Fan, Songmiao, Xiaodan Chen, and Judah Cohen, June 2024: Boreal winter extratropical weather regime changes during 1979–2019 and their weather impacts and possible linkages to sea-ice in the Nordic Seas. JGR Atmospheres, 129(11), DOI:10.1029/2023JD039868. Abstract
Previous studies have suggested possible connections between the decreasing Arctic sea-ice and long-duration (>5 days, LD) cold weather events in Eurasia and North America. Here we document the occurrences of weather regimes in winter by their durations, based on the empirical orthogonal function analyses of the daily geopotential height fields at 500 hPa (z500) for the months of November–March 1979–2019. Significant changes in the occurrence frequency and persistence of Ural ridge (UR) and weak stratospheric polar vortex (PV) were found between winters following high and low autumn sea-ice covers (SIC) in the Barents and Kara seas. It is shown that a strengthening of the UR is accompanied with a weakening of the PV, and a weak PV favors Greenland ridge (GR). Cold spells in East Asia persist for 5 more days after an LDUR. Cold spells from Canada to the U.S. occur 2–5 days after an LD Ural trough (UT) and are associated with a z500 anomaly dipole centered over Alaska (+) and Hudson Bay (−). Cold spells in the eastern U.S. occur 1–4 days after an LDGR due to circulations resembling the Pacific-North America pattern. Increased occurrences of UR in winter are associated with a decreased eastward propagation of synoptic waves from the North Atlantic to Japan and the North Pacific.
Fan, Songmiao, May 2022: The influence of extratropical weather regimes on wintertime temperature variations in the Arctic during 1979-2019. Atmosphere, 13(6), 880, DOI:10.3390/atmos13060880. Abstract
In this study, the Arctic sea ice cover in the sector 30° W–60° E in February, and the monthly mean temperature (averaged over the polar cap north of 70° N and 700–1000 hPa, Tcap) in winter during 1979–2019 were analyzed using established change-point detection methods. Step changes were found in 2004, with lower sea ice cover and higher air temperature during 2005–2019 than 1979–2004 (with Tcap anomalies of 1.05 K and −0.63 K, respectively). Two combinations of weather regimes were associated with the anomalously warm months (1.61 K): (1) Scandinavian trough and Ural blocking, and (2) Atlantic ridge and Ural blocking. The first causes a “polar express” for the poleward transport of heat and moisture from mid-latitude East Europe. The second causes a “two-stage heat pump” that transports heat and moisture from the subarctic Atlantic. Their opposite combinations were associated with the anomalously cold months (−0.73 K), which occurred more frequently during 1979–2004. These trends in weather regimes could account for 25% of the step-change in Arctic winter temperature, with the remainder likely caused by changes in sea ice cover, ocean heat transport, and concentrations of aerosol and greenhouse gases.
We describe the model performance of a new global coupled climate model configuration, CM4-MG2. Beginning with the Geophysical Fluid Dynamics Laboratory's fourth-generation physical climate model (CM4.0), we incorporate a two-moment Morrison-Gettelman bulk stratiform microphysics scheme with prognostic precipitation (MG2), and a mineral dust and temperature-dependent cloud ice nucleation scheme. We then conduct and analyze a set of fully coupled atmosphere-ocean-land-sea ice simulations, following Coupled Model Intercomparison Project Phase 6 protocols. CM4-MG2 generally captures CM4.0's baseline simulation characteristics, but with several improvements, including better marine stratocumulus clouds off the west coasts of Africa and North and South America, a reduced bias toward “double” Intertropical Convergence Zones south of the equator, and a stronger Madden-Julian Oscillation (MJO). Some degraded features are also identified, including excessive Arctic sea ice extent and a stronger-than-observed El Nino-Southern Oscillation. Compared to CM4.0, the climate sensitivity is reduced by about 10% in CM4-MG2.
A two-moment Morrison-Gettelman bulk cloud microphysics with prognostic precipitation (MG2), together with a mineral dust and temperature-dependent ice nucleation scheme, have been implemented into the Geophysical Fluid Dynamics Laboratory's Atmosphere Model version 4.0 (AM4.0). We refer to this configuration as AM4-MG2. This paper describes the configuration of AM4-MG2, evaluates its performance, and compares it with AM4.0. It is shown that the global simulations with AM4-MG2 compare favorably with observations and reanalyses. The model skill scores are close to AM4.0. Compared to AM4.0, improvements in AM4-MG2 include (a) better coastal marine stratocumulus and seasonal cycles, (b) more realistic ice fraction, and (c) dominant accretion over autoconversion. Sensitivity tests indicate that nucleation and sedimentation schemes have significant impacts on cloud liquid and ice water fields, but higher horizontal resolution (about 50 km instead of 100 km) does not.
This contribution describes the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL‐ESM4.1), assesses GFDL‐ESM4.1's capacity to capture observed ocean biogeochemical patterns, and documents its response to increasing atmospheric CO2. Notable differences relative to the previous generation of GFDL ESM's include enhanced resolution of plankton food web dynamics, refined particle remineralization, and a larger number of exchanges of nutrients across Earth system components. During model spin‐up, the carbon drift rapidly fell below the 10 Pg C per century equilibration criterion established by the Coupled Climate‐Carbon Cycle Model Intercomparison Project (C4MIP). Simulations robustly captured large‐scale observed nutrient distributions, plankton dynamics, and characteristics of the biological pump. The model overexpressed phosphate limitation and open ocean hypoxia in some areas but still yielded realistic surface and deep carbon system properties, including cumulative carbon uptake since preindustrial times and over the last decades that is consistent with observation‐based estimates. The model's response to the direct and radiative effects of a 200% atmospheric CO2 increase from preindustrial conditions (i.e., years 101–120 of a 1% CO2 yr−1 simulation) included (a) a weakened, shoaling organic carbon pump leading to a 38% reduction in the sinking flux at 2,000 m; (b) a two‐thirds reduction in the calcium carbonate pump that nonetheless generated only weak calcite compensation on century time‐scales; and, in contrast to previous GFDL ESMs, (c) a moderate reduction in global net primary production that was amplified at higher trophic levels. We conclude with a discussion of model limitations and priority developments.
Mixed-phase clouds are frequently observed in the atmosphere. Here we present a parameterization for ice crystal concentration and ice nucleation rate based on parcel model simulations for mixed-phase stratocumulus clouds, in complement to a previous parameterization for stratus clouds. The parcel model uses a singular (time-independent) description for deposition nucleation and a time-dependent description for condensation nucleation and immersion freezing on mineral dust particles. The mineral dust and temperature-dependent parameterizations have been implemented in the Geophysical Fluid Dynamics Laboratory atmosphere model AM4.0 (new), while the standard AM4.0 (original) uses a temperature-dependent parameterization. Model simulations with the new and original AM4.0 show significant changes in cloud properties and radiative effects. In comparison to measurements, cloud-phase (i.e., liquid and ice partitioning) simulation appears to be improved in the new AM4.0 model. More supercooled liquid cloud is predicted in the new model, it is sustained even at temperatures lower than -25 °C unlike in the original model. A more accurate accounting of ice nucleating particles and ice crystals is essential for improved cloud phase simulation in the global atmosphere.
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.
The wintertime Arctic temperature (T, surface-400 hPa) decreased from 1979-1997 and increased rapidly from 1998-2012, in contrast to the global mean surface air temperature. Here we examine aspects of circulation variability that are associated with these temperature changes using the NCEP/NCAR and ERA-Interim reanalysis products. It is found that the Nordic-Siberia seesaw of meridional winds near 70°N is associated with 2/3 of the variance of the Arctic winter mean T, possibly contributing to the cooling and warming trends. We suggest that the seesaw accounts for much of the difference in Arctic amplification between observations and climate models. Growth of sea ice in winter is hindered by southerly winds over the Nordic region (0°-60°E). Through modulation of the wind seesaw, the Eastern Atlantic (EA) pattern is found to be significantly associated with Arctic and East Asia winter climate variations. In one phase of the EA pattern, a mid-latitude North Atlantic ridge anomaly is associated with a poleward shift of the mean storm track, a weakened eddy-driven jet over Eurasia, and above normal sea-level pressures (SLP) over Siberia, most significantly in the region to the northwest of Lake Baikal. The EA pattern is associated with 2/3 of the variance of winter-average SLP over Siberia.
Fan, Songmiao, D A Knopf, A Heymsfield, and Leo J Donner, November 2017: Modeling of aircraft measurements of ice crystal concentration in the Arctic and a parameterization for mixed-phase cloud. Journal of the Atmospheric Sciences, 74(11), DOI:10.1175/JAS-D-17-0037.1. Abstract
In this study two parameterizations of ice nucleation rate on dust particles are used in a parcel model to simulate aircraft measurements of ice crystal number concentration (Ni) in the Arctic. The parcel model has detailed microphysics for droplet and ice nucleation, growth and evaporation with prescribed vertical air velocities. Three dynamic regimes are considered including large scale ascent, cloud-top generating cells and their combination. With observed meteorological conditions and aerosol concentrations, the parcel model predicts the number concentrations of size-resolved ice crystals, which may be compared to aircraft measurements. Model results show rapid changes with height/time in relative humidity, Ni and thermodynamic phase partitioning, which is not resolved in current climate and weather forecasting models. Parameterizations for ice number and nucleation rate in mixed-phase stratus clouds are thus developed based on the parcel model results to represent the time-integrated effect of some microphysical processes in large-scale models.
Li, Z, Junfeng Liu, D L Mauzerall, X Li, Songmiao Fan, and Larry W Horowitz, et al., March 2017: A potential large and persistent black carbon forcing over Northern Pacific inferred from satellite observations. Scientific Reports, 7, 43429, DOI:10.1038/srep43429. Abstract
Black carbon (BC) aerosol strongly absorbs solar radiation, which warms climate. However, accurate estimation of BC’s climate effect is limited by the uncertainties of its spatiotemporal distribution, especially over remote oceanic areas. The HIAPER Pole-to-Pole Observation (HIPPO) program from 2009 to 2011 intercepted multiple snapshots of BC profiles over Pacific in various seasons, and revealed a 2 to 5 times overestimate of BC by current global models. In this study, we compared the measurements from aircraft campaigns and satellites, and found a robust association between BC concentrations and satellite-retrieved CO, tropospheric NO2, and aerosol optical depth (AOD) (R2 > 0.8). This establishes a basis to construct a satellite-based column BC approximation (sBC*) over remote oceans. The inferred sBC* shows that Asian outflows in spring bring much more BC aerosols to the mid-Pacific than those occurring in other seasons. In addition, inter-annual variability of sBC* is seen over the Northern Pacific, with abundances varying consistently with the springtime Pacific/North American (PNA) index. Our sBC* dataset infers a widespread overestimation of BC loadings and BC Direct Radiative Forcing by current models over North Pacific, which further suggests that large uncertainties exist on aerosol-climate interactions over other remote oceanic areas beyond Pacific.
The soluble fraction of aerosol Fe, mainly Fe(II), represents a large source of nutrient iron to the open ocean. Fe(II) may also play an important role in the adverse health effects of ambient aerosols. Our current understanding of the reduction of Fe(III) to Fe(II) in aerosols suggests that the major pathway is the photoreduction of Fe(III)–oxalate complexes, but this pathway cannot explain the observed nighttime Fe(II) in ambient aerosols and is also limited by the supply of oxalate. Here we propose a new pathway initiated by gaseous HO2 uptake, followed by Cu–Fe redox coupling, which can sustain nighttime Fe(II) and also dominate Fe(III) reduction in the absence of Fe(III)–oxalate complexes. Consequently, aqueous OH production is substantially enhanced via the Fenton reaction and sustained by the influx of HO2 from the gas phase. This mechanism is potentially the major mechanism for sustaining soluble Fe(II) in ambient aerosols and can be tested by a combination of modeling and aerosol Fe speciation measurements. We hypothesize that this mechanism may also be relevant to mineral Fe dissolution in dust aerosols.
Paulot, Fabien, Songmiao Fan, and Larry W Horowitz, January 2017: Contrasting seasonal responses of sulfate aerosols to declining SO2 emissions in the Eastern US: implications for the efficacy of SO2 emission controls. Geophysical Research Letters, 44(1), DOI:10.1002/2016GL070695. Abstract
Stringent controls have reduced US SO2 emissions by over 60% since the late 1990s. These controls have been more effective at reducing surface [ inline image] in summer (JJA) than in winter (DJF), a seasonal contrast that is not robustly captured by CMIP5 global models. We use the GFDL AM3 chemistry-climate model to show that oxidant limitation during winter causes [ inline image] (DJF) to be sensitive to primary inline image emissions, in-cloud titration of H2O2, and in-cloud oxidation by O3. The observed contrast in the seasonal response of [ inline image] to decreasing SO2 emissions is best explained by the O3 reaction, whose rate coefficient has increased over the past decades as a result of increasing NH3 emissions and decreasing SO2 emissions, both of which lower cloud water acidity. The fraction of SO2 oxidized to inline image is projected to keep increasing in future decades, delaying improvements in wintertime air quality.
We update and evaluate the treatment of nitrate aerosols in the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model (AM3). Accounting for the radiative effects of nitrate aerosols generally improves the simulated aerosol optical depth, although nitrate concentrations at the surface are biased high. This bias can be reduced by increasing the deposition of nitrate to account for the near-surface volatilization of ammonium nitrate or by neglecting the heterogeneous production of nitric acid to account for the inhibition of N2O5 reactive uptake at high nitrate concentrations. Globally, uncertainties in these processes can impact the simulated nitrate optical depth by up to 25 %, much more than the impact of uncertainties in the seasonality of ammonia emissions (6 %) or in the uptake of nitric acid on dust (13 %). Our best estimate for present-day fine nitrate optical depth at 550 nm is 0.006 (0.005–0.008). We only find a modest increase of nitrate optical depth (< 30 %) in response to the projected changes in the emissions of SO2 (−40 %) and ammonia (+38 %) from 2010 to 2050. Nitrate burden is projected to increase in the tropics and in the free troposphere, but to decrease at the surface in the midlatitudes because of lower nitric acid concentrations. Our results suggest that better constraints on the heterogeneous chemistry of nitric acid on dust, on tropical ammonia emissions, and on the transport of ammonia to the free troposphere are needed to improve projections of aerosol optical depth.
The Arctic troposphere has warmed faster than the global average over the last several decades. It was suggested that atmospheric northward energy transport (ANET) into the Arctic had contributed to tropospheric warming in the Arctic. Here we calculate ANET based on the NCEP/NCAR reanalysis data from 1979 to 2012. During this period the zonally integrated energy flux into the Arctic has decreased rather than increased in all seasons. However, the trends are statistically insignificant except for the winter and annual mean fluxes. For the winter season, the transient eddy flux of energy increases over Greenland and the Greenland Sea and decreases over west-central Siberia (WCS). Trends in meridional wind variance and vorticity also indicate increasing transient eddy activity over Northern Canada, the Greenland Sea and the Norwegian Sea and decreasing activity over WCS. Inter-winter variations in local vorticity over the WCS are significantly anti-correlated with the Arctic climate.
A potent greenhouse gas and biological irritant, tropospheric ozone is also the primary source of atmospheric hydroxyl radicals, which remove numerous hazardous trace gases from the atmosphere. Tropospheric ozone levels have increased in spring at remote sites in the mid-latitudes of the Northern Hemisphere over the past few decades; this increase has been attributed to a growth in Asian precursor emissions. In contrast, 40 years of continuous measurements at Mauna Loa Observatory in Hawaii reveal little change in tropospheric ozone levels during spring (March–April), but a rise in autumn (September–October). Here we examine the contribution of decadal shifts in atmospheric circulation patterns to decadal variability in tropospheric ozone levels at Mauna Loa using a suite of chemistry–climate model simulations. We show that the flow of ozone-rich air from Eurasia towards Hawaii during spring weakened in the 2000s as a result of La-Niña-like decadal cooling in the eastern equatorial Pacific Ocean. During autumn, in contrast, the flow of ozone-rich air from Eurasia to Hawaii strengthened in the mid-1990s onwards, coincident with the positive phase of the Pacific–North American pattern. We suggest that these shifts in atmospheric circulation patterns can reconcile observed trends in tropospheric ozone levels at Mauna Loa and the northern mid-latitudes in recent decades. We conclude that decadal variability in atmospheric circulation patterns needs to be considered when attributing observed changes in tropospheric ozone levels to human-induced trends in precursor emissions.
Long-range transport of black carbon (BC) is a growing concern as a result of the efficiency of BC in warming the climate and its adverse impact on human health. We study transpacific transport of BC during HIPPO-3 using a combination of inverse modeling and sensitivity analysis. We use the GEOS-Chem chemical transport model and its adjoint to constrain Asian BC emissions and estimate the source of BC over the North Pacific. We find that different sources of BC dominate the transport to the North Pacific during the southbound (29 March 2010) and northbound (13 April 2010) measurements in HIPPO-3. While biomass burning in Southeast Asia (SE) contributes about 60% of BC in March, more than 90% of BC comes from fossil fuel and biofuel combustion in East Asia (EA) during the April mission. GEOS-Chem simulations generally resolve the spatial and temporal variation of BC concentrations over the North Pacific, but are unable to reproduce the low and high tails of the observed BC distribution. We find that the optimized BC emissions derived from inverse modeling fail to improve model simulations significantly. This failure indicates that uncertainties in BC transport, rather than in emissions, account for the major biases in GEOS-Chem simulations of BC.
The aging process, transforming BC from hydrophobic into hydrophilic form, is one of the key factors controlling wet scavenging and remote concentrations of BC. Sensitivity tests on BC aging suggest that the aging time scale of anthropogenic BC from EA is several hours, faster than assumed in most global models, while the aging process of biomass burning BC from SE may occur much slower, with a time scale of a few days. To evaluate the effects of BC aging and wet deposition on transpacific transport of BC, we develop an idealized model of BC transport. We find that the mid-latitude air masses sampled during HIPPO-3 may have experienced a series of precipitation events, particularly near the EA and SE source region. Transpacific transport of BC is sensitive to BC aging when the aging rate is fast; this sensitivity peaks when the aging time scale is in the range of 1–1.5 d. Our findings indicate that BC aging close to the source must be simulated accurately at a process level in order to simulate better the global abundance and climate forcing of BC.
Fan, Songmiao, October 2013: Modeling of observed mineral dust aerosols in the arctic and the impact on winter season low-level clouds. Journal of Geophysical Research: Atmospheres, 118(19), DOI:10.1002/jgrd.50842. Abstract
Mineral dust aerosol is the main ice nucleus (IN) in the Arctic. Observed dust concentrations at Alert, Canada, are lowest in winter and summer and highest in spring and autumn. In this study, we simulate transport and deposition of dust in a global chemical transport model. The model predicts the spring maximum caused by natural dust from desert sources in Asia and Sahara but underestimates the observations in autumn. Both natural and pollution sources contribute to the wintertime dust burden, as suggested by previous measurements of elemental compositions. Cloud parcel model simulations were carried out to study the impact of dust aerosol on the formation of mixed-phase and ice clouds in the Arctic lower troposphere. The liquid water path of low-level cloud is most sensitive to dust aerosol concentration from winter to early spring when air temperature is at its lowest in the annual cycle. The global and parcel models together suggest that low concentrations and acid coating of dust particles are favorable conditions for occurrence of mixed-phase clouds and that anthropogenic pollution can cause significant perturbations to Arctic IN and clouds in winter.
Secondary organic aerosols (SOA) exert a significant influence on ambient air quality and regional climate. Recent field, laboratorial and modeling studies have confirmed that in-cloud processes contribute to a large fraction of SOA production. This study evaluates the key factors that govern the production of cloud-process SOA (SOAcld) in a global scale based on the GFDL coupled chemistry-climate model AM3 in which full cloud chemistry is employed. The association between SOAcld production rate and six factors (i.e. liquid water content (LWC), total carbon chemical loss rate (TCloss), temperature, VOC/NOx, OH, and O3) is examined. We find that LWC alone determines the spatial pattern of SOAcld production, particularly over the tropical, subtropical and temperate forest regions, and is strongly correlated with SOAcld production. TCloss ranks the second and mainly represents the seasonal variability of vegetation growth. Other individual factors are essentially uncorrelated to SOAcld production. We find that the rate of SOAcld production is simultaneously determined by both LWC and TCloss, but responds linearly to LWC and nonlinearly (or concavely) to TCloss. A parameterization based on LWC and TCloss can capture well the spatial and temporal variability of the process-based SOAcld formation (R2=0.5) and can be easily applied to global three dimensional models to represent the SOA production from cloud processes.
Mao, Jingqiu, Songmiao Fan, Marta Abalos, and K R Travis, January 2013: Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols. Atmospheric Chemistry and Physics, 13(2), DOI:10.5194/acp-13-509-2013. Abstract
The hydroperoxyl radical (HO2) is a major precursor of OH and tropospheric ozone. OH is the main atmospheric oxidant, while tropospheric ozone is an important surface pollutant and greenhouse gas. Standard gas-phase models for atmospheric chemistry tend to overestimate observed HO2 concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO2 uptake by aerosol involve conversion to H2O2, but this is of limited efficacy as an HO2 sink because H2O2 can photolyze to regenerate OH and from there HO2. Joint atmospheric observations of HO2 and H2O2 suggest that HO2 uptake by aerosols may in fact not produce H2O2. Here we propose a catalytic mechanism involving coupling of the transition metal ions (TMI) Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO2 to H2O in aerosols. The implied HO2 uptake significantly affects global model predictions of tropospheric OH, ozone, and other species, improving comparisons to observations, and may have a major and previously unrecognized impact on atmospheric oxidant chemistry.
Biomass burning is one of the largest sources of trace gases and aerosols to the atmosphere, and has profound influence on tropospheric oxidants and radiative forcing. Using a fully coupled chemistry-climate model (GFDL AM3), we find that co-emission of trace gases and aerosol from present-day biomass burning increases the global tropospheric ozone burden by 5.1%, and decreases global mean OH by 6.3%. Gas and aerosol emissions combine to increase CH4 lifetime non-linearly. Heterogeneous processes are shown to contribute partly to the observed lower ΔO3/ΔCO ratios in northern high latitudes versus tropical regions. The radiative forcing from biomass burning is shown to vary non-linearly with biomass burning strength. At present-day emission levels, biomass burning produces a net radiative forcing of −0.19 W/m2 (−0.29 from short-lived species, mostly aerosol direct and indirect effects, +0.10 from CH4 and CH4-induced changes in O3 and stratospheric H2O), but increasing emissions to over 5 times present levels would result in a positive net forcing.
Fan, Songmiao, J P Schwarz, Junfeng Liu, D W Fahey, Paul Ginoux, Larry W Horowitz, Hiram Levy II, Yi Ming, and J R Spackman, December 2012: Inferring ice formation processes from global-scale black carbon profiles observed in the remote atmosphere and model simulations. Journal of Geophysical Research: Atmospheres, 117, D23205, DOI:10.1029/2012JD018126. Abstract
Black carbon (BC) aerosol absorbs solar radiation and can act as cloud condensation nucleus and ice formation nucleus. The current generation of climate models have difficulty in accurately predicting global-scale BC concentrations. Previously, an ensemble of such models was compared to measurements, revealing model biases in the tropical troposphere and in the polar troposphere. Here, global aerosol distributions are simulated using different parameterizations of wet removal and model results are compared to BC profiles observed in the remote atmosphere to explore the possible sources of these biases. The model-data comparison suggests a slow removal of BC aerosol during transport to the Arctic in winter and spring, because ice crystal growth causes evaporation of liquid cloud via the Bergron process and, hence, release of BC aerosol back to ambient air. By contrast, more efficient model wet removal is needed in the cold upper troposphere over the tropical Pacific. Parcel model simulations with detailed droplet and ice nucleation and growth processes suggest that ice formation in this region may be suppressed due to a lack of ice nuclei (mainly insoluble dust particles) in the remote atmosphere, allowing liquid and mixed-phase clouds to persist under freezing temperatures, and forming liquid precipitation capable of removing aerosol incorporated in cloud water. Falling ice crystals can scavenge droplets in lower clouds, which also results in efficient removal of cloud condensation nuclei. The combination of models with global-scale BC measurements in this study has provided new, latitude-dependent information on ice formation processes in the atmosphere, and highlights the importance of a consistent treatment of aerosol and moist physics in climate models.
Liu, Junfeng, Larry W Horowitz, Songmiao Fan, Hiram Levy II, and A G Carlton, August 2012: Global in-cloud production of secondary organic aerosols: Implementation of a detailed chemical mechanism in the GFDL atmospheric model AM3. Journal of Geophysical Research: Atmospheres, 117, D15303, DOI:10.1029/2012JD017838. Abstract
Secondary organic aerosols (SOA) constitute a significant fraction of ambient aerosols, but their global source is only beginning to be understood. Substantial evidence has shown that oxidation of water-soluble organic species in the liquid cloud leads to the formation of SOA. To evaluate this global source and explore its sensitivity to various assumptions concerning cloud properties, we simulate in-cloud SOA (IC-SOA) formation based on detailed multi-phase chemistry incorporated into the newly developed Geophysical Fluid Dynamics Laboratory (GFDL) coupled chemistry-climate model AM3. We find global IC-SOA production is around 20-30 Tg∙yr-1 between 1999 and 2001. Depending on season and location, oxalic acid accounts for 40-90% of the total IC-SOA source (particularly between 800hPa - 400hPa), and glyoxylic acid and oligomers (formed by glyoxal and methylglyoxal in evaporating clouds) each contribute an additional 10-20%. Besides glyoxal and methylglyoxal (extensively studied by previous research), glycolaldehyde and acetic acid are among the most important precursors leading to the formation of IC-SOA, particularly oxalic acid. Different implementations of cloud fraction or cloud lifetime in global climate models could potentially modify estimates of IC-SOA mass production by 20-30%. Dense IC-SOA production occurs in the tropical and mid-latitude regions of the lower troposphere (surface to 500hPa). In DJF, IC-SOA production is concentrated over the western Amazon and southern Africa. In JJA, substantial IC-SOA production occurs over southern China and boreal forest regions. This study confirms a significant in-cloud source of SOA, which will directly and indirectly influence global radiation balance and regional climate.
Surface ocean iron speciation is simulated using a time-dependent box-model of lightmediated
redox cycling over a range of aeolian inputs of soluble iron in the stratified epipelagic ocean.
At steady-state, Dissolved iron (DFe) concentration increases with aeolian input of soluble iron up to
0.1 μmol m-2 d-1, and is limited by the solubility of ferric hydroxide at higher fluxes which causes the
formation of colloidal iron. We demonstrate that even in the presence of ample excess ligand, rapid
conversion of dissolved iron between oxidized and reduced forms in the tropical surface ocean exposes
DFe to colloid formation and scavenging. This result provides an explanation for the much smaller
range of interregional variability in DFe measurements (0.05-0.4 nM) than soluble Fe fluxes (0.01-1
μmol m-2 d-1) and dust fluxes (0.1-10 g m-2 d-1) predicted by atmospheric models. We incorporate the
critical behavior of the full chemical speciation model into a reduced, computationally efficient model
suitable for large scale calculations.
This study evaluates the sensitivity of long-range transport of black carbon (BC)
from mid- and high-latitude source regions to the Arctic to aging, dry deposition and
wet removal processes using the GFDL coupled chemistry and climate model (AM3).
We derive a simple parameterization for BC aging (i.e., coating with soluble materials)
which allows the rate of aging to vary diurnally and seasonally. Slow aging during
winter permits BC to remain largely hydrophobic throughout transport from
mid-latitude source regions to the Arctic. In addition, we apply surface-dependent dry
deposition velocities and reduce the wet removal efficiency of BC in ice clouds. The
inclusion of the above parameterizations significantly improves simulated magnitude,
seasonal cycle and vertical profile of BC over the Arctic compared with those in the
base model configuration. In particular, wintertime concentrations of BC in the Arctic
are increased by a factor of 100 throughout the tropospheric column. Based on
sensitivity tests involving each process, we find that the transport of BC to the Arctic
is a synergistic process. A comprehensive understanding of microphysics and
chemistry related to aging, dry and wet removal processes is thus essential to the
simulation of BC concentrations over the Arctic.
Moxim, Walter, Songmiao Fan, and Hiram Levy II, February 2011: The meteorological nature of variable soluble iron transport and deposition within the North Atlantic Ocean basin. Journal of Geophysical Research: Atmospheres, 113, D03203, DOI:10.1029/2010JD014709. Abstract
Aerosol transport from the Sahara desert to the North Atlantic Ocean
generates the largest annual flux of mineral dust and total Fe found in the global oceans,
enriching the mixed layer with soluble iron. We use the Geophysical Fluid Dynamics
Laboratory (GFDL) Global Chemical Transport model (GCTM) to examine the transport
and deposition of bio-available iron on time scales ranging from seasonal to daily. The
model is compared with observed mineral dust concentrations, depositions, and soluble
Fe fractions. It is shown that simulated cumulative soluble Fe deposition (SFeD)
employing a variable Fe solubility parameterization compares well with observed shortterm
changes of dissolved iron (dFe) within a thermally stratified surface mixed layer,
while assuming a constant two percent solubility does not. The largest year to year
variability of seasonal SFeD (45 to 90%) occurs throughout winter and spring in the
central and northeast Atlantic Ocean. It is strongly linked to the North Atlantic
Oscillation (NAO) index, producing substantially more SFeD during the positive phase
than the negative phase. The ratio of wet to total SFeD increases with distance from the
Saharan source region and is especially large when concentrations are small during the
negative NAO. In summer, the relatively steady circulation around the “Azores” high
results in low inter-annual variability of SFeD (< 30%), however, regional short-term
events are found to be highly episodic and daily deposition rates can be a factor of four or
more higher than the monthly mean flux. Three-dimensional backward trajectories are
used to determine the origin and evolution of a specific SFeD event. It is shown that the
dust mass-mean sedimentation rate should be incorporated into the “air parcel”
dynamical vertical velocity for a more precise transport path.
Net community production in the Southern Ocean is correlated with simulated local dust deposition, and more so with modeled deposition of soluble iron. Model simulations of the latter two properties are consistent with observations in both hemispheres. These results provide strong evidence that aerosol iron deposition is a first-order control on net community production and export production over large areas of the Southern Ocean.
Fan, Songmiao, February 2008: Photochemical and biochemical controls on reactive oxygen and iron speciation in the pelagic surface ocean. Marine Chemistry, 109(1-2), DOI:10.1016/j.marchem.2008.01.005. Abstract
A time-dependent chemistry model is used to predict reactive oxygen species (ROS = H2O2 + O2−) and dissolved Fe (DFe) speciation in the surface ocean. A new feature of the model is inclusion of biological sources of superoxide. The model suggests that biochemistry mediated by phytoplankton cells is as important as photochemistry for the formation of ROS. Formation of stable organic Fe(III) complexes (FeL) maintains the concentration of DFe in seawater. Iron speciation in the model is also controlled by biochemical and photochemical processes, and is far from thermodynamic equilibrium. During light periods, photo-reduction of FeL produces dissolved inorganic iron much more than thermal decomposition and cell-surface reduction of FeL, thus facilitating phytoplankton uptake of iron in the ocean. During the nighttime, O2−produced by reductases on cell surfaces both reacts with FeL, producing Fe(II), and retards the oxidation of Fe(II) and subsequent formation of FeL; therefore significant levels of bio-available Fe is maintained through this period. Photo-reduction nearly balances the formation of FeL in the model, and may control bioavailability of dissolved iron. This suggests a possible extracellular mechanism of iron and light colimitation to primary productivity. A phytoplankton growth limitation by FeL photo-reduction depends on its rate coefficient for which we need extensive measurements in natural seawater.
Law, R M., W Peters, C Aulagnier, D J Bergmann, Songmiao Fan, and Shian-Jiann Lin, et al., 2008: TransCom model simulations of hourly atmospheric CO2: Experimental overview and diurnal cycle results for 2002. Global Biogeochemical Cycles, 22, GB3009, DOI:10.1029/2007GB003050. Abstract
A
forward atmospheric transport modeling experiment has been coordinated by
the TransCom group to investigate synoptic and diurnal variations in CO2 .
Model simulations were run for biospheric, fossil, and air-sea exchange of
CO2
and for SF6
and radon for 2000–2003. Twenty-five models or model variants participated
in the comparison. Hourly concentration time series were submitted for 280
sites along with vertical profiles, fluxes, and meteorological variables at
100 sites. The submitted results have been analyzed for diurnal variations
and are compared with observed CO2 in 2002. Mean summer diurnal cycles vary widely in amplitude across models.
The choice of sampling location and model level account for part of the
spread suggesting that representation errors in these types of models are
potentially large. Despite the model spread, most models simulate the
relative variation in diurnal amplitude between sites reasonably well. The
modeled diurnal amplitude only shows a weak relationship with vertical
resolution across models; differences in near-surface transport simulation
appear to play a major role. Examples are also presented where there is
evidence that the models show useful skill in simulating seasonal and
synoptic changes in diurnal amplitude.
Patra, Prabir K., Songmiao Fan, and Shian-Jiann Lin, et al., December 2008: TransCom model simulations of hourly atmospheric CO2 : Analysis of synoptic-scale variations for the period 2002-2003. Global Biogeochemical Cycles, 22, GB4013, DOI:10.1029/2007GB003081. Abstract
The
ability to reliably estimate CO2
fluxes from current in situ atmospheric CO2
measurements and future satellite CO2
measurements is dependent on transport model performance at synoptic and
shorter timescales. The TransCom continuous experiment was designed to
evaluate the performance of forward transport model simulations at hourly,
daily, and synoptic timescales, and we focus on the latter two in this
paper. Twenty-five transport models or model variants submitted hourly time
series of nine predetermined tracers (seven for CO2 )
at 280 locations. We extracted synoptic-scale variability from daily
averaged CO2
time series using a digital filter and analyzed the results by comparing
them to atmospheric measurements at 35 locations. The correlations between
modeled and observed synoptic CO2
variabilities were almost always largest with zero time lag and
statistically significant for most models and most locations. Generally, the
model results using diurnally varying land fluxes were closer to the
observations compared to those obtained using monthly mean or daily average
fluxes, and winter was often better simulated than summer. Model results at
higher spatial resolution compared better with observations, mostly because
these models were able to sample closer to the measurement site location.
The amplitude and correlation of model-data variability is strongly model
and season dependent. Overall similarity in modeled synoptic CO2
variability suggests that the first-order transport mechanisms are fairly
well parameterized in the models, and no clear distinction was found between
the meteorological analyses in capturing the synoptic-scale dynamics.
Biogeochemical rate processes in the Southern Ocean have an important impact on the global environment. Here, we summarize an extensive set of published and new data that establishes the pattern of gross primary production and net community production over large areas of the Southern Ocean. We compare these rates with model estimates of dissolved iron that is added to surface waters by aerosols. This comparison shows that net community production, which is comparable to export production, is proportional to modeled input of soluble iron in aerosols. Our results strengthen the evidence that the addition of aerosol iron fertilizes export production in the Southern Ocean. The data also show that aerosol iron input particularly enhances gross primary production over the large area of the Southern Ocean downwind of dry continental areas.
Atmospheric deposition of mineral dust supplies much of the essential nutrient iron to the ocean. Presumably only the readily soluble fraction is available for biological uptake. Previous ocean models assumed this fraction was constant. Here the variable solubility of Fe in aerosols and precipitation is parameterized with a two-step mechanism, the development of a sulfate coating followed by the dissolution of iron (hydr)oxide on the dust aerosols. The predicted soluble Fe fraction increases with transport time from the source region and with the corresponding decrease in dust concentration. The soluble fraction is ~1 percent near sources, but often 10–40 percent farther away producing a significant increase in soluble Fe deposition in remote ocean regions. Our results may require more rapid biological and physicochemical scavenging of Fe than used in current ocean models. We further suggest that increasing SO2 emission alone could have caused significant Fe fertilization in the modern northern hemisphere oceans.
Patra, Prabir K., Jasmin G John, Jorge L Sarmiento, and Songmiao Fan, et al., March 2006: Sensitivity of inverse estimation of annual mean CO2 sources and sinks to ocean-only sites versus all-sites observational networks. Geophysical Research Letters, 33, L05814, DOI:10.1029/2005GL025403. Abstract
Inverse estimation of carbon dioxide (CO2) sources and sinks uses atmospheric CO2 observations, mostly made near the Earth's surface. However, transport models used in such studies lack perfect representation of atmospheric dynamics and thus often fail to produce unbiased forward simulations. The error is generally larger for observations over the land than those over the remote/marine locations. The range of this error is estimated by using multiple transport models (16 are used here). We have estimated the remaining differences in CO2 fluxes due to the use of ocean-only versus all-sites (i.e., over ocean and land) observations of CO2 in a time-independent inverse modeling framework. The fluxes estimated using the ocean-only networks are more robust compared to those obtained using all-sites networks. This makes the global, hemispheric, and regional flux determination less dependent on the selection of transport model and observation network.
Calculations from a microphysics model are shown which indicate the factors that control droplet nucleation scavenging of hydrophilic mineral dust particles over a large range of conditions including the size, chemical composition, and number density of particles in both cumulus and stratus clouds. We focus specifically on the activation threshold radius (ATR) for droplet nucleation which determines the particles that are activated and those available for further transport and subsequent iron deposition to the remote ocean. Results suggest: the ATR is typically found in the range of clay-sized particles (radius = .1 to 1. µm), a spectrum over which the amount of dust removed declines ~60% both in surface area and particle number; nucleation of silt-sized particles (1.–10. µm) occurs under most conditions; larger fractions of mineral aerosols are removed in cumulus clouds than in stratus; and while acid coating of dust particles in polluted environments acts to decrease the ATR, the effect is reduced by competition with soluble aerosols. Regional mineral dust environments exhibit potentially diverse aerosol wet removal impacts. The ATR representative of the tropical Atlantic ocean basin (<.2 µm) indicates ~80% removal of the total dust surface area, while in the pristine southern hemisphere mid latitudes an ATR ~.5 µm implies ~60%. In contrast, varying conditions in the polluted region of East Asia suggest a large ATR spectrum (.2 to 3. µm) with dust surface area removal ranging from >80% to <10%.
Kaufman, Y J., Ilan Koren, L Remer, D Tanré, Paul Ginoux, and Songmiao Fan, 2005: Dust transport and deposition observed from the Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) spacecraft over the Atlantic Ocean. Journal of Geophysical Research, 110, D10S12, DOI:10.1029/2003JD004436. Abstract
Meteorological observations, in situ data, and satellite images of dust episodes were used already in the 1970s to estimate that 100 Tg of dust are transported from Africa over the Atlantic Ocean every year between June and August and are deposited in the Atlantic Ocean and the Americas. Desert dust is a main source of nutrients to oceanic biota and the Amazon forest, but it deteriorates air quality, as shown for Florida. Dust affects the Earth radiation budget, thus participating in climate change and feedback mechanisms. There is an urgent need for new tools for quantitative evaluation of the dust distribution, transport, and deposition. The Terra spacecraft, launched at the dawn of the last millennium, provides the first systematic well-calibrated multispectral measurements from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument for daily global analysis of aerosol. MODIS data are used here to distinguish dust from smoke and maritime aerosols and to evaluate the African dust column concentration, transport, and deposition. We found that 240 ± 80 Tg of dust are transported annually from Africa to the Atlantic Ocean, 140 ± 40 Tg are deposited in the Atlantic Ocean, 50 Tg fertilize the Amazon Basin (four times as previous estimates, thus explaining a paradox regarding the source of nutrition to the Amazon forest), 50 Tg reach the Caribbean, and 20 Tg return to Africa and Europe. The results are compared favorably with dust transport models for maximum particle diameter between 6 and 12 ìm. This study is a first example of quantitative use of MODIS aerosol for a geophysical research.
Mineral dust aerosols originating from arid regions are simulated in an atmospheric global chemical transport model. Based on model results and observations of dust concentration, we hypothesize that air pollution increases the scavenging of dust by producing high levels of readily soluble materials on the dust surface, which makes dust aerosols effective cloud condensation nuclei (CCN). This implies that air pollution could have caused an increase of dust deposition to the coastal oceans of East Asia and a decrease by as much as 50% in the eastern North Pacific.
Battle, M, M Bender, M B Hendricks, D T Ho, R Mika, Galen McKinley, Songmiao Fan, T Blaine, and Ralph F Keeling, August 2003: Measurements and models of the atmospheric Ar/N2 ratio. Geophysical Research Letters, 30(15), 1786, DOI:10.1029/2003GL017411. Abstract
The Ar/N2 ratio of air measured at 6 globally distributed sites shows annual cycles with amplitudes of 12 to 37 parts in 106. Summertime maxima reflect the atmospheric Ar enrichment driven by seasonal warming and degassing of the oceans. Paired models of air-sea heat fluxes and atmospheric tracer transport predict seasonal cycles in the Ar/N2ratio that agree with observations, within uncertainties.
Gao, Y, Songmiao Fan, and Jorge L Sarmiento, April 2003: Aeolian iron input to the ocean through precipitation scavenging: A modeling perspective and its implication for natural iron fertilization in the ocean. Journal of Geophysical Research, 108(D7), 4221, DOI:10.1029/2002JD002420. Abstract
Aeolian dust input may be a critical source of dissolved iron for phytoplankton growth in some oceanic regions. We used an atmospheric general circulation model (GCM) to simulate dust transport and removal by dry and wet deposition. Model results show extremely low dust concentrations over the equatorial Pacific and Southern Ocean. We find that wet deposition through precipitation scavenging accounts for ~40% of the total deposition over the coastal oceans and ~60% over the open ocean. Our estimates suggest that the annual input of dissolved Fe by precipitation scavenging ranges from 0.5 to 4 × 1012 g yr-1, which is 4-30% of the total aeolian Fe fluxes. Dissolved Fe input through dry deposition is significantly lower than that by wet deposition, accounting for only 0.6-2.4 % of the total Fe deposition. Our upper limit estimate on the fraction of dissolved Fe in the total atmospheric deposition is thus more than three times higher than the value of 10% currently considered as an upper limit for dissolved Fe in Aeolian fluxes. As iron input through precipitation may promote episodic phytoplankton growth in the ocean, measurements of dissolved iron in rainwater over the oceans are needed for the study of oceanic biogeochemical cycles.
Gurney, K R., A Lauer, A S Denning, P Rayner, D F Baker, Philippe Bousquet, Lori Bruhwiler, Yu-Han Chen, Philippe Ciais, Songmiao Fan, I Y Fung, M Gloor, M Heimann, K Higuchi, Jasmin G John, Eva Kowalczyk, T Maki, Shamil Maksyutov, P Peylin, Michael J Prather, B Pak, Jorge L Sarmiento, S Taguchi, T Takahashi, and C-W Yuen, 2003: TransCom 3 CO2 inversion intercomparison: 1. Annual mean control results and sensitivity to transport and prior flux information. Tellus B, 55B(2), 555-579. Abstract PDF
Spatial and temporal variations of atmospheric CO2 concentration contain information about surface sources and sinks, which can be quantitatively interpreted through tracer transport inversion. Previous CO2 inversion calculations obtained differing results due to different data, methods and transport models used. To isolate the sources of uncertainty, we have conducted a set of annual mean inversion experiments in which 17 different transport models or model variants were used to calculate regional carbon sources and sinks from the same data with a standardized method. Simulated transport is a significant source of uncertainty in these calculations, particularly in the response to prescribed "background" fluxes due to fossil fuel combustion, a balanced terrestrial biosphere, and air-sea gas exchange. Individual model-estimated fluxes are often a direct reflection of their response to these background fluxes. Models that generate strong surface maxima near background exchange locations tend to require larger uptake near those locations. Models with weak surface maxima tend to have less uptake in those same regions but may infer small sources downwind. In some cases, individual model flux estimates cannot be analyzed through simple relationships to background flux responses but are likely due to local transport differences or particular responses at individual CO2 observing locations. The response to the background biosphere exchange generates the greatest variation in the estimated fluxes, particularly over land in the Northern Hemisphere. More observational data in the tropical regions may help in both lowering the uncertain tropical land flux uncertainties and constraining the northern land estimates because of compensation between these two broad regions in the inversion. More optimistically, examination of the model-mean retrieved fluxes indicates a general insensitivity to the prior fluxes and the prior flux uncertainties. Less uptake in the Southern Ocean than implied by oceanographic observations, and an evenly distributed northern land sink, remain in spite of changes in this aspect of the inversion setup.
McKinley, Galen, M J Follows, J Marshall, and Songmiao Fan, February 2003: Interannual variability of air-sea O2 fluxes and the determination of CO2sinks using atmospheric O2/N2. Geophysical Research Letters, 30(3), 1101, DOI:10.1029/2002GL016044. Abstract
Motivated by the use of atmospheric O2/N2 to determine CO2 sinks under the assumption of negligible interannual variability in air-sea O2 fluxes, we examine interannual fluctuations of the global air-sea flux of O2 during the period of 1980-1998 using a global ocean circulation and biogeochemistry model along with an atmospheric transport model. It is found that both the El Niño/ Southern Oscillation (ENSO) cycle and wintertime convection in the North Atlantic are primary drivers of global air-sea oxygen flux interannual variability. Model estimated extremes of O2 flux variability are -70/+100 x 1012 mol/yr (Tmol/yr), where positive fluxes are to the atmosphere. O2/N2 variability could cause an up to ±1.0 PgC/yr error in estimates of interannual variability in land and ocean CO2 sinks derived from atmospheric O2/N2 observations
Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models 1-9. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive in transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network with the tropics.
Donner, Leo J., Charles J Seman, Richard S Hemler, and Songmiao Fan, 2001: A cumulus parameterization including mass fluxes, convective vertical velocities, and mesoscale effects: thermodynamic and hydrological aspects in a general circulation model. Journal of Climate, 14(16), 3444-3463. Abstract PDF
A cumulus parameterization based on mass fluxes, convective-scale vertical velocities, and mesoscale effects has been incorporated in an atmospheric general circulation model (GCM). Most contemporary cumulus parameterizations are based on convective mass fluxes. This parameterization augments mass fluxes with convective-scale vertical velocities as a means of providing a method for incorporating cumulus microphysics using vertical velocities at physically appropriate (subgrid) scales. Convective-scale microphysics provides a key source of material for mesoscale circulations associated with deep convection, along with mesoscale in situ microphysical processes. The latter depend on simple, parameterized mesoscale dynamics. Consistent treatment of convection, microphysics, and radiation is crucial for modeling global-scale interactions involving clouds and radiation.
Thermodynamic and hydrological aspects of this parameterization in integrations of the Geophysical Fluid Dynamics Laboratory SKYHI GCM are analyzed. Mass fluxes, phase changes, and heat and moisture transport by the mesoscale components of convective systems are found to be large relative to those of convective (deep tower) components, in agreement with field studies. Partitioning between the convective and mesoscale components varies regionally with large-scale flow characteristics and agrees well with observations from the Tropical Rainfall Measuring Mission (TRMM) satellite.
The effects of the mesoscale components of convective systems include stronger Hadley and Walker circulations, warmer upper-tropospheric Tropics, and moister Tropics. The mass fluxes for convective systems including mesoscale components differ appreciably in both magnitude and structure from those for convective systems consisting of cells only. When mesoscale components exist, detrainment is concentrated in the midtroposphere instead of the upper troposphere, and the magnitudes of mass fluxes are smaller. The parameterization including mesoscale components is consistent with satellite observations of the size distribution of convective systems, while the parameterization with convective cells only is not.
The parameterization of convective vertical velocities is an important control on the intensity of the mesoscale stratiform circulations associated with deep convection. The mesoscale components are less intense than in TRMM observations if spatially and temporally invariant convective vertical velocities are used instead of parameterized, variable velocities.
Gruber, Nicolas, M Gloor, Songmiao Fan, and Jorge L Sarmiento, 2001: Air-sea flux of oxygen estimated from bulk data: Implications for the marine and atmospheric oxygen cycles. Global Biogeochemical Cycles, 15(4), 783-803. Abstract PDF
We estimate the annual net air-sea fluxes of oxygen for 13 regions on the basis of a steady state inverse modeling technique that is independent of air-sea gas exchange parameterizations. The inverted data consist of the observed oceanic oxygen concentration after a correction has been applied to account for biological cycling. We find that the tropical oceans (13°S-13°N) emit ~212 Tmol O2 yr -1 , which is compensated by uptake of 148 Tmol yr-1 in the Northern Hemisphere (>13°N) and by uptake of 65 Tmol yr-1 in the Southern Hemisphere (<13°S). These results imply that the dominant feature of oxygen transport in the combined ocean-atmosphere system is the existence of a closed circulation cell in each hemisphere. These two cells consist of O2 uptake by the ocean in the middle and high latitudes of both hemispheres and transport in the ocean toward the tropics, where O2 is lost to the atmosphere and transported in the atmosphere back toward the poles. We find an asymmetry in the two cells involving O2 uptake in the temperate regions of the Northern Hemisphere versus loss of O2 in the temperate regions of the Southern Hemisphere. There is an additional asymmetry between the Atlantic basin, which has a net southward transport at all latitudes north of 36°S, in agreement with independent transport estimates, versus the Indian and Pacific Oceans, which have a strong equatorward transport everywhere. We find that these inverse estimates are relatively insensitive to details in the inversion scheme but are sensitive to biases in the ocean general circulation model that provides the linkage between surface fluxes and ocean interior concentrations. Forward simulations of O2 in an atmospheric tracer transport model using our inversely estimated oxygen fluxes as a boundary condition agree reasonably well with observations of atmospheric potential oxygen (APO O2 + CO2 ). Our results indicate that the north-south asymmetry in the strength of the two hemispheric cells coupled with a strong asymmetry in fossil fuel emissions can explain much of the observed interhemispheric gradient in APO. Therefore it might not be necessary to invoke the existence of a large southward interhemispheric transport of O2 in the ocean, such as proposed by Stephens et al. [1998]. However, we find that uncertainties in the modeled APO distribution stemming from seasonal atmospheric rectification effects and the limited APO data coverage prevent the currently available APO data from providing strong constraints on the magnitude of interhemispheric transport.
For the period 1980–89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 1015 grams of carbon). The net carbon flux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net flux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980–89 are similar to those for 1985–89 and 1990–94, indicating a relatively stable U.S. sink throughout the period.
Gloor, M, Songmiao Fan, Stephen W Pacala, and Jorge L Sarmiento, 2000: Optimal sampling of the atmosphere for purpose of inverse modeling: A model study. Global Biogeochemical Cycles, 14(1), 407-428. Abstract PDF
The 66 stations of the GLOBALVIEW-CO2 sampling network (GLOBALVIEW-CO2: Cooperative Atmospheric Data Integration Project - Carbon Dioxide, (1997)) are located primarily remotely from continents where signals of fossil fuel consumption and biospheric exchange are diluted. It is thus not surprising that inversion studies are able to estimate terrestrial sources and sinks only to a very limited extent. The poor constraint on terrestrial fluxes propagates to the oceans and strongly limits estimates of oceanic fluxes as well, at least if no use is made of other information such as isotopic ratios. We analyze here the resolving power of the GLOBALVIEW-CO2 network, compare the efficiency of different measurement strategies, and determine optimal extensions to the present network. We find the following: (1) GLOBALVIEW-CO2 is well suited to characterize the meridional distribution of sources and sinks but is poorly suited to separate terrestrial from oceanic sinks at the same latitude. The most poorly constrained regions are South America, Africa, and southern hemispheric oceans. (2) To improve the network, observing stations need to be positioned on the continents near to the largest biospheric signals despite the large diurnal and seasonal fluctuations associated with biological activity and the dynamics of the PBL. The mixing in the atmosphere is too strong to allow positioning of stations remote from large fluxes. Our optimization results prove to be fairly insensitive to the details of model transport and the inversion model with the addition of ~ 10 optimally positioned stations. (3) The best measurement strategy among surface observations, N-S airplane transects, and vertical profiles proves to be vertical profiles. (4) Approximately 12 optimally positioned vertical profiles or 30 surface stations in addition to GLOBALVIEW-CO2 would reduce estimate uncertainties caused by insufficient data coverage from ~ 1 Pg C yr -1 per region to ~ 0.2 Pg C yr -1 per region.
Hamilton, Kevin P., and Songmiao Fan, 2000: Effects of the stratospheric quasi-biennial oscillation on long-lived greenhouse gases in the troposphere. Journal of Geophysical Research, 105(D16), 20,581-20,587. Abstract
An analysis is presented of results of an extended integration with a global general circulation model that includes treatment of a long-lived tropospheric trace constituent as well as a momentum source that forces a realistic stratospheric quasi-biennial oscillation (QBO). It is shown that the dynamical QBO may modulate stratosphere-troposphere exchange in such a manner as to produce a QBO in global-mean tropospheric tracer mixing ratio. For the growth rate of tropospheric methane this transport-induced QBO is expected to have a peak-to-peak amplitude of 1-2 ppbv yr-1, which, while modest compared with the full range of observed variability in growth rate, is still significant. The observed methane growth rate time series during 1983-1999 is shown to be consistent with the predicted QBO effect, although the record is definitely dominated by other interannual variations.
Denning, A S., and Songmiao Fan, et al., 1999: Three-dimensional transport and concentration of SF6: A model intercomparison study (TransCom 2). Tellus B, 51B(2), 266-297. Abstract PDF
Sulfur hexafluoride (SF6) is an excellent tracer of large-scale atmospheric transport, because it has slowly increasing sources mostly confined to northern midlatitudes, and has a lifetime of thousands of years. We have simulated the emissions, transport, and concentration of SF6 for a 5-year period, and compared the results with atmospheric observations. In addition, we have performed an intercomparison of interhemispheric transport among 11 models to investigate the reasons for the differences among the simulations. Most of the models are reasonably successful at simulating the observed meridional gradient of SF6 in the remote marine boundary layer, though there is less agreement at continental sites. Models that compare well to observations in the remote marine boundary layer tend to systematically overestimate SF6 at continental locations in source regions, suggesting that vertical trapping rather than meridional transport may be a dominant control on the simulated meridional gradient. The vertical structure of simulated SF6 in the models supports this interpretation. Some of the models perform quite well in terms of the simulated seasonal cycle at remote locations, while others do not. Interhemispheric exchange time varies by a factor of 2 when estimated from 1-dimensional meridional profiles at the surface, as has been done for observations. The agreement among models is better when the global surface mean mole fraction is used, and better still when the full 3-dimensional mean mixing ratio is used. The ranking of the interhemispheric exchange time among the models is not sensitive to the change from station values to surface means, but is very sensitive to the change from surface means to the full 3-dimensional tracer fields. This strengthens the argument that vertical redistribution dominates over interhemispheric transport in determining the meridional gradient at the surface. Vertically integrated meridional transport in the models is divided roughly equally into transport by the mean motion, the standing eddies and the transient eddies. The vertically integrated mass flux is a good index of the degree to which resolved advection vs. parameterized diffusion accomplishes the meridional transport of SF6. Observational programs could provide a much better constraint on simulated chemical tracer transport if they included regular sampling of vertical profiles in the middle to upper troposphere. Further analysis of the SF6 simulations will focus on the subgrid-scale parameterized transports.
Fan, Songmiao, T Blaine, and Jorge L Sarmiento, 1999: Terrestrial carbon sink in the Northern Hemisphere estimated from the atmospheric CO2 difference between Mauna Loa and the South Pole since 1959. Tellus B, 51B(5), 863-870. Abstract PDF
The difference between Mauna Loa and South Pole atmospheric CO2 concentrations from 1959 to the present scales linearly with CO2 emissions from fossil fuel burning and cement production (together called fossil CO2). An extrapolation to zero fossil CO2 emission has been used to suggest that the atmospheric CO2 concentration at Mauna Loa was 0.8 ppm less than that at the South Pole before the industrial revolution, associated with a northward atmospheric transport of about 1 Gt C yr-1 (Keeling et al., 1989a). Mass conservation requires an equal southward transport in the ocean. However, our ocean general circulation and biogeochemistry model predicts a much smaller pre-industrial carbon transport. Here, we present a new analysis of the Mauna Loa and South Pole CO2 data, using a general circulation model and a 2-box model of the atmosphere. It is suggested that the present CO2 difference between Mauna Loa and the South Pole is caused by, in addition to fossil CO2 sources and sinks, a pre-industrial interhemispheric flux of 0.5-0.7 Gt C yr-1 , and a terrestrial sink of 0.8-1.2 Gt C yr-1 in the mid-latitude Northern Hemisphere, balanced by a tropical deforestation source that has been operating continuously in the period from 1959 to the present.
Fan, Songmiao, Jorge L Sarmiento, M Gloor, and Stephen W Pacala, 1999: On the use of regularization techniques in the inverse modeling of atmospheric carbon dioxide. Journal of Geophysical Research, 104(D17), 21,503-21,512. Abstract PDF
The global distribution of carbon sources and sinks is estimated from atmospheric CO2 measurements using an inverse method based on the Geophysical Fluid Dynamics Laboratory SKYHI atmospheric general circulation model. Applying the inverse model without any regularization yields unrealistically large CO2 fluxes in the tropical regions. We examine the use of three regularization techniques that are commonly used to stabilize inversions: truncated singular value decomposition, imposition of a priori flux estimates, and use of a quadratic inequality constraint. The regularization techniques can all be made to minimize the unrealistic fluxes in the tropical regions. This brings inversion estimated CO2 fluxes for oceanic regions in the tropics and in the Southern Hemisphere into better agreement with independent estimates of the air-sea exchange. However, one cannot assume that stabilized inversions give accurate estimates, as regularization merely holds the fluxes to a priori estimates or simply reduces them in magnitude in regions that are not resolvable by observations. By contrast, estimates of flux and uncertainty for the temperate North Atlantic, temperate North Pacific, and boreal and temperate North American regions are far less sensitive to the regularization parameters, consistent with the fact that these regions are better constrained by the present observations.
Gloor, M, Songmiao Fan, Stephen W Pacala, Jorge L Sarmiento, and M Ramonet, 1999: A model-based evaluation of inversions of atmospheric transport, using annual mean mixing ratios, as a tool to monitor fluxes of nonreactive trace substances like CO2 on a continental scale. Journal of Geophysical Research, 104(D12), 14,245-14,260. Abstract PDF
The inversion of atmospheric transport of CO2 may potentially be a means for monitoring compliance with emission treaties in the future. There are two types of errors though, which may cause errors in inversions: (1) amplification of high-frequency data variability given the information loss in the atmosphere by mixing and (2) systematic errors in the CO2flux estimates caused by various approximations used to formulate the inversions. In this study we use simulations with atmospheric transport models and a time independent inverse scheme to estimate these errors as a function of network size and the number of flux regions solved for. Our main results are as follows: (1) When solving for 10-20 source regions, the average uncertainty of flux estimates caused by amplification of high-frequency data variability alone decreases strongly with increasing number of stations for up to ~150 randomly positions stations and then levels off (for 150 stations of the order of ±0.2 Pg C yr-1). As a rule of thumb, about 10 observing stations are needed per region to be estimated. (2) Of all the sources of systematic errors, modeling error is the largest. Our estimates of SF6 emissions from five continental regions simulated with 12 different AGCMs differ by up to a factor of 2. The number of observations needed to overcome the information loss due to atmospheric mixing is hence small enough to permit monitoring of fluxes with inversions on a continental scale in principle. Nevertheless errors in transport modeling are still too large for inversions to be a quantitatively reliable option for flux monitoring.
Fan, Songmiao, M Gloor, Jerry D Mahlman, Stephen W Pacala, Jorge L Sarmiento, T Takahashi, and P P Tans, 1998: A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models. Science, 282(5388), 442-446. Abstract PDF
Atmospheric carbon dioxide increased at a rate of 2.8 petagrams of carbon per year (Pg C year-1) during 1988 to 1992 (1 Pg = 1015 grams). Given estimates of fossil carbon dioxide emissions, and net oceanic uptake, this implies a global terrestrial uptake of 1.0 to 2.2 Pg C year-1. The spatial distribution of the terrestrial carbon dioxide uptake is estimated by means of the observed spatial patterns of the greatly increased atmospheric carbon dioxide data set available from 1988 onward, together with two atmospheric transport models, two estimates of the sea-air flux, and an estimate of the spatial distribution of fossil carbon dioxide emissions. North America is the best constrained continent, with a mean uptake of 1.7 ± 0.5 Pg C year-1, mostly south of 51 degrees north. Eurasia-North Africa is relatively weakly constrained, with a mean uptake of 0.1 ± 0.6 Pg C year-1. The rest of the world's land surface is poorly constrained, with a mean source of 0.2 ± 0.9 Pg C year-1.
Munger, J W., Songmiao Fan, P Bakwin, M L Goulden, A H Goldstein, A S Colman, and S C Wofsy, 1998: Regional budgets for nitrogen oxides from continental sources:Variations of rates for oxidation and deposition with season and distance from source regions. Journal of Geophysical Research, 103(D7), 8355-8368. Abstract PDF
Measurements of nitrogen deposition and concentrations of NO, NO2, NOy (total oxidized N), and O3 have been made at Harvard Forest in central Massachusetts since 1990 to define the atmospheric budget for reactive N near a major source region. Total (wet plus dry) reactive N deposition for the period 1990-1996 averaged 47 mmol m-2 yr-1 (126 umol m-2 d-1, 6.4 kg N ha-1 yr-1), with 34% contributed by dry deposition. Atmospheric input adds about 12% to the N made available annually by mineralization in the forest soil. The corresponding deposition rate at a distant site, Schefferville, Quebec, was 20 mmol m-2 d-1 during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert NOx to HNO3 in the boundary layer. HNO3 is subsequently removed rapidly by either dry deposition or precipitation. The characteristic (e-folding) time for NOx oxidation ranges from 0.30 days in summer, when OH radical is abundant, to ~1.5 days in the winter, when heterogeneous reactions are dominant and O3 concentrations are lowest. The characteristic time for removal of NOx oxidation products (defined as NOy minus NOx) from the boundary layer by wet and dry deposition is ~1 day, except in winter when it decreases to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir species, such as peroxyacetylnitrate, that may be exported from the region. A simple model assuming pseudo first-order rates for oxidation of NOx, followed by deposition, predicts that 45% of NOx in the northeastern U.S. boundary layer is removed in 1 day during summer and 27% is removed in winter. It takes 3.5 and 5 days for 95% removal in summer and winter, respectively.
