and R John Wilson, 2004: Simulation of the Martian dust cycle with the GFDL Mars GCM. Journal of Geophysical Research, 109, E11906, DOI:10.1029/2004JE002243. Abstract
The Martian seasonal dust cycle is examined with a general circulation model (GCM) that treats dust as a radiatively and dynamically interactive trace species. Dust injection is parameterized as being due to convective processes (such as dust devils) and model-resolved wind stresses. Size-dependent dust settling, transport by large-scale winds and subgrid scale diffusion, and radiative heating due to the predicted dust distribution are treated. Multiyear Viking and Mars Global Surveyor air temperature data are used to quantitatively assess the simulations. Varying the three free parameters for the two dust injection schemes (rate parameters for the two schemes and a threshold for wind-stress lifting), we find that the highly repeatable northern spring and summer temperatures can be reproduced by the model if the background dust haze is supplied by either convective lifting or by stress lifting with a very low threshold and a low injection rate. Dust injection due to high-threshold, high-rate stress lifting must be added to these to generate spontaneous and variable dust storms. In order to supply the background haze, widespread and ongoing lifting is required by the model. Imaging data provide a viable candidate mechanism for convective lifting, in the form of dust devils. However, observed nonconvective lifting systems (local storms, etc.) appear insufficiently frequent and widespread to satisfy the role. On the basis of the model results and thermal and imaging data, we suggest that the background dust haze on Mars is maintained by convective processes, specifically, dust devils. Combining the convective scheme and high-threshold stress lifting, we obtain a “best fit” multiyear simulation, which produces a realistic thermal state in northern spring and summer and, for the first time, spontaneous and interannually variable global dust storms.
Hinson, D P., and R John Wilson, 2004: Temperature inversions, thermal tides, and water ice clouds in the Martian tropics. Journal of Geophysical Research, 109, E01002, DOI:10.1029/2003JE002129. Abstract
We report new results on the structure and dynamics of the tropical atmosphere of Mars derived from a combination of radio occultation measurements by Mars Global Surveyor and simulations by a Mars general circulation model (MGCM). Radio occultation experiments sounded the equatorial atmosphere at latitudes of 36°N to 30°S during midsummer of the Northern Hemisphere (Ls = 134°–162°), sampling the predawn thermal structure at a local time of ~0412. Elevated temperature inversions are a conspicuous feature of these observations. They appear at pressures between 30 and 200 Pa, well above the surface, and their magnitude exceeds 6 K in 34% of the temperature profiles in this latitude band. The properties and spatial distribution of these elevated inversions are organized across the tropics on planetary scales. Inversions are strongest and occur most frequently above elevated terrain, achieving a peak magnitude of ~30 K near Tharsis, and their altitude generally increases toward the south. According to MGCM simulations, which closely resemble the observations, these temperature inversions arise from zonally modulated thermal tides. The best simulation includes an interactive hydrologic cycle, which results in strong coupling between the thermal tides and radiatively active water ice clouds. Prominent clouds form in response to wave-induced adiabatic cooling and evolve in a pattern closely correlated with the thermal structure of the tides. The tides in turn are intensified by radiative forcing from the clouds. This tide-cloud coupling imposes strong diurnal modulation on the properties of clouds in the tropics.
Banfield, D, B J Conrath, M D Smith, P R Christensen, and R John Wilson, February 2003: Forced waves in the martian atmosphere from MGS TES nadir data. Icarus, 161(2), 319-345. Abstract
We have analyzed the temperature retrievals from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) nadir spectra to yield latitude-height resolved maps of various atmospheric forced wave modes as a function of season for a full Mars year. Among the isolated wave modes is the zonal mean, time mean temperature, which we used to derive zonal mean zonal winds and stationary wave quasi-geostrophic indices of refraction, diagnostic of their propagation. The diurnal Kelvin wave was isolated in the data, with results roughly consistent with models (Wilson and Hamilton, 1996. J. Atmos. Sci. 33, 1290-1326). The s = 1 and s = 2 stationary waves were found to have significant amplitude in ducts extending up the winter polar jets, while the s = 3 stationary wave was found to be confined to near the surface. The s = 1 stationary wave was found to have little phase tilt with height during northern winter, but significant westward phase tilt with height in the southern winter. This indicates that the wave carries heat poleward, slightly more than that found in Barnes et al. (1996, J. Geophs. Res. 101, 12,753-12,776). The s = 1 stationary wave is likely the dominant mechanism for eddy meridional heat transport for the southern winter. We noted that the phase of the s = 2 stationary wave is nearly constant with time, but that the s = 1 stationary wave moved 90° of longitude from fall to winter and back in spring in the North. While interannual variability is not yet addressed, overall, these results provide the first comprehensive benchmark for forced waves in Mars atmosphere against which future atmospheric models of Mars can be computed.
Hinson, D P., R John Wilson, M D Smith, and A Abe-Ouchi, 2003: Stationary planetary waves in the atmosphere of Mars during southern winter. Journal of Geophysical Research, 108(E1), 5004, DOI:10.1029/2002JE001949. Abstract
We report new observations of stationary planetary waves in the Southern Hemisphere of Mars by Mars Global Surveyor (MGS). We focus on a period during midwinter (Ls = 134° – 160°) when independent observations were acquired by two techniques. Radio occultation experiments sounded the atmosphere at essentially fixed latitude (~68°S) and local time (~1030), yielding profiles of geopotential and temperature between the surface and the 9 Pa pressure level. Observations by the Thermal Emission Spectrometer (TES) included systematic limb sounding at nine discrete latitudes and two local times (~0200 and ~1500), yielding temperature profiles at pressures of 1–100 Pa. We supplemented these data with a simulation by a Mars general circulation model (MGCM), which provides an accurate synthesis of the observations. These stationary planetary waves have significant amplitudes at zonal wave numbers s =1 and 2. The s =1 component propagates vertically, as reflected by a westward tilt with increasing height in the geopotential and temperature fields and a net poleward eddy heat flux. The peak amplitude at s =1 is ~1 km in geopotential height and ~7 K in temperature. The geopotential field of the s =2 component is "barotropic" in character, which results through hydrostatic balance in a distinctive temperature field. The peak amplitude at s =2 is ~700 m in geopotential height and 4 – 6 K in temperature.
Horinouchi, T, S Pawson, K Shibata, U Langematz, E Manzini, M A Giorgetta, F Sassi, R John Wilson, J De Grandpré, and Adam A Scaife, 2003: Tropical cumulus convection and upward-propagating waves in middle-atmospheric GCMs. Journal of the Atmospheric Sciences, 60(22), 2765-2782. Abstract
It is recognized that the resolved tropical wave spectrum can vary considerably among general circulation models (GCMs) and that these differences can have an important impact on the simulated climate. A comprehensive comparison of low-latitude waves is presented for the December-January-February period using high-frequency data from nine GCMs participating in the GCM Reality Intercomparison Project for Stratospheric Processes and Their Role in Climate (GRIPS; SPARC). Quantitative measures of the wavenumber-frequency structure of resolved waves and their impacts on the zonal mean circulation are given. Space-time spectral analysis reveals that the wave spectrum throughout the middle atmosphere is linked to the variability of convective precipitation, which is determined by the parameterized convection. The variability of the precipitation spectrum differs by more than an order of magnitude among the models, with additional changes in the spectral distribution (especially the frequency). These differences can be explained primarily by the choice of different cumulus parameterizations: quasi-equilibrium mass-flux schemes tend to produce small variability, while the moist-convective adjustment scheme is the most active. Comparison with observational estimates of precipitation variability suggests that the model values are scattered around the observational estimates. Among the models, only those that produce the largest precipitation variability can reproduce the equatorial quasi-biennial oscillation (QBO). This implies that in the real atmosphere, the forcing from the waves, which are resolvable with the typical resolutions of present-day GCMs, is insufficient to drive the QBO. Parameterized cumulus convection also impacts the nonmigrating tides in the equatorial region. In most of the models, momentum transport by diurnal nonmigrating tides in the mesosphere is comparable to or larger than that by planetary-scale Kelvin waves, being more significant than has been thought. It is shown that the westerly accelerations in the equatorial semi-annual oscillation in the models examined are driven mainly by gravity waves with periods shorter than 3 days, with some contribution from parameterized gravity waves, and that the contribution from the wavenumber-1 Kelvin waves is negligible. These results provide a state-of-the-art assessment of the links between convective parameterizations and middle-atmospheric waves in present-day middle-atmosphere climate models.
Leroy, S S., Yuk L Yung, M Richardson, and R John Wilson, 2003: Principal modes of variability of Martian atmospheric surface pressure. Geophysical Research Letters, 30(13), 1707, DOI:10.1029/2002GL015909. Abstract PDF
An analysis of daily-to-interannual variability in the surface pressure field of the Martian nothern hemisphere as given by a Martian climate model is presented. In an empirical orthogonal function (EOF) decomposition, the dominant first two modes of variability comprise a zonal wavenumber 1 feature centered at 70 N latitude moving eastward with a period of 6 to 8 sols. This feature is a baroclinic wave and accounts for 53% of the northern hemisphere non-stationary surface pressure variability, and, when active, has an amplitude of up to 2% of local surface pressure. The third mode of the EOF decomposition is annular about the Martian north pole, is null southward of 70 N, and accounts for 7% of the northern hemisphere non-stationary surface pressure variability. The baroclinic wave (EOFs 1 & 2) is active during northern hemisphere winter and spring, consistent with models of the Martian atmospheric circulation, and the annular mode (EOF 3) is active only at the onset and demise of the baroclinic feature. When active, it is not uncommon for the annular mode to reside in either its positive or negative state stably for 20 to 30 sols. It is postulated that baroclinic waves with longitudinal wavenumber 2, 3, and 4 act as a pump for the annular mode. The annular mode should not be present in MGS TES data.
Liu, J, M Richardson, and R John Wilson, August 2003: An assessment of the global, seasonal, and interannual spacecraft record of Martian climate in the thermal infrared. Journal of Geophysical Research, 108(E8), 5089, DOI:10.1029/2002JE001921. Abstract
Intercomparison of thermal infrared data collected by Mariner 9, Viking, and Mars Global Surveyor (MGS) is presented with a specific focus on air temperatures, dust opacities, and water ice opacities. Emphasis is placed on creating a uniform data set to most effectively reduce interinstrument biases and offsets. The annual cycle consistently shows a strong asymmetry about the equinoxes, with northern spring and summer exhibiting relatively low temperatures, very high year-to-year repeatability, and essentially no short-term (tens of days) variability. The globally averaged Martian nighttime air temperatures close annually to within a Kelvin during northern spring and summer. Daytime temperatures show more variability (3-6 K). The difference in repeatability of daytime versus nighttime temperatures is not understood. Viking and MGS air temperatures are essentially indistinguishable for this period, suggesting that the Viking and MGS eras are characterized by essentially the same climatic state. Southern summer is characterized by strong dust storm activity and hence strong year-to-year air temperature variability. Dust opacity shows a remarkable degree of interannual variability in southern spring and summer, associated with the intermittent activity of regional and planet-encircling dust storms, but exhibits high year-to-year repeatability in northern spring and summer. Specifically, late northern spring and early northern summer dust opacities appear to be completely insensitive to the occurrence (or not) of major dust storms in the previous southern spring or summer. We show that both Viking and MGS data sets exhibit significant (and similar) polar cap edge dust storm activity. The origins of the various major dust storms can be identified in the thermal infrared data from Viking and MGS, including the transport of dust from the northern autumn baroclinic zone into the southern hemisphere tropics, which has also been identified in visible imaging. We also note that the period around Ls = 225° is characterized by very high dust opacities associated with dust storm development or decay in every year thus far observed by spacecraft. Water ice opacities have been retrieved from Viking infrared data for the first time. We show that the northern spring and summer tropical cloud belt structure and evolution are essentially the same in each of the multiple years observed by Viking and MGS. Relatively subtle spatial features recur in the cloud belt from year to year, suggesting the influence of surface topography and thermophysical properties and a reasonably consistent supply of water vapor. The seasonal evolution of the tropical cloud belt through northern spring and summer is shown, with the only significant deviations between years occurring from Lsub>s = 140° to 160°, where opacities fall in the second MGS year associated with a small dust storm. Polar hood clouds are observed in Viking and MGS observations with similar timing and extent. Interactions between dust and water ice were highlighted in the Hellas basin region during the southern spring 1977a and 2001 dust storms. The observations demonstrate that the Martian atmosphere executes a very "repeatable" annual cycle of atmospheric phenomena. However, a major part of this cycle is the occurrence of highly variable and potentially major dust storm events. After such dust storm events the atmosphere rapidly relaxes to its stable, repeatable state.
Mischna, M, M Richardson, R John Wilson, and D J McCleese, 2003: On the orbital forcing of Martian water and CO2 cycles: A general circulation model study with simplified volatile schemes. Journal of Geophysical Research, 108(E6), 5062, DOI:10.1029/2003JE002051. Abstract PDF
Variations in the Martian water and CO2 cycles with changes in orbital and rotational parameters are examined using the Geophysical Fluid Dynamics Laboratory Mars General Circulation Model. The model allows for arbitrary specification of obliquity, eccentricity, and argument of perihelion as well as the position and thickness of surface ice. Exchange of CO2 between the surface and atmosphere is modeled, generating seasonal cycles of surface ice and surface pressure. Water is allowed to exchange between the surface and atmosphere, cloud formation is treated, and both cloud and vapor are transported by modeled winds and diffusion. Exchange of water and CO2 with the subsurface is not allowed, and radiative effects of water vapor and clouds are not treated. The seasonal cycle of CO2 is found to become more extreme at high obliquity, as suggested by simple heat balance models. Maximum pressures remain largely the same, but the minima decrease substantially as more CO2 condenses in the more extensive polar night. Vapor and cloud abundances increase dramatically with obliquity. The stable location for surface ice moves equatorward with increasing obliquity, such that by 45° obliquity, water ice is stable in the tropics only. Ice is not spatially uniform, but rather found preferentially in regions of high thermal inertia or high topography. Eccentricity and argument of perihelion can provide a second-order modification to the distribution of surface ice by altering the temporal distribution of insolation at the poles. Further model simulations reveal the robustness of these distributions for a variety of initial conditions. Our findings shed light on the nature of near-surface, ice-rich deposits at midlatitudes and low-latitudes on Mars.
Wang, H, M Richardson, R John Wilson, A Ingersoll, A D Toigo, and R W Zurek, 2003: Cyclones, tides, and the origin of a cross-equatorial dust storm on Mars. Geophysical Research Letters, 30(9), 1488, DOI:10.1029/2002GL016828. Abstract
We investigate the triggering mechanism of a cross-equatorial dust storm observed by Mars Global Surveyor in 1999. This storm, which had a significant impact on global mean temperatures, was seen in visible and infrared data to commence with the transport of linear dust fronts from the northern high latitudes into the southern tropics. However, other similar transport events observed in northern fall and winter did not lead to large dust storms. Based on off-line Lagrangian particle transport analysis using a high resolution Mars general circulation model, we propose a simple explanation for the diurnal, seasonal and interannual variability of this type of frontal activity, and of the resulting dust storms, that highlights the cooperative interaction between northern hemisphere fronts associated with low pressure cyclones and tidally-modified return branch of the Hadley circulation.
Mars Global Surveyor is providing the first observations of transient eddies in the southern hemisphere of Mars. We derive basic properties of the traveling eddies that appear in midwinter (Ls = 134°–148°) through analysis of radio occultation measurements at 67°–70°S latitude. The dominant mode has a period of ∼2 solar days and a zonal wavenumber s = 3. Strong zonal variations in eddy amplitude signal the presence of a possible “storm zone” at 150°–330°E longitude. Within this longitude band the eddies achieve peak amplitudes at the 300-Pa pressure level of ∼7 K in temperature and 10–15 m s−1 in meridional wind speed. The minimum temperature associated with the eddies is ∼2 K colder than saturation of CO2, close to the threshold where nucleation and growth of new ice particles can occur. A simulation by a Mars general circulation model produces traveling eddies that closely resemble the observations.
Richardson, M, and R John Wilson, 2002: Investigation of the nature and stability of the Martian seasonal water cycle with a general circulation model. Journal of Geophysical Research, 107(E5), DOI:10.1029/2001JE001536. Abstract
We describe the first use of a general circulation model to study the Martian water cycle. Water is treated as a passive tracer, except for ice-albedo coupling. The model is used to assess which mechanisms and water reservoirs are critical to the seasonal evolution of water and specifically the attainment of an interannually repeatable steady state. The model comes to a reasonable steady state with active surface ice and atmospheric vapor and ice reservoirs. A regolith is not necessary. The mechanism of equilibration results from independent parameters controlling the transport of water between the northern polar and the extratropical atmospheres at different seasons. Water export from the northern summer pole results from weak mixing across a strong vapor gradient, dependent upon northern cap temperatures. Import at other seasons depends on stronger mixing and weak vapor gradients, which are history dependent. Equilibration is achieved when the fluxes balance, minus a small net loss to the south. We find that with a southern residual CO2 cap, the water cycle cannot be completely closed. We conclude that the northern summer cap temperature determines the bulk humidity of the atmosphere, all else being equal. We proceed to show that a water cap exposed in southern summer would be unstable with respect to the north for dynamical as well as thermal reasons. At high obliquity (45°), much higher vapor abundances result in more widespread surface ice with seasonal ice caps overlapping in the equinoctial subtropics, producing year-round stability of water ice just north of the equator.
Richardson, M, and R John Wilson, 2002: A topographically forced asymmetry in the martian circulation and climate. Nature, 416(6878), 298-301. Abstract
Large seasonal and hemispheric asymmetries in the martian climate system are generally ascribed to variations in solar heating associated with orbital eccentricity. As the orbital elements slowly change (over a period of >10 years), characteristics of the climate such as dustiness and the vigour of atmospheric circulation are thought to vary, as should asymmetries in the climate (for example, the deposition of water ice at the northern versus the southern pole). Such orbitally driven climate change might be responsible for the observed layering in Mars' polar deposits by modulating deposition of dust and water ice. Most current theories assume that climate asymmetries completely reverse as the angular distance between equinox and perihelion changes by 180°. Here we describe a major climate mechanism that will not precess in this way. We show that Mars' global north-south elevation difference forces a dominant southern summer Hadley circulation that is independent of perihelion timing. The Hadley circulation, a tropical overturning cell responsible for trade winds, largely controls interhemispheric transport of water and the bulk dustiness of the atmosphere. The topography therefore imprints a strong handedness on climate, with water ice and the active formation of polar layered deposits more likely in the north
Richardson, M, R John Wilson, and A V Rodin, 2002: Water ice clouds in the Martian atmosphere: General circulation model experiments with a simple cloud scheme. Journal of Geophysical Research, 107(E9), 5064, DOI:10.1029/2001JE001804. Abstract
We present the first comprehensive general circulation model study of water ice condensation and cloud formation in the Martian atmosphere. We focus on the effects of condensation in limiting the vertical distribution and transport of water and on the importance of condensation for the generation of the observed Martian water cycle. We do not treat cloud ice radiative effects, ice sedimentation rates are prescribed, and we do not treat interactions between dust and cloud ice. The model generates cloud in a manner consistent with earlier one-dimensional (1-D) model results, typically evolving a uniform (constant mass mixing ratio) vertical distribution of vapor, which is capped by cloud at the level where the condensation point temperature is reached. Because of this vertical distribution of water, the Martian atmosphere is generally very far from fully saturated, in contrast to suggestions based upon interpretation of Viking data. This discrepancy results from inaccurate representation of the diurnal cycle of air temperatures in the Viking Infrared Thermal Mapper (IRTM) data. In fact, the model suggests that only the northern polar atmosphere in summer is consistently near its column-integrated holding capacity. In this case, the column amount is determined primarily by the temperature of the northern polar ice cap. Comparison of the water cycle generated by the model with and without atmospheric ice condensation and precipitation shows two major roles for water ice cloud. First, clouds are essential to the observed rapid return of atmospheric water to the surface in late northern summer, as ice sedimentation forces the water column to shrink in response to the downward motion of the condensation level, concentrating water near surface sinks. Second, ice sedimentation limits the amount of water that is transported between the hemispheres through the Hadley circulation. This latter effect is used to greatly improve the model simulation of the annual water cycle by increasing ice sedimentation rates. The model is thus shown to be able to reasonably reproduce the annual cycles of vapor and ice cloud as compared to Viking data. In addition, the model is shown able to reproduce near-instantaneous maps of water ice derived from Hubble Space Telescope images. The seasonal evolution of the geographic distribution of water ice compares reasonably well with Viking and Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) observations, except in the prediction of a weak tropical cloud belt in southern summer. Finally, it is shown that the tropical cloud belt is generated in the model by the cooling of water vapor entrained in the upwelling branch of the Hadley cell. Decline of the tropical cloud belt in mid northern summer is shown to be related to an increase in air temperatures, rather than to decreases in water vapor supply or the vigor of Hadley cell ascent. By equinox, the cloud belt experiences a second major decline event, this time due to a reduction in vapor supply. The ability of the model to emulate many aspects of observed cloud behavior is encouraging, as is the ability of enhanced ice sedimentation to improve the overall quality of the water cycle simulation. However, significant work remains to be done before all observational constraints can be matched simultaneously. Specifically, in order for the generally good fit to all other data to be attained, cloud ice particle sizes about an order of magnitude too large must be used.
Toigo, A D., M Richardson, R John Wilson, H Wang, and A Ingersoll, 2002: A first look at dust lifting and dust storms near the south pole of Mars with a mesoscale model. Journal of Geophysical Research, 107(E7), DOI:10.1029/2001JE001592. Abstract
Surface wind stresses and dust lifting in the south polar region of Mars are examined with a three-dimensional numerical model. The focus of this study is the middle to late southern spring period when cap-edge dust lifting events are observed. Mesoscale model simulations of high southern latitudes are conducted at three dates within this season (Ls = 225°, 255°, and 310°). Assuming that dust injection is related to the saltation of sand-sized grains or aggregates, the Mars MM5 mesoscale model predicts surface wind stresses of sufficient strength to initiate movement of sand-sized particles (~100 µm), and hence dust lifting, during all three periods. The availability of dust and/or sand-sized particles is not addressed within this study. Instead, the degree to which the existence of sufficiently strong winds limit dust injection is examined. By eliminating forcing elements from the model, the important dynamical modes generating high wind stresses are isolated. The direct cap-edge thermal contrast (and topographic slopes in some locations) provides the primary drive for high surface wind stresses at the cap edge, while sublimation flow is not found to be particularly important, at these three dates. Simulations in which dust is injected into the lowest model layer when wind stresses exceed a threshold show similar patterns of atmospheric dust to those seen in recent observations. Comparison between these simulations and those without active dust injection shows no signs of consistent positive or negative feedback due to dust clouds on the surface wind stress fields during the late spring season examined here.
Wilson, R J., 2002: Evidence for nonmigrating thermal tides in the Mars upper atmosphere from the Mars Global Surveyor Accelerometer Experiment. Geophysical Research Letters, 29(7), DOI:10.1029/2001GL013975. Abstract
Mars Global Surveyor Accelerometer Experiment density measurements indicate the presence of planetary-scale wave structure in the Mars upper atmosphere (~130 km). In particular, Phase 2 aerobraking observations reveal large amplitude zonal wave 2 and 3 variations in dayside density between ± 60° latitude. These spatial variations (in a fixed local solar time reference) can be qualitatively reproduced by a Mars general circulation model and are identified as a manifestation of eastward propagating nonmigrating thermal tides with long vertical wavelengths. The simulated wave 2 variation is dominated by a diurnal period wave 1 Kelvin mode while the principal components of the simulated zonal wave 3 structure are a diurnal period wave 2 Kelvin mode and a wave 1 semidiurnal tide. The characterization of these waves is important for understanding the structure and variability of the martian atmosphere at aerobraking altitudes.
Analysis of temperature retrievals from Mars Global Surveyor Thermal Emission Spectrometer data has revealed the presence of regular, eastward propagating waves in the Northern Hemisphere. A large amplitude, zonal wave 1 with a long (~20 sol) period is particularly prominent during early winter (Ls = 220-270° ). After Ls = 270 °, a weaker and more rapidly propagating (6.5 sol period) zonal wave 1 is dominant. These waves have a deep vertical structure (>40 km) correlated with the axis of the winter hemisphere westerly jet. Simulations with a Mars general circulation model suggest that the fast wave is associated with baroclinic instability due to the strong meridional temperature gradient at the surface and is consistent with surface pressure oscillations seen in Viking Lander data. By contrast, the slow wave has the appearance of a large-amplitude Rossby wave that is coupled with an inertially unstable region in the subtropics.
Hamilton, Kevin P., R John Wilson, and Richard S Hemler, 2001: Spontaneous stratospheric QBO-like oscillations Simulated by the GFDL SKYHI general circulation model. Journal of the Atmospheric Sciences, 58(21), 3271-3292. Abstract
The tropical stratospheric mean flow behavior in a series of integrations with high vertical resolution versions of the Geophysical Fluid Dynamics Laboratory (GFDL) "SKYHI" model is examined. At sufficiently high vertical and horizontal model resolution, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed quasi-biennial oscillation (QBO) in many respects, although the simulated oscillation has a period less than half that of the real QBO. The same basic mean flow oscillation appears in both seasonally varying and perpetual equinox versions of the model, and most of the analysis in this paper is focused on the perpetual equinox cases. The mean flow oscillation is shown to be largely driven by eddy momentum fluxes associated with a broad spectrum of vertically propagating waves generated spontaneously in the tropical troposphere of the model. Several experiments are performed with the model parameters perturbed in various ways. The period of the simulated tropical stratospheric mean flow oscillation is found to change in response to large alterations in the sea surface temperatures (SSTs) employed. This is a fairly direct demonstration of the link between the stratospheric mean flow behavior and tropical convection that is inherent in current theories of the QBO. It is also shown in another series of experiments that the oscillation is affected by the coefficients used for the subgrid-scale diffusion parameterization. These experiments demonstrate that at least one key reason why reasonably fine horizontal resolution is needed for the model to simulate a mean flow oscillation is the smaller horizontal diffusion that can be used at high resolution.
Hinson, D P., G L Tyler, J L Hollingsworth, and R John Wilson, 2001: Radio occultation measurements of forced atmospheric waves on Mars. Journal of Geophysical Research, 106(E1), 1463-1480. Abstract
Mars Global Surveyor performed a series of radio occultaion experiments in December 1998, resulting in 36 profiles of the neutral atmosphere in late northern spring (Ls = 74.1° - 77.3°). The measurements are confined in latitude (64.6° - 67.2°N) and local time (0321-0418), but their distribution in longitude is fairly uniform. We used least squares spectral analysis to characterize the zonal structure of the atmosphere and constructed longitude-height cross sections of both temperature and geopotential. Zonal variations of temperature exceed 12 K near the surface but are much smaller (2-3 K) at higher latitudes. Zonal variations of geopotential are ~200 m throughout the vertical range of the measurements. These patterns of temperature and geopotential appear to be stationary relative to the surface with little day-to-day variation within the 7-sol span of the measurements. We relied heavily on Mars general circulation models (GCMs) for guidance in understanding these data. Stationary planetary waves are responsible for some aspects of the temperature and geopotential fields, particularly at pressures exceeding 100-200 Pa. On the basis of strong similarities between a GCM simulation and the observations, we conclude that the disturbance takes the form of a planetary wave train excited by Alba Patera. The data also include the signature of non-Sun-synchronous thermal tides, which produce a pattern that appears to be stationary when sampled at fixed local time. Comparison between a GCM simulation and the measured geopotential field provides evidence for the presence of the resonantly enhanced, diurnal, wave-1 Kelvin mode.
Clancy, R T., B J Sandor, M J Wolff, P R Christensen, M D Smith, J C Pearl, B J Conrath, and R John Wilson, 2000: An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere. Journal of Geophysical Research, 105(E4), 9553-9571. Abstract
During the period October 1997 to September 1999 we obtained and analyzed over 100 millimeter-wave observations of Mars atmospheric CO line absorption for atmospheric temperature profiles. These measurements extend through one full Mars year (solar longitudes Ls of 190° in 1997 to 180°in 1999) and coincide with atmospheric temperature profile and dust column measurements from the Thermal Emission Spectrometer (TES) experiment on board the Mars Global Surveyor (MGS) spacecraft. A comparison of Mars atmospheric temperatures retrieved by these distinct methods provides the first opportunity to place the long-term (1982-1999) millimeter retrievals of Mars atmospheric temperatures within the context of contemporaneous, spatially mapped spacecraft observations. Profile comparisons of 0-30 km altitude atmospheric temperatures retrieved with the two techniques agree typically to within the 5 K calibration accuracy of the millimeter observations. At the 0.5 mbar pressure level (~25 km altitude) the 30°N/30°S average for TES infrared temperatures and the disk-averaged millimeter temperatures are also well correlated in their seasonal and dust-storm-related variations over the 1997-1999 period. This period includes the Noachis Terra regional dust storm, which led to very abrupt heating (~15 K at 0.5 mbar) of the global Mars atmosphere at Ls = 224° in 1997 [Christensen et al., 1998: Conrath et al., this issue; Smith et al., this issue]. Much colder (10-20 K) global atmospheric temperatures were observed during the 1997 versus 1977 perihelion periods (Ls = 200°-330°), consistent with the much (2 to 8 times) lower global dust loading of the atmosphere during the 1997 perihelion dust storm season versus the Viking period of the 1977a,b storms. The 1998-1999 Mars atmosphere revealed by both the millimeter and TES observations is also 10-15 K colder than presented by the Viking climatology during the aphelion season (Ls = 0°-180°, northern spring/summer) of Mars. We reassess the observational basis of the Viking dusty-warm climatology for this season to conclude that the global aphelion atmosphere of Mars is colder, less dusty, and cloudier than indicated by the established Viking climatology even for the Viking period. We also conclude that Mars atmospheric temperatures exhibit their most significant interannual variations during the perihelion dust storm season (10-20 K for Ls = 200°-340°) and during the post-aphelion northern summer season (5-10 K for Ls = 100°-200°).
Pawson, S, K Kodera, Kevin P Hamilton, T G Shepherd, S R Beagley, B A Boville, J D Farrara, T D A Fairlie, A Kitoh, W A Lahoz, U Langematz, E Manzini, R John Wilson, and Jerry D Mahlman, et al., 2000: The GCM-reality intercomparison project for SPARC (GRIPS): scientific issues and initial results. Bulletin of the American Meteorological Society, 81(4), 781-796. Abstract
To investigate the effects of the middle atmosphere on climate, the World Climate Research Programme is supporting the project "Stratospheric Processes and their Role in Climate" (SPARC). A central theme of SPARC, to examine model simulations of the coupled troposphere-middle atmosphere system, is being performed through the initiative called GRIPS (GCM-Reality Intercomparison Project for SPARC). In this paper, an overview of the objectives of GRIPS is given. Initial activities include an assessment of the performance of middle atmosphere climate models, and preliminary results from this evaluation are presented here. It is shown that although all 13 models evaluated represent most major features of the mean atmospheric state, there are deficiencies in the magnitude and location of the features, which cannot easily be traced to the formulation (resolution or the parameterizations included) of the models. Most models show a cold bias in all locations, apart from the tropical tropopause region where they can be either too warm or too cold. The strengths and locations of the major jets are often misrepresented in the models. Looking at three-dimensional fields reveals, for some models, more severe deficiencies in the magnitude and positioning of the dominant structures (such as the Aleutian high in the stratosphere), although undersampling might explain some of these differences from observations. All the models have shortcomings in their simulations of the present-day climate, which might limit the accuracy of predictions of the climate response to ozone change and other anomalies forcing.
Wilson, R J., 2000: Evidence for diurnal period Kelvin waves in the Martial atmosphere from Mars Global Surveyor TES data. Geophysical Research Letters, 27(23), 3889-3892. Abstract
Midlevel (~25 km) atmospheric temperatures derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectra indicate the presence of stationary waves and thermal tides. Stationary waves are prominent at middle to high latitudes where westerly zonal flow is indicated by the meridional temperature gradient. Longitudinal variability within 30°S to 30°N is dominated by topographically-forced nonmigrating thermal tides that have westward and eastward propagating components. The MGS mapping data are available at two fixed local times so that is it not possible to distinguish between these components or isolate the sun-synchronous tide. A comparison with Mars general circulation model (MGCM) simulations suggests that the observed wave patterns are consistent with the presence of eastward propagating, diurnal period Kelvin waves with zonal wavenumbers one and two. These waves can propagate to great heights and may account for observed zonal variations in thermospheric density.
Wilson, R J., and M Richardson, 2000: The martian atmosphere during the Viking Mission, I: Infrared measurements of atmospheric temperatures revisited. Icarus, 145(2), 555-579. Abstract
The Viking Infrared Thermal Mapper 15-µm channel brightness temperature observations (IRTM T15)provide extensive spatial and temporal coverage of martian atmospheric temperatures on diurnal to seasonal time scales. The15-µm channel was designed so that these temperatures would be representative of a deep layer of atmosphere centered at 0.5 mb (~25 km). Our re-examination of the IRTM data indicates that the 15-µmchannel was additionally sensitive to surface radiance so that air temperature determinations (nominal T15)are significantly biased when the thermal contrast between the surface and atmosphere is large. This bias is suggested by the strong correlation between the diurnal variation of tropical T15 and surface temperatures for non-dust-storm conditions. We show that numerical modeling of the thermal tides provides a basis for distinguishing between the surface and atmospheric contributions to IRTM T15 and thus allows the atmospheric component to be estimated. The resulting bias amounts to a ~15-Koffset for midday atmospheric temperatures at subsolar latitudes during relatively clear periods and is negligible at night. The proposed temperature correction results in close agreement between the stimulated and observed patterns of diurnal variation for conditions ranging from clear to dusty.
A major consequence of this work is the improved definition of the diurnal, latitudinal, and seasonal variation of martian atmosphere temperatures during the Viking mission. An accounting for the surface temperature bias resolves much of the discrepancy between IRTM and corresponding microwave observations, indicating that there is relatively little interannual variability in global temperatures during the aphelion season (Ls~ 40°-100°). We find further support for this argument in a comparison with T15 temperatures synthesized from Mariner9 Infrared Interferometer Spectrometer spectra. The significantly reduced diurnal temperature variations in this season are consistent with the relatively clear atmosphere that is implied by the cooler temperatures. Cooler temperatures and reduced diurnal variation will likely be of significance for the modeling of water ice cloud dynamics in this season.
Hamilton, Kevin P., R John Wilson, and Richard S Hemler, 1999: Middle atmosphere simulated with high vertical and horizontal resolution versions of a GCM: Improvements in the cold pole bias and generation of a QBO-like oscillation in the tropics. Journal of the Atmospheric Sciences, 56(22), 3829-3846. Abstract
The large-scale circulation in the Geophysical Fluid Dynamics Laboratory "SKYHI" troposphere-stratosphere-mesosphere finite-difference general circulation model is examined as a function of vertical and horizontal resolution. The experiments include one with horizontal grid spacing of ~35 km and another with ~100 km horizontal grid spacing but very high vertical resolution (160 levels between the ground and about 85 km). The simulation of the middle-atmospheric zonal-mean winds and temperatures in the extratropics is found to be very sensitive to horizontal resolution. For example, in the early Southern Hemisphere winter the South Pole near 1 mb in the model is colder than observed, but the bias is reduced with improved horizontal resolution (from ~70°C in a version with ~300 km grid spacing to less than 10°C in the ~35 km version). The extratropical simulation is found to be only slightly affected by enhancements of the vertical resolution. By contrast, the tropical middle atmospheric simulation is extremely dependent on the vertical resolution employed. With level spacing in the lower stratosphere ~1.5 km, the lower stratospheric zonal-mean zonal winds in the equatorial region are nearly constant in time. When the vertical resolution is doubled, the simulated stratospheric zonal winds exhibit a strong equatorially centered oscillation with downward propagation of the wind reversals and with formation of strong vertical shear layers. This appears to be a spontaneous internally generated oscillation and closely resembles the observed QBO in many respects, although the simulated oscillation has a period less than half that of the real QBO.
Rodin, A V., R T Clancy, and R John Wilson, 1999: Dynamical properties of Mars waterice clouds and their interaction with atmospheric dust and radiation. Advances in Space Research, 23, 1577-1585.
Jones, P, Kevin P Hamilton, and R John Wilson, 1997: A very high resolution general circulation model simulation of the global circulation in Austral Winter. Journal of the Atmospheric Sciences, 54(8), 1107-1116. Abstract
This paper discusses a simulation obtained with the Geophysical Fluid Dynamics Laboratory "SKYHI" troposphere-stratosphere-mesosphere general circulation model run at very high horizontal resolution (~60-km grid spacing) and without any parameterization of subgrid-scale gravity wave drag. The results are for a period around the austral winter solstice, and the emphasis is on the simulated Southern Hemisphere (SH) winter circulation. Comparisons are made with results obtained from lower horizontal resolution versions of the same model.
The focus in this paper is on two particularly striking features of the high-resolution simulation: the extratropical surface winds and the winter polar middle atmospheric vortex. In the extratropical SH, the simulated surface westerlies and meridional surface pressure gradients in the high-resolution model are considerably stronger than observed and are stronger than those simulated at lower horizontal resolution. In the middle atmosphere, the high-resolution model produces a simulation of the zonal mean winter polar vortex that is considerably improved over that found with lower resolution models (although it is still significantly affected by the usual cold pole bias). Neither the improvement of the middle atmospheric polar vortex simulation nor the deterioration of the simulation of surface winds with increased model resolution shows a clear convergence, even at the ~60-km grid spacing employed here .
Wilson, R J., 1997: A general circulation model simulation of the Martian polar warming. Geophysical Research Letters, 24(2), 123-126. Abstract
This paper reports on a successful general circulation model simulation of a rapid warming phenomenon observed in the Martian winter polar atmosphere during global dust storm conditions. The model includes a self-consistent simulation of the dust distribution which is forced with a prescribed surface source. The warming is shown to be largely a response to the development of a pole-to-pole solstitial Hadley circulation resulting from the greatly increased dust loading. A crucial aspect of the simulation is a sufficiently deep computational domain that allows for the full development of this circulation. These simulations indicate that the thermal tides play a contributing role by providing zonal momentum flux divergence at the winter pole and by the advection of aerosol.
Wilson, R J., and Kevin P Hamilton, 1996: Comprehensive model simulation of thermal tides in the Martian atmosphere. Journal of the Atmospheric Sciences, 53(9), 1290-1326. Abstract
This paper discusses the thermotidal oscillations in simulations performed with a newly developed comprehensive general circulation model of the Martian atmosphere. With reasonable assumptions about the effective thermal inertia of the planetary surface and about the distribution of radiatively active atmospheric aerosol, the model produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude. With any reasonable aerosol distribution, the simulated diurnal pressure oscillation has a very strong zonal variation, in particular a very pronounced zonal wavenumber-2 modulation. This results from a combination of the prominent wave-2 component in the important boundary forcings (topography and surface thermal inertia) and from the fact that the eastward-propagating zonal wave-1 Kelvin normal mode has a period near 1 sol (a Martian mean solar day of 88 775 s). The importance of global resonance is explicitly demonstrated with a series of calculations in which the global mean temperature is arbitrarily altered. The resonant enhancement of the diurnal wave-1 Kelvin mode is predicted to be strongest in the northern summer season. In the model simulations there is also a strong contribution to the semidiurnal tide from a near-resonant eastward-propagating wave-2 Kelvin mode. It is shown that this is significantly forced by a nonlinear steepening of the diurnal Kelvin wave. The daily variations of near-surface winds in the model are also examined. The results show that the daily march of wind at any location depends strongly on the topography, even on the smallest horizontal scales resolved in the model (~ few hundred km). The global tides also play an important role in determining the near-surface winds, especially so in very dusty atmospheric conditions.
The results for the diurnal and semidiurnal surface pressure oscillations in seasonal integrations of the model are compared in detail with the observations at the two Viking Lander sites (22°N and 48°N). The observations over much of the year can be reasonably reproduced in simulations with a globally uniform aerosol mixing ratio (and assuming more total aerosol in the northern winter season, when the largest dust storms are generally observed). There are features of the Viking observations that do not seem to be explainable in this way, however. In particular, in early northern summer, the model predicts amplitudes for the diurnal pressure oscillation at both lander sites that are at least a factor of 2 larger than observed. Results are presented showing that the low amplitudes observed could be explained if the dust distribution tended to be concentrated over the highlands, rather than being uniformly mixed. Annual cycle simulations with a version of the model with an interactive dust transport do in fact reveal the tendency of the circulation to organize so that larger dust mixing ratios occur over highlands, particularly near subsolar latitudes. When the model includes globally uniform surface dust injection and parameterized dust sedimentation, the annual cycle of the diurnal and semidiurnal tides at both lander sites can be rather well reproduced, except for the periods of global dust storms. The attempts to simulate the observed rapid evolution of the tidal pressure oscillations during the onset of a global dust storm also demonstrate the importance of a nonuniform dust concentration. Simulations with the version of the model incorporating interactive dust are able to roughly reproduce the Viking observations when a strong zonally uniform dust injection is prescribed in the Southern Hemisphere Tropics and subtropics.
Hamilton, Kevin P., R John Wilson, Jerry D Mahlman, and L Umscheid, 1995: Climatology of the SKYHI troposphere-stratosphere-mesosphere general circulation model. Journal of the Atmospheric Sciences, 52(1), 5-43. Abstract
The long-term mean climatology obtained from integrations conducted with different resolutions of the GFDL "SKYHI" finite-difference general circulation model is examined. A number of improvements that have been made recently in the model are also described. The versions considered have <3°C x <3.6°C, <2° x <2.4°C, and <1° x <1.2°C latitude-longitude resolution, and in each case the model is run with 40 levels from the ground to 0.0096 mb. The integrations all employ a fixed climatological cycle of sea surface temperature. Over 25 years of integration with the <3°C model and shorter integrations with the higher-resolution versions are analyzed. Attention is focused on the December-February and June-August periods.
The model does a reasonable job of representing the atmospheric flow in the troposphere and lower stratosphere. The simulated tropospheric climatology has an interesting sensitivity to horizontal resolution. In common with several spectral GCMs that have been examined earlier, the surface zonal-mean westerlies in the SKYHI extratropics become stronger with increasing horizontal resolution. However, this "zonalization" of the flow with resolution is not as prominent in the upper troposphere of SKYHI as it is in some spectral models. It is noteworthy that--without parameterized gravity wave drag--the SKYHI model at all three resolutions can simulate a realistic separation of the subtropical and polar night jet streams and a fairly realistic strength of the lower-stratospheric winter polar vortex.
The geographical distribution of the annual-mean and seasonal precipitation are reasonably well simulated. When compared against observations in an objective manner, the SKYHI global precipitation simulation is found to be as good or better than that obtained by other state-of-the-art general circulation models. However, some significant shortcomings remain, most notably in the summer extratropical land areas and in the tropical summer monsoon regions. The time-mean precipitation simulation is remarkably insensitive to the horizontal model resolution employed. The other tropospheric feature examined in detail is the tropopause temperature. The whole troposphere suffers from a cold bias of the order of a few degrees Celcius, but in the <3°C SKYHI model this grows to about <6°C at 100 mb. Interestingly, the upper-tropospheric bias is reduced with increasing horizontal resolution, despite that the cloud parameters in the radiation code are specified identically in each version.
The simulated polar vortex in the Northern Hemisphere winter in the upper stratosphere is unrealistically confined to high latitudes, although the maximum zonal-mean zonal wind is close to observed values. Near the strato- pause the June-August mean temperatures at the South Pole are colder than observations by ~<65°C, < 50°C, and <30°C in the < 3°C, <2°C, and < 1°C simulations, respectively. The corresponding zonal-mean zonal wind patterns display an unrealistically strong polar vortex. The extratropical stratospheric stationary wave field in the Northern Hemisphere winter is examined in some detail using the multi- year averages available from the <3°C SKYHI integration. Comparison with comparable long-term mean observations suggests that the model captures the amplitude and phase of the stationary waves rather well.
The SKYHI model simulates the reversed equator-pole temperature gradient near the summer mesopause. The simulated summer polar mesopause temperatures decrease with increasing horizontal resolution, although even at <1 degree C resolution the predicted temperatures are still warmer than observed. The increasing resolution is accompanied by increased westerly driving of the mean flow in the summer mesosphere by dissipating gravity waves. The present results suggest that the SKYHI model does explicitly resolve a significant component of the gravity waves required to produce the observed summer mesopause structure. The seminannual oscillation near the tropical stratopause is reasonably well simulated in the < 3°C version. The main deficiency is in the westerly phase, which is not as strong as observed. There is also a second peak in the amplitude of the semiannual wind oscillation at the top model level (0.0096 mb) corresponding to the observed mesopause semiannual oscillation. This simulated mesopause oscillation is weaker (by a factor of ~3) than that observed. The simulation in the tropical stratopause and mesosphere changes quite significantly with increasing resolution, however, in the tropical lower stratosphere of the <3°C model the zonal-mean zonal wind displays a very weak (~3 m s-1 peak to peak) interannual variation, which--while rather irregular--does display a roughly biennial period and the downward phase propagation that is characteristic of the observed quasibiennial oscillation.
Hamilton, Kevin P., R John Wilson, and Hans Vahlenkamp, 1994: Three-dimensional visualization of the polar stratospheric vortex. Canadian Meteorological and Oceanographic Society (CMOS) Bulletin, 22(4), 4-6.
The stability and genesis of the vortices associated with long solitary divergent Rossby waves - the Rossby vortices - are studied numerically using the single-layer (SL) model with Jovian parameters. Vortex behavior depends on location and on balances among the translation, twisting, steepening, dispersion and advection processes. Advection is the main preserver of vortices. The solutions provide an explanation for the origin, uniqueness and longevity of the Great Red Spot (GRS).
In midlatitudes, stable anticyclones exist in a variety of sizes and balances: from the large planetary-geostrophic (PG) and medium intermediate-geostrophic (IG) vortices that propagate westward, to the small quasi-geostrophic (QG) vortices that migrate equatorward. These vortices all merge during encounters. Geostrophic vortices in the fo-plane system adjust toward symmetry by rotating; those on the sphere adjust by rotating and propagating. Stable cyclones exist mainly at the QG scale or on the fo-plane.
In low latitudes stable anticyclones exist only when a strong equatorial westerly jet and a significant easterly current are present to elininate the highly dispersive equatorial modes. The permanence of a GRS-like, low-latitude vortex in a Jovian flow configuration is established by a 100-year simulation. At the equator, stable anticyclones exist only when they have the Hermite latitudinal form and the Korteweg-DeVries longitudinal form and amplitude range as prescribed by Boyd (1980). Soliton interactions occur between equatorial vortices of similar order.
Vortices can be generated at the equator by the collapse of low-latitude anticyclones. In mid or low latitudes, unstable easterly jets generate vortices whose final number depends mainly on the interaction history. Stochastically forced eddies cascade by wave interactions into zonal currents and by eddy mergers into a single Rossby vortex that thrives on the turbulence. Directly forced ageostrophic jets can make vortex drift more westerly and can change it from free state values of -10 ms-1 to forced state values of -5 ms-1 (as the GRS) or of +5 ms-1 (as the Large Ovals).
