In this paper, U.S. landfalling tropical cyclone (TC) activity is projected for the late twenty-first century using a two-step dynamical downscaling framework. A regional atmospheric model, is run for 27 seasons, to generate tropical storm cases. Each storm case is -resimulated (up to 15 days) using the higher-resolution Geophysical Fluid Dynamics Laboratory hurricane model. Thirteen CMIP3 or CMIP5 climate change scenarios are explored. Robustness of projections is assessed using statistical significance tests and comparing changes across models. The proportion of TCs making U.S. landfall increases for the warming scenarios, due, in part, to an increases in the percentage of TC genesis near the U.S. coast and a change in climatological steering flows favoring more U.S. landfall events. The increases in U.S. landfall proportion leads to an increase in U.S. landfalling category 4–5 hurricane frequency, averaging about + 400% across the models; 10 of 13 models/ensembles project an increase (which is statistically significant in three of 13 models). We have only tentative confidence in this latter increase, which occurs despite a robust decrease in Atlantic basin category 1–5 hurricane frequency, no robust change in Atlantic basin category 4–5 and U.S. landfalling category 1–5 hurricane frequency, and no robust change in U.S. landfalling hurricane intensities. Rainfall rates, averaged within a 100-km radius of the storms, are projected to increase by about 18% for U.S. landfalling TCs. Important caveats to the study include low correlation (skill) for interannual variability of modeled vs. observed U.S. TC landfall frequency and model bias of excessive TC genesis near and east of the U.S. east coast in present-day simulations.
Successful collaborations played a pivotal role in transitioning the GFDL hurricane research model into a long-standing state-of-the-art operational system that provided critical guidance for over 20 years.
The hurricane project at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) was established in 1970. By the mid 1970s pioneering research had led to the development of a new hurricane model. As the reputation of the model grew, GFDL was approached in 1986 by the director of the National Meteorological Center about establishing collaboration between the two Federal organizations to transition the model into an operational modeling system. After a multi-year effort by GFDL scientists to develop a system that could support rigorous requirements of operations, and multi-year testing had demonstrated its superior performance compared to existing guidance products, operational implementation was made in 1995. Through collaboration between GFDL and the US Navy, the model was also made operational at Fleet Numerical Meteorology and Oceanography Center in 1996. GFDL scientists continued to support and improve the model during the next two decades by collaborating with other scientists at GFDL, the NCEP Environmental Modeling Center (EMC), the National Hurricane Center, the US Navy, the University of Rhode Island (URI), Old Dominion University, and the NOAA Hurricane Research Division. Scientists at GFDL, URI, and EMC collaborated to transfer key components of the GFDL model to the NWS new Hurricane Weather and Research Forecast (HWRF) model that became operational in 2007. The purpose of the article is to highlight the critical role of these collaborations. It is hoped that the experiences of the authors will serve as an example of how such collaboration can benefit the nation with improved weather guidance products.
The GFDL hurricane modelling system, initiated in the 1970s, has progressed from a research tool to an operational system over four decades. This system is still in use today in research and operations, and its evolution will be briefly described. This study used an idealized version of the 2014 GFDL model to test its sensitivity across a wide range of three environmental factors that are often identified as key factors in tropical cyclone (TC) evolution: SST, atmospheric stability (upper air thermal anomalies), and vertical wind shear (westerly through easterly). A wide range of minimum central pressure intensities resulted (905 to 980hPa). The results confirm that a scenario (e.g., global warming) in which the upper troposphere warms relative to the surface will have less TC intensification than one with a uniform warming with height. TC rainfall is also investigated for the SST-stability parameter space. Rainfall increases for combinations of SST increase and increasing stability similar to global warming scenarios, consistent with climate change TC downscaling studies with the GFDL model. The forecast system’s sensitivity to vertical shear was also investigated. The idealized model simulations showed weak disturbances dissipating under strong easterly and westerly shear of 10 m s-1. A small bias for greater intensity under easterly sheared versus westerly sheared environments was found at lower values of SST. The impact of vertical shear on intensity was different when a strong vortex was used in the simulations. In this case none of the initial disturbances weakened, and most intensified to some extent.
Global projections of intense tropical cyclone activity are derived from the Geophysical Fluid Dynamics Laboratory (GFDL) HiRAM (50 km grid) atmospheric model and the GFDL Hurricane Model using a two-stage downscaling procedure. First, tropical cyclone genesis is simulated globally using the HiRAM atmospheric model. Each storm is then downscaled into the GFDL Hurricane Model, with horizontal grid-spacing near the storm of 6 km, and including ocean coupling (e.g., ‘cold wake’ generation). Simulations are performed using observed sea surface temperatures (SSTs) (1980-2008); for a “control run” with 20 repeating seasonal cycles; and for a late 21st century projection using an altered SST seasonal cycle obtained from a CMIP5/RCP4.5 multi-model ensemble. In general agreement with most previous studies, projections with this framework indicate fewer tropical cyclones globally in a warmer late-21st-century climate, but also an increase in average cyclone intensity, precipitation rates, and in the number and occurrence-days of very intense category 4-5 storms. While these changes are apparent in the globally averaged tropical cyclone statistics, they are not necessarily present in each individual basin. The inter-basin variation of changes in most of the tropical cyclone metrics we examined is directly correlated to the variation in magnitude of SST increases between the basins. Finally, the framework is shown capable of reproducing both the observed global distribution of outer storm size--albeit with a slight high bias--and its inter-basin variability. Projected median size is found to remain nearly constant globally, with increases in most basins offset by decreases in the Northwest Pacific.
In this extended abstract, we report on progress in two areas of research at GFDL relating to Indian Ocean regional climate and climate change. The first topic is an assessment of regional surface temperature trends in the Indian Ocean and surrounding region. Here we illustrate the use of a multi-model approach (CMIP3 or CMIP5 model ensembles) to assess whether an anthropogenic warming signal has emerged in the historical data, including identification of where the observed trends are consistent or not with current climate models. Trends that are consistent with All Forcing runs but inconsistent with Natural Forcing Only runs are ones which we can attribute, at least in part, to anthropogenic forcing.
Twenty-first-century projections of Atlantic climate change are downscaled to explore the robustness of potential changes in hurricane activity. Multimodel ensembles using the phase 3 of the Coupled Model Intercomparison Project (CMIP3)/Special Report on Emissions Scenarios A1B (SRES A1B; late-twenty-first century) and phase 5 of the Coupled Model Intercomparison Project (CMIP5)/representative concentration pathway 4.5 (RCP4.5; early- and late-twenty-first century) scenarios are examined. Ten individual CMIP3 models are downscaled to assess the spread of results among the CMIP3 (but not the CMIP5) models. Downscaling simulations are compared for 18-km grid regional and 50-km grid global models. Storm cases from the regional model are further downscaled into the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model (9-km inner grid spacing, with ocean coupling) to simulate intense hurricanes at a finer resolution.
A significant reduction in tropical storm frequency is projected for the CMIP3 (−27%), CMIP5-early (−20%) and CMIP5-late (−23%) ensembles and for 5 of the 10 individual CMIP3 models. Lifetime maximum hurricane intensity increases significantly in the high-resolution experiments—by 4%–6% for CMIP3 and CMIP5 ensembles. A significant increase (+87%) in the frequency of very intense (categories 4 and 5) hurricanes (winds ≥ 59 m s−1) is projected using CMIP3, but smaller, only marginally significant increases are projected (+45% and +39%) for the CMIP5-early and CMIP5-late scenarios. Hurricane rainfall rates increase robustly for the CMIP3 and CMIP5 scenarios. For the late-twenty-first century, this increase amounts to +20% to +30% in the model hurricane’s inner core, with a smaller increase (~10%) for averaging radii of 200 km or larger. The fractional increase in precipitation at large radii (200–400 km) approximates that expected from environmental water vapor content scaling, while increases for the inner core exceed this level.
Several recent models suggest that the frequency of Atlantic tropical cyclones could decrease as the climate warms. However, these models are unable to reproduce storms of category 3 or higher intensity. We explored the influence of future global warming on Atlantic hurricanes with a downscaling strategy by using an operational hurricane-prediction model that produces a realistic distribution of intense hurricane activity for present-day conditions. The model projects nearly a doubling of the frequency of category 4 and 5 storms by the end of the 21st century, despite a decrease in the overall frequency of tropical cyclones, when the downscaling is based on the ensemble mean of 18 global climate-change projections. The largest increase is projected to occur in the Western Atlantic, north of 20°N.
Knutson, Thomas R., and Robert E Tuleya, May 2008: Tropical cyclones and climate change: Revisiting recent studies at GFDL In Climate Extremes and Society, Diaz, H.F. and R.J. Murnane, Eds., New York, NY, Cambridge University Press, 120-144. Abstract
In this chapter, we revisit two recent studies performed at the Geophysical Fluid Dynamics Laboratory (GFDL), with a focus on issues relevant to tropical cyclones and climate change. The first study was a model-based assessment of twentieth-century regional surface temperature trends. The tropical Atlantic Main Development Region (MDR) for hurricane activity was found to have warmed by several tenths of a degree Celsius over the twentieth century. Coupled model historical simulations using current best estimates of radiative forcing suggest that the century-scale warming trend in the MDR may contain a significant contribution from anthropogenic forcing, including increases in atmospheric greenhouse gas concentrations. The results further suggest that the low-frequency variability in the MDR, apart from the trend, may contain substantial contributions from both radiative forcing (natural and anthropogenic) and internally generated climate variability. The second study used the GFDL huyrricane model, in an idealized setting, to simulate the impact of a pronounced CO2-induced warming on hurricane intensities and precipitation. A 1.75°C warming increases the intensities of hurricanes in the model by 5.8% in terms of surface wind speeds, 14% in terms of central pressure fall, or about one half category on the Saffir-Simpson Hurricane Scale. A revised storm-core accumulated (six-hour) rainfall measure shows a 21.6% increase under high CO2 conditions. Our simulated storm intensities are substantially less sensitive to sea surface temperature (SST) changes than recently reported historical observational trends are - a difference we are not able to completely reconcile at this time.
The past decade has been marked by significant advancements in numerical weather prediction of hurricanes, which have greatly contributed to the steady decline in forecast track error. Since its operational implementation by the U.S. National Weather Service (NWS) in 1995, the best-track model performer has been NOAA’s regional hurricane model developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The purpose of this paper is to summarize the major upgrades to the GFDL hurricane forecast system since 1998. These include coupling the atmospheric component with the Princeton Ocean Model, which became operational in 2001, major physics upgrades implemented in 2003 and 2006, and increases in both the vertical resolution in 2003 and the horizontal resolution in 2002 and 2005. The paper will also report on the GFDL model performance for both track and intensity, focusing particularly on the 2003 through 2006 hurricane seasons. During this period, the GFDL track errors were the lowest of all the dynamical model guidance available to the NWS Tropical Prediction Center in both the Atlantic and eastern Pacific basins. It will also be shown that the GFDL model has exhibited a steady reduction in its intensity errors during the past 5 yr, and can now provide skillful intensity forecasts. Tests of 153 forecasts from the 2004 and 2005 Atlantic hurricane seasons and 75 forecasts from the 2005 eastern Pacific season have demonstrated a positive impact on both track and intensity prediction in the 2006 GFDL model upgrade, through introduction of a cloud microphysics package and an improved air–sea momentum flux parameterization. In addition, the large positive intensity bias in sheared environments observed in previous versions of the model is significantly reduced. This led to the significant improvement in the model’s reliability and skill for forecasting intensity that occurred in 2006.
In
this study, a new modeling framework for simulating Atlantic hurricane
activity is introduced. The model is an 18-km-grid nonhydrostatic regional
model, run over observed specified SSTs and nudged toward observed
time-varying large-scale atmospheric conditions (Atlantic domain wavenumbers
0–2) derived from the National Centers for Environmental Prediction (NCEP)
reanalyses. Using this “perfect large-scale model” approach for 27 recent
August–October seasons (1980–2006), it is found that the model successfully
reproduces the observed multidecadal increase in numbers of Atlantic
hurricanes and several other tropical cyclone (TC) indices over this period.
The correlation of simulated versus observed hurricane activity by year
varies from 0.87 for basin-wide hurricane counts to 0.41 for U.S.
landfalling hurricanes. For tropical storm count, accumulated cyclone
energy, and TC power dissipation indices the correlation is 0.75, for major
hurricanes the correlation is 0.69, and for U.S. landfalling tropical
storms, the correlation is 0.57. The model occasionally simulates hurricanes
intensities of up to category 4 (942 mb) in terms of central pressure,
although the surface winds (< 47 m s-1 ) do not exceed category-2
intensity. On interannual time scales, the model reproduces the observed
ENSO-Atlantic hurricane covariation reasonably well. Some notable aspects of
the highly contrasting 2005 and 2006 seasons are well reproduced, although
the simulated activity during the 2006 core season was excessive. The
authors conclude that the model appears to be a useful tool for exploring
mechanisms of hurricane variability in the Atlantic (e.g., shear versus
potential intensity contributions). The model may be capable of making
useful simulations/projections of pre-1980 or twentieth-century Atlantic
hurricane activity. However, the reliability of these projections will
depend on obtaining reliable large-scale atmospheric and SST conditions from
sources external to the model.
Marchok, Timothy, Robert Rogers, and Robert E Tuleya, 2007: Validation Schemes for Tropical Cyclone Quantitative Precipitation Forecasts: Evaluation of Operational Models for U.S. Landfalling Cases. Weather and Forecasting, 22(4), DOI:10.1175/WAF1024.1. Abstract
A scheme for validating quantitative precipitation forecasts (QPFs) for landfalling tropical cyclones is developed and presented here. This scheme takes advantage of the unique characteristics of tropical cyclone rainfall by evaluating the skill of rainfall forecasts in three attributes: the ability to match observed rainfall patterns, the ability to match the mean value and volume of observed rainfall, and the ability to produce the extreme amounts often observed in tropical cyclones. For some of these characteristics, track-relative analyses are employed that help to reduce the impact of model track forecast error on QPF skill. These characteristics are evaluated for storm-total rainfall forecasts of all U.S. landfalling tropical cyclones from 1998 to 2004 by the NCEP operational models, that is, the Global Forecast System (GFS), the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model, and the North American Mesoscale (NAM) model, as well as the benchmark Rainfall Climatology and Persistence (R-CLIPER) model. Compared to R-CLIPER, all of the numerical models showed comparable or greater skill for all of the attributes. The GFS performed the best of all of the models for each of the categories. The GFDL had a bias of predicting too much heavy rain, especially in the core of the tropical cyclones, while the NAM predicted too little of the heavy rain. The R-CLIPER performed well near the track of the core, but it predicted much too little rain at large distances from the track. Whereas a primary determinant of tropical cyclone QPF errors is track forecast error, possible physical causes of track-relative differences lie with the physical parameterizations and initialization schemes for each of the models. This validation scheme can be used to identify model limitations and biases and guide future efforts toward model development and improvement.
Tuleya, Robert E., M DeMaria, and R J Kuligowski, February 2007: Evaluation of GFDL and simple statistical model rainfall forecasts for U.S. landfalling tropical storms. Weather and Forecasting, 22(1), DOI:10.1175/WAF972.1. Abstract
To date, little objective verification has been performed for rainfall predictions from numerical forecasts of landfalling tropical cyclones. Until 2001, digital output from the operational version of the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane forecast model was available only on a 1° grid. The GFDL model was rerun or reanalyzed for 25 U.S. landfalling tropical cyclones from 1995 to 2002 to obtain higher resolution (1/3°) output. Several measures of forecast quality were used to evaluate the predicted rainfall from these runs, using daily rain gauge data as ground truth. The overall quality was measured by the mean error and bias averaged over all the gauge sites. An estimate of the quality of the forecasted pattern was obtained through the correlation coefficient of the model versus gauge values. In addition, more traditional precipitation verification scores were calculated including equitable threat and bias scores. To evaluate the skill of the rainfall forecasts, a simple rainfall climatology and persistence (R-CLIPER) model was developed, where a climatological rainfall rate is accumulated along either the forecasted or observed storm track. Results show that the R-CLIPER and GFDL forecasts had comparable mean absolute errors of 0.9 in. (23 mm) for the 25 cases. The GFDL model exhibited a higher pattern correlation with observations than R-CLIPER, but still only explained 30% of the spatial variance. The GFDL model also had higher equitable threat scores than R-CLIPER, partially because of a low bias of R-CLIPER for rainfall amounts larger than 0.5 in. (13 mm). A large case-to-case variability was found that was dependent on both synoptic conditions and track error.
A response is made to the comments of Michaels et al. concerning a recent study by the authors. Even after considering Michaels et al.'s comments, the authors stand behind the conclusions of the original study. In contrast to Michaels et al., who exclusively emphasize uncertainties that lead to smaller future changes, uncertainties are noted that could lead to either smaller or larger changes in future intensities of hurricanes than those summarized in the original study, with accompanying smaller or larger societal impacts.
Knutson, Thomas R., and Robert E Tuleya, 2004: Impact of CO2-induced warming on simulated hurricane intensity and precipitation: Sensitivity to the choice of climate model and convective parameterization. Journal of Climate, 17(18), 3477-3495. Abstract PDF
Previous studies have found that idealized hurricanes, simulated under warmer, high-CO2 conditions, are more intense and have higher precipitation rates than under present-day conditions. The present study explores the sensitivity of this result to the choice of climate model used to define the CO2-warmed environment and to the choice of convective parameterization used in the nested regional model that simulates the hurricanes. Approximately 1300 five-day idealized simulations are performed using a higher-resolution version of the GFDL hurricane prediction system (grid spacing as fine as 9 km, with 42 levels). All storms were embedded in a uniform 5 m s−1 easterly background flow. The large-scale thermodynamic boundary conditions for the experiments— atmospheric temperature and moisture profiles and SSTs—are derived from nine different Coupled Model Intercomparison Project (CMIP2+) climate models. The CO2-induced SST changes from the global climate models, based on 80-yr linear trends from +1% yr−1 CO2 increase experiments, range from about +0.8° to +2.4°C in the three tropical storm basins studied. Four different moist convection parameterizations are tested in the hurricane model, including the use of no convective parameterization in the highest resolution inner grid. Nearly all combinations of climate model boundary conditions and hurricane model convection schemes show a CO2-induced increase in both storm intensity and near-storm precipitation rates. The aggregate results, averaged across all experiments, indicate a 14% increase in central pressure fall, a 6% increase in maximum surface wind speed, and an 18% increase in average precipitation rate within 100 km of the storm center. The fractional change in precipitation is more sensitive to the choice of convective parameterization than is the fractional change of intensity. Current hurricane potential intensity theories, applied to the climate model environments, yield an average increase of intensity (pressure fall) of 8% (Emanuel) to 16% (Holland) for the high-CO2 environments. Convective available potential energy (CAPE) is 21% higher on average in the high-CO2 environments. One implication of the results is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas–induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms.
Persing, J, M T Montgomery, and Robert E Tuleya, 2002: Environmental interactions in the GFDL Hurricane Model for Hurricane Opal. Monthly Weather Review, 130(2), 298-317. Abstract PDF
Hurricane Opal (1995) crossed the Gulf of Mexico rapidly intensifying to a 130-kt storm, then fortunately weakening before landfall on the Florida panhandle. This intensification was underforecast by the National Hurricane Center. Forecast fields from the 1997 version of the Geophysical Fluid Dynamics Laboratory Hurricane Prediction System (GFDL model) for Hurricane Opal are used to diagnose the rapid intensification of the tropical cyclone. While falling short of the realized peak intensity, the simulation did capture the phase of intensification. This study presents the first step toward diagnosing the mechanisms for intensification within a moderate resolution (~15 km) hydrostatic model and testing the extant hypotheses in the literature.
Using a mean tangential wind budget, and the Eliassen balanced vortex model, positive eddy vorticity fluxes aloft are identified in the vicinity (~600 km) of Opal, but are not found to aid intensification. A detailed examination of each of the terms of the budget (mean and eddy vorticity flux, mean and eddy vertical advection, and "friction") shows for the most rapidly intensifying episodes a greater forcing for mean tangential winds near the center of the storm, particularly from the mean vertical advection and mean vorticity flux terms. Variations in these mean terms can be primarily attributed to variations in the heating rate. Upper-level divergence exhibits significant vertical structure, such that single-level or layer-average analysis techniques do not capture the divergence signature aloft. Far from the storm (400 km), divergence features near 200 mb are significantly influenced by convective events over land that are, perhaps, only indirectly influenced by the hurricane.
While there is a trough interaction simulated within the model, we suggest that the hurricane develops strongly without an important interaction with the trough. A synthetic removal of specific potential vorticity features attributed to the trough is proposed to test this hypothesis. Imposed shear is proposed to weaken the storm at later times, which is at odds with other recent "nontrough" theories for the behavior of Opal.
Shen, W, Isaac Ginis, and Robert E Tuleya, 2002: A numerical investigation of land surface water on landfalling hurricanes. Journal of the Atmospheric Sciences, 59(4), 789-802. Abstract PDF
Little is known about the effects of surface water over land on the decay of landfalling hurricanes. This study, using the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory hurricane model, examines the surface temperature changes due to hurricane-land surface water interactions, and their effects on the surface heat fluxes, hurricane structure, and intensity. Different water depths and surface conditions are incorporated for a variety of experiments starting with a hurricane bogus embedded in a uniform easterly mean flow of 5 m s-1.
A salient feature of hurricane-land surface water interaction is the local surface cooling near the hurricane core with the largest cooling behind and on the right side of the hurricane center. Unlike the surface cooling due to hurricane-ocean interaction, the largest cooling in hurricane-land surface water interaction can be much closer to the hurricane core. Without solar radiation during night, the surface evaporation dominates the local surface cooling. This causes a surface temperature contrast between the core area and its environment. During the day, the surface temperature contrast is enhanced due to additional influence from the reduced solar radiation under the core. Related to the local surface cooling, there is a significant reduction of surface evaporation with a near cutoff behind the hurricane center. A layer of half-meter water can noticeably reduce landfall decay although the local surface temperature around the hurricane core region is more than 4°C lower than in its environment. Further experiments indicate that an increase of roughness reduces the surface winds but barely changes the surface temperature and evaporation patterns and their magnitudes since the increase of roughness also increases the efficiency of surface evaporation.
Tuleya, Robert E., and Thomas R Knutson, 2002: Impact of climate change on tropical cyclones In Atmosphere-Ocean Interactions, Vol. 1, Southampton, UK, WIT Press, 293-312. Abstract
One of the possible impacts of global warming is on tropical cyclones, on their formation, track, intensity and decay rates. One of the consequences of global warming appears to be not only an increase in sea surface temperature, but more importantly a potential increase in the overall energy flux at the tropical ocean surface. Theoretical considerations imply that this increased surface disequilibrium may lead to more intense tropical storms. Three-dimensional numerical modeling is another approach to evaluating these potential consequences. Since global models are still rather limited in simulating mesoscale storm structure, this paper describes a regional modeling approach utilizing a multiple nested technique which has already been shown to be practical in operational forecasts. These 3-D model results confirm theoretical methods that indicate an increase of 3 to 10% in maximum wind speeds for a CO2 tropical SST warming of ~2.5°C. Perhaps more importantly, model results indicate a 20 to 30% increase in hurricane-related precipitation. Furthermore, the resulting increases in intensity and precipitation appear to be qualitatively insensitive to changes in convective parameterization. This paper emphasizes the impact of global warming on storm intensity and precipitation. The question of the possible impact on tropical storm frequency and track is still problematic.
Knutson, Thomas R., Robert E Tuleya, W Shen, and Isaac Ginis, 2001: Impact of CO2-induced warming on hurricane intensities simulated in a hurricane model with ocean coupling. Journal of Climate, 14(11), 2458-2468. Abstract PDF
This study explores how a carbon dioxide (CO2) warming-induced enhancement of hurricane intensity could be altered by the inclusion of hurricane-ocean coupling. Simulations are performed using a coupled version of the Geophysical Fluid Dynamics Laboratory hurricane prediction system in an idealized setting with highly simplified background flow fields. The large-scale atmospheric boundary conditions for these high-resolution experiments (atmospheric temperature and moisture profiles and moisture profiles and SSTs) are derived from control and high-CO2 climatologies obtained from a low-resolution (R30) global coupled ocean-atmosphere climate model. The high-CO2 conditions are obtained from years 71-120 of a transient +1% yr -1 CO2-increase experiment with the global model. The CO2-induced SST changes from the global climate model range from +2.2° to +2.7°C in the six tropical storm basins studied. In the storm simulations, ocean coupling significantly reduces the intensity of simulated tropical cyclones, in accord with previous studies. However, the net impact of ocean coupling on the simulated CO2 warming-induced intensification of tropical cyclones is relatively minor. For both coupled and uncoupled simulations, the percentage increase in maximum surface wind speeds averages about 5%-6% over the six basins and varies from about 3% to 10% across the different basins. Both coupled and uncoupled simulations also show strong increases of near-storm precipitation under high-CO2 climate conditions, relative to control (present day) conditions.
Soden, Brian J., C S Velden, and Robert E Tuleya, 2001: The impact of satellite winds on experimental GFDL hurricane model forecasts. Monthly Weather Review, 129(4), 835-852. Abstract PDF
A series of experimental forecasts are performed to evaluate the impact of enhanced satellite-derived winds on numerical hurricane track predictions. The winds are derived from Geostationary Operational Environmental Satellite-8 (GOES-8) multispectral radiance observations by tracking cloud and water vapor patterns from successive satellite images. A three-dimensional optimum interpolation method is developed to assimilate the satellite winds directly into the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane prediction system. A series of parallel forecasts are then performed, both with and without the assimilation of GOES winds. Except for the assimilation of the satellite winds, the model integrations are identical in all other respects. A strength of this study is the large number of experiments performed. Over 100 cases are examined from 11 different storms covering three seasons (1996–98), enabling the authors to account for and examine the case-to-case variability in the forecast results when performing the assessment. On average, assimilation of the GOES winds leads to statistically significant improvements for all forecast periods, with the relative reductions in track error ranging from ~5% at 12 h to ~12% at 36 h. The percentage of improved forecasts increases following the assimilation of the satellite winds, with roughly three improved forecasts for every two degraded ones. Inclusion of the satellite winds also dramatically reduces the westward bias that has been a persistent feature of the GFDL model forecasts, implying that much of this bias may be related to errors in the initial conditions rather than a deficiency in the model itself. Finally, a composite analysis of the deep-layer flow fields suggests that the reduction in track error may be associated with the ability of the GOES winds to more accurately depict the strength of vorticity gyres in the environmental flow. These results offer compelling evidence that the assimilation of satellite winds can significantly improve the accuracy of hurricane track forecasts.
Vitart, Frederic, Jeffrey L Anderson, Joseph J Sirutis, and Robert E Tuleya, 2001: Sensitivity of tropical storms simulated by a general circulation model to changes in cumulus parameterization. Quarterly Journal of the Royal Meteorological Society, 127(571), 25-51. Abstract PDF
A number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surface temperatures. Many GCMs have demonstrated an ability to simulate some aspects of the observed interannual variability of tropical storms, in particular, variability in storm frequency. This has led to nascent attempts to use GCMs as part of programs to produce operational seasonal forecasts of tropical-storm numbers.
In this study, the sensitivity of the statistics of GCM-simulated tropical storms to changes in the model's physical parameterization is examined. After preliminary results indicated that these statistics were most sensitive to details of the convective parameterization, GCM simulations with identical dynamical cores but different convective parameterizations were created. The parameterizations examined included moist convective adjustment, two variants of the Arakawa-Schubert scheme, and several variants of the relaxed Arakawa-Schubert (RAS) scheme; the impact of including a shallow-convection parameterization was also examined.
The simulated tropical -storm frequency, intensity, structure, and interannual variability were all found to exhibit significant sensitivities to changes in convective parameterization. A particularly large sensitivity was found when the RAS and Arakawa-Schubert parameterizations were modified to place restrictions on the production of deep convection.
Climatologies of the GCM tropical atmosphere and composites of tropical storms were examined to address the question of whether the tropical-storm statistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parameterization-induced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model , initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropical storms nevertheless produce a much more realistic interannual variability of storms, apparently due to an improved tropical mean climate.
A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to the occurrence of tropical storms in the low-resolution GCMs, although this may not be the case with observations. If the tropical CAPE is too low, tropical storms in the low-resolution GCMs cannot form with realistic frequency.
DeMaria, M, and Robert E Tuleya, 2000: Evaluation of quantitative precipitation forecasts from the GFDL Hurricane Model In Symposium on Precipitation Extremes, Boston, MA:, American Meteorological Society, 340-343.
Shen, W, Robert E Tuleya, and Isaac Ginis, 2000: A sensitivity study of the thermodynamic environment on GFDL model hurricane intensity: Implications for global warming. Journal of Climate, 13(1), 109-121. Abstract PDF
In this study, the effect of thermodynamic environmental changes on hurricane intensity is extensively investigated with the National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory hurricane model for a suite of experiments with different initial upper-tropospheric temperature anomalies up to ±4ºC and sea surface temperatures ranging from 26º to 31ºC given the same relative humidity profile.
The results indicate that stabilization in the environmental atmosphere and sea surface temperature (SST) increase cause opposing effects on hurricane intensity. The offsetting relationship between the effects of atmospheric stability increase (decrease) and SST increase (decrease) is monotonic and systematic in the parameter space. This implies that hurricane intensity increase due to a possible global warming associated with increased CO2 is considerably smaller than that expected from warming of the oceanic waters alone. The results also indicate that the intensity of stronger (weaker) hurricanes is more (less) sensitive to atmospheric stability and SST changes. The model-attained hurricane intensity is found to be well correlated with the maximum surface evaporation and the large-scale environmental convective available potential energy. The model-attained hurricane intensity is highly correlated with the energy available from wet-adiabatic ascent near the eyewall relative to a reference sounding in the undisturbed environment for all the experiments. Coupled hurricane-ocean experiments show that hurricane intensity becomes less sensitive to atmospheric stability and SST changes since the ocean coupling causes larger (smaller) intensity reduction for stronger (weaker) hurricanes. This implies less increase of hurricane intensity related to a possible global warming due to increased CO2.
Knutson, Thomas R., and Robert E Tuleya, 1999: Increased hurricane intensities with CO2 -induced global warming as simulated using the GFDL hurricane prediction system. Climate Dynamics, 15, 503-519. Abstract PDF
The impact of CO2 -induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO2 increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO2 conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO2 storms, with SSTs warmer by about 2.2° C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3-7 m/s (5%-11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO2 sample. In terms of storm tracks, the high CO2 sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO2 storm than for the control,and the high CO2 storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.
Hurricanes can inflict catastrophic property damage and loss of human life. Thus, it is important to determine how the character of these powerful storms could change in response to greenhouse gas-induced global warming. The impact of climate warming on hurricane intensities was investigated with a regional, high-resolution, hurricane prediction model. In a case study, 51 western Pacific storm cases under present-day climate conditions were compared with 51 storm cases under high-CO2 conditions. More idealized experiments were also performed. The large-scale initial conditions were derived from a global climate model. For a sea surface temperature warming of about 2.2°C, the simulations yielded hurricanes that were more intense by 3 to 7 meters per second (5 to 12 percent) for wind speed and 7 to 20 millibars for central surface pressure.
Kurihara, Yoshio, and Robert E Tuleya, 1998: A prospect of improvement in the forecast of hurricane landfall In 16th Conference on Weather Analysis & Forecasting and Symposium on the Research Foci of the U.S. Weather Research Program, Boston, MA, American Meteorological Society, 524-525.
Kurihara, Yoshio, Robert E Tuleya, and Morris A Bender, 1998: Application and improvement of the GFDL Hurricane Prediction System In Research Activities in Atmospheric and Oceanic Modelling, WMO/TD No. 865, Geneva, Switzerland, World Meteorological Organization, 5.31.
The Geophysical Fluid Dynamics Laboratory (GFDL) Hurricane Prediction System was adopted by the U.S. National Weather Service as an operational hurricane prediction model in the 1995 hurricane season. The framework of the prediction model is described with emphasis on its unique features. The model uses a multiply nested movable mesh system to depict the interior structure of tropical cyclones. For cumulus parameterization, a soft moist convective adjustment scheme is used. The model initial condition is defined through a method of vortex replacement. It involves generation of a realistic hurricane vortex by a scheme of controlled spinup. Time integration of the model is carried out by a two-step iterative method that has a characteristic of frequency-selective damping.
The outline of the prediction system is presented and the system performance in the 1995 hurricane season is briefly summarized. Both in the Atlantic and the eastern Pacific, the average track forecast errors are substantially reduced by the GFDL model, compared with forecasts by other models, particularly for the forecast periods beyond 36 h. Forecasts of Hurricane Luis and Hurricane Marilyn were especially skillful. A forecast bias is noticed in cases of Hurricane Opal and other storms in the Gulf of Mexico. The importance of accurate initial conditions, in both the environmental flow and the storm structure, is argued.
Knutson, Thomas R., Robert E Tuleya, and Yoshio Kurihara, 1997: Exploring the sensitivity of hurricane intensity to CO2-induced global warming using the GFDL Hurricane Prediction System In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 587-588.
Kurihara, Yoshio, Morris A Bender, and Robert E Tuleya, 1997: For hurricane intensity forecast: Formulation of a new initialization method for the GFDL Hurricane Prediction Model In 22nd Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 543-544.
Kurihara, Yoshio, Robert E Tuleya, and Morris A Bender, 1997: Improvement of the GFDL Hurricane Prediction System In CAS/JSC Working Group on Numerical Experimentation - Research Activities in Atmospheric & Oceanic Modelling, Report No. 25, WMO/TD-No. 792, Geneva, Switzerland, World Meteorological Organization, 5.22.
Tuleya, Robert E., and S J Lord, 1997: The impact of dropwindsonde data on GFDL hurricane model forecasts using global analyses. Weather and Forecasting, 12(2), 307-323. Abstract PDF
The National Centers for Environmental Prediction (NCEP) and the Hurricane Research Division (HRD) of NOAA have collaborated to postprocess Omega dropwindsonde (ODW) data into the NCEP operational global analysis system for a series of 14 cases of Atlantic hurricanes (or tropical storms) from 1982 to 1989. Objective analyses were constructed with and without ingested ODW data by the NCEP operational global system. These analyses were then used as initial conditions by the Geophysical Fluid Dynamics Laboratory (GFDL) high-resolution regional forecast model.
This series of 14 experiments with and without ODWs indicated the positive impacts of ODWs on track forecasts using the GFDL model. The mean forecast track improvement at various forecast periods ranged from 12% to 30% relative to control cases without ODWs: approximately the same magnitude as those of the NCEP global model and higher than those of the VICBAR barotropic model for the same 14 cases. Mean track errors were reduced by 12 km at 12 h, by ~50 km for 24-60 h, and by 127 km at 72 h (nine cases). Track improvements were realized with ODWs at ~75% of the verifying times for the entire 14-case ensemble.
With the improved analysis using ODWs, the GFDL model was able to forecast the interaction of Hurricane Floyd (1987) with an approaching midlatitude trough and the storm's associated movement from the western Caribbean north, then northeastward from the Gulf of Mexico into the Atlantic east of Florida. In addition, the GFDL model with ODWs accurately forecasted the rapid approach and landfall of Hurricane Hugo (1989) onto the U.S. mainland. An assessment of the differences between analyses indicates that the impact of ODWs can be attributable in part to differences of ~1 m s-1 in steering flow of the initial state.
In addition to track error, the skill of intensity prediction using the ODW dataset was also investigated. Results indicate a positive impact on intensity forecasts with ODW analyses. However, the overall skill relative to the National Hurricane Center statistical model SHIFOR is shown only after 2 or 3 days. It is speculated that with increased data coverage such as ODWs both track and intensity error can be further reduced provided that data sampling can be optimized and objective analysis techniques utilizing asynoptic data can be developed and improved.
Bender, Morris A., Robert E Tuleya, Yoshio Kurihara, and S J Lord, 1996: Results of the operational GFDL hurricane model at NCEP In 11th Conference on Numerical Weather Prediction, Boston, MA, American Meteorological Society, 393-395.
Burpee, R W., J L Franklin, A Abe-Ouchi, Robert E Tuleya, and S D Aberson, 1996: The impact of Omega dropwindsondes on operational hurricane track forecast models. Bulletin of the American Meteorological Society, 77(5), 925-933. Abstract PDF
Since 1982, the Hurricane Research Division (HRD) has conducted a series of experiments with research aircraft to enhance the number of observations in the environment and the core of hurricanes threatening the United States. During these experiments, the National Oceanic and Atmospheric Administration WP-3D aircraft crews release Omega dropwindsondes (ODWs) at 15-20 min. intervals along the flight track to obtain profiles of wind, temperature, and humidity between flight level and the sea surface. Data from the ODWs are transmitted back to the aircraft and then sent via satellite to the Tropical Prediction Center and the National Centers for Environmental Prediction (NCEP), where the observations become part of the operational database.
This paper tests the hypothesis that additional observations improve the objective track forecast models that provide operational guidance to the hurricane forecasters. The testing evaluates differences in forecast tracks from models run with and without the ODW data in a research mode at HRD, NCEP, and the Geophysical Fluid Dynamics Laboratory. The middle- and lower-tropospheric ODW data produce statistically significant reductions in 12-60-h mean forecast errors. The error reductions, which range from 16% to 30%, are at least as large as the accumulated improvement in operational forecasts achieved over the last 20-25 years. This breakthrough provides strong experimental evidence that more comprehensive observations in the hurricane environment and core will lead to immediate improvements in operational forecast guidance.
Kurihara, Yoshio, Robert E Tuleya, and Morris A Bender, 1996: Simulation studies of tropical cyclones In Research Activities in Atmospheric and Oceanic Modelling, CAS/JSC Working Group on Numerical Experimentation, Report No. 23 WMO/TD No. 734, World Meteorological Organization, 5.17-5.18.
Soden, Brian J., Robert E Tuleya, and C S Velden, 1996: Improving hurricane forecasts through the assimilation of satellite-derived winds In 15th Conference on Weather Analysis and Forecasting, Boston, MA, American Meteorological Society, 505.
Tuleya, Robert E., Morris A Bender, and Yoshio Kurihara, 1996: Prediction of hurricane landfall using the GFDL model In 11th Conference on Numerical Weather Prediction, Boston, MA, American Meteorological Society, 407-408.
Kurihara, Yoshio, Morris A Bender, and Robert E Tuleya, 1995: Performance evaluation of the GFDL Hurricane Prediction System in the 1994 hurricane season In 21st Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 41-43.
The hurricane model initialization scheme developed at GFDL was modified to improve the representation of the environmental fields in the initial condition. The filter domain defining the extent of the tropical cyclone in the global analysis is determined from the distribution of the low-level disturbance winds. The shape of the domain is generally not circular in order to minimize the removal of important nonhurricane features near the storm region. An optimum interpolation technique is used to determine the environmental fields within the filter domain. Outside of the domain, the environmental fields are identical to the original global analysis. The generation process of the realistic and mode-compatible vortex has also undergone some minor modifications so that reasonable vortices are produced for various data conditions. The upgraded hurricane prediction system was tested for a number of cases and compared against the previous version and yielded an overall improvement in the forecasts of storm track. The system was run in an automated semioperational mode during the 1993 hurricane season for 36 cases in the Atlantic and 36 cases in the eastern Pacific basin. It demonstrated satisfactory skill in the storm track forecasts in many cases, including the abrupt recurvature of Hurricane Emily in the Atlantic and the landfall of Hurricane Lidia onto the Pacific coast of Mexico.
Tuleya, Robert E., 1994: Tropical storm development and decay: sensitivity to surface boundary conditions. Monthly Weather Review, 122(2), 291-304. Abstract PDF
Hurricane models have rarely been used to investigate the observational fact that tropical disturbances seldom form, develop, or intensify over land. Furthermore, rather ad hoc assumptions have been made when modeling landfall. The general conclusion is that energy supplied primarily through surface fluxes is necessary for tropical cyclone development and maintenance. In the past, rather a priori assumptions have been made such as the elimination of surface sensible and latent heat fluxes over land or the reduction of surface land temperature. By incorporating an improved version of the Geophysical Fluid Dynamics Laboratory (GFDL) tropical cyclone model with diurnal radiation and a bulk subsurface layer with explicit prediction of land temperature, a series of experiments was performed to test the sensitivity of surface boundary conditions to tropical cyclone development and decay as landfall.
A triply nested version of the GFDL model was used in an idealized setting in which a tropical disturbance, taken from the incipient stage of Gloria (1985), was superposed on a uniform easterly flow of 5 m s-1. A control case was performed for ocean conditions of fixed 302-K SST in which the initial disturbance of about 998 hPa developed to a quasi-steady state of of 955 hPa after one day of integration. Using identical atmospheric conditions, a series of experiments was performed in which the underlying land surface was specified with different values of thermal property, roughness, and wetness. By systematically changing the thermal property (i.e., heat capacity and conductivity) of the subsurface from values typical of a mixed-layer ocean to those of land, a progressively weaker tropical system was observed. It was found that the initial disturbance over land failed to intensify below 985 hPa, even when evaporation was specified at the potential rate. The reduction of evaporation over land, caused primarily by the reduction of surface land temperature near the storm core, was responsible for the inability of the tropical disturbance to develop to any large extent. Under land conditions, the known positive feedback between storm surface winds and surface evaporation was severely disrupted.
In sensitivity experiences analogous to the all-land cases, a series of landfall simulations were performed in which land conditions were specified for a region of the domain so that a strong mature tropical cyclone similar to the ocean control case encountered land. Again as in the all-land case, the demise of the landfalling storm takes place due to the suppression of the potential evaporation and the associated reduction of surface temperatures beneath the landfalling cyclone. Even when evaporation was prescribed at the potential rate, a realistic rapid filling (36 hPa in 12 h) ensued despite the idealized nature of the simulations. Although not critical for decay, it was found that surface roughness and reduced relative wetness do enhance decay at landfall.
The initialization scheme designed at GFDL to specify a more realistic initial storm structure of tropical cyclones was tested on four real data cases using the GFDL high-resolution multiply nested movable mesh hurricane model. Three of the test cases involved Hurricane Gloria (1985) in the Atlantic basin; the fourth involved Hurricane Gilbert (1988) in the Gulf of Mexico. The initialization scheme produced an initial vortex that was well adapted to the forecast model and was much more realiztic in size and intensity than the storm structure obtained from the NMC T80 global analysis. As a result, the erratic storm motion seen in previous intergrations of the GFDL model has been nearly eliminated with dramatic improvements in track forecasts during the first 48 h of the prediction. Using the new scheme, the average 24-h and 48-h forecast error for the four test cases was 58 and 94 km, respectively, compared with 143 and 191 km for the noninitialized forecasts starting from the global analysis. The average National Hurricane Center operational forecast error at 24 and 48 h was 118 and 212 km for the same four cases. After 48 h the difference in the average track error became small between the integrations starting from the global analysis and the forecasts starting from the fields obtained by the initialization scheme
With accurate specification of the initial vortex structure, changes in the storm intensity were also well predicted in these cases. The model correctly forecasted the rapid intensification of Hurricane Gloria just after the system was first upgraded to a hurricane. The model storm intensification also ceased at approximately the same time as observed, with gradual weakening as the storm moved north and approached the east coast of the United States. In the forecast of Hurricane Gilbert, the model storm initially weakened as it moved over the Yucatan Peninsula and underwent only moderate reintensification after moving over the Gulf of Mexico, in good agreement with observations
Finally, in the case where the track of Hurricane Gloria was well forecast, the distribution of the maximum low-level winds was accurately predicted as the storm moved up the east coast of the United States. During this period the model successfully reproduced many observed features such as large asymmetries in the wind field, with strongest winds occurring well east of the storm center, and a sharp decrease of the wind speed at the coast. Although asymmetry in the wind distribution was reproduced to a first order in the forecast starting with the global analysis, the agreement with observations was much better with the specified vortex, primarily due to a more realistic radius of maximum wind and storm intensity.
Kurihara, Yoshio, Robert E Tuleya, Morris A Bender, and R Ross, 1993: Advanced modeling of tropical cyclones In Tropical Cyclone Disasters, Proceedings of ICSU/WMO International Symposium, October 12-16, 1992, Beijing, China, Peking University Press, 190-201. Abstract
Advanced tropical cyclone models of sufficiently fine resolution are capable of representing important internal structure of the vortex. In the model, a vortex should interact with ocean and land in a realistic manner. Interaction with the ocean can significantly moderate the storm intensity. Inclusion of the heat budget of the soil layer retards the storm intensity over land. How to improve the treatment of deep convection is an issue which is wide open for future study. Specification of a realistic, yet model-adapted vortex in the initial condition of the model is essential for improvement of tropical cyclone track and intensity prediction.
Kurihara, Yoshio, Morris A Bender, Robert E Tuleya, and R Ross, 1993: Hurricane forecasting with the GFDL automated prediction system In 20th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 323-326.
Bender, Morris A., R Ross, Yoshio Kurihara, and Robert E Tuleya, 1991: Improvements in tropical cyclone track and intensity forecasts using a bogus vortex In 19th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 324-325. PDF
Tuleya, Robert E., 1991: Sensitivity studies of tropical storm genesis using a numerical model. Monthly Weather Review, 119(3), 721-733. Abstract PDF
This study investigates two cases of the FGGE III-B tropical cyclone genesis study of Tuleya (1988) in more detail. These two cases occurred within a week of one another in the tropical North Atlantic in August 1979. One disturbance developed into Hurricane David, the other did not develop past the depression stage. At one point in their evolution the disturbances had quite similar values of low-level vorticity. In the developing case of Hurricane David, the disturbance propagated along in a low-level wave trough with an accompanying high wind maximum. In the nondeveloping case the initial disturbance was also embedded in a wave trough with an associated wind maximum. This low-level wave propagated westward leaving the depression in its wake. The different environmental flow was responsible for the different behavior. Synoptic and budget analyses revealed significant differences in disturbance structure and vorticity and equivalent potential temperature tendencies at the time of approximate equal strength of the two disturbances. The evolution of these two disturbances was quite robust even to reasonable increases to the initial relative humidity.
Supplementary experiments of the developing case were performed by altering the sea surface temperature and surface evaporation. It was found that the difference in storm evolution was minor in a case when climatological mean values of sea surface temperatures were specified.
The climatological mean values were ~0.5 K lower than the August 1979 mean used in the control simulation. In addition, an experiment without evaporation led to a propagating easterly wave with little development. Furthermore, when the evaporation was specified to a climatological constant value, there was intensification into a weak tropical storm with a rather peculiar structure. Apparently, at least in this case, processes other than evaporation-wind feedback led to moderate storm intensification.
The prediction capability of the GFDL triply nested, movable mesh model, with finest grid resolution of 1/6 degree, was investigated using several case studies of Hurricane Gloria (1985) during the period that the storm approached and moved up the east coast of the United States. The initial conditions for these experiments were interpolated from an NMC T80 global analysis at 0000 UTC 25 September and 1200 UTC 22 September. The integrations starting from 0000 UTC 25 September were run 72 h, while those starting on 1200 UTC 22 September were run 132 h. The lateral boundary conditions were obtained from either an integration of the NMC T80 forecast model or the T80 global analysis, or were fixed to the initial value.
The model's predicted track of Gloria for each integration was compared against the best track determined by the National Hurricane Center (NHC). For the case starting from 0000 UTC 25 September using a forecasted boundary condition, the model successfully forecasted significant acceleration of the storm's movement after 48 h. The 72 h forecast error was about 191 km, compared to 480 km for the official track forecast made by the NHC.
To examine the model's skill in simulating the storm structure, distributions of the low level maximum wind and total storm rainfall during passage of the model storm are shown and compared with observed values. The model successfully reproduced many observed features such as the occurrence of strong winds well east of the storm center, with an abrupt decrease of the wind field along the coastline. When the storm track was accurately forecasted, the total storm rainfall amounts agreed well with the observed values. In both the model integration and observations, a significant structural change took place as the storm accelerated toward the north with little significant precipitation occurring south of the storm center and heavy precipitation spreading well north of the storm. It appears that the gross features of the structure of the storm's outer region resulted from the interaction of the vortex with its environment.
Sensitivity of the model forecast to the lateral boundary condition and the horizontal resolution was also investigated. The storm's track error was greatly affected after the boundary error propagated by advection to the storm region. The impact of the horizontal resolution on the forecast was such that the model with one degree resolution produced a fairly good track forecast up to 48 h, but failed to simulate some of the main structural features.
In the experiments starting from the 0000 UTC September 25 initial field, the interior storm structure did not develop, and the storm exhibited too large a radius of maximum wind throughout the integration. However, the integrations starting from 1200 UTC September 22 developed a more intense storm, with a more realistic radius of maximum wind. These differences were due to the spinup time necessary for the storm to develop in the model when starting from a coarse resolution global analysis which did not adequately resolve the fine structure of the storm interior. This indicates the importance of proper specification of the storm in the initial field.
Tuleya, Robert E., 1988: A numerical study of the genesis of tropical storms observed during the FGGE year. Monthly Weather Review, 116(5), 1188-1208. Abstract PDF
This study utilizes the First GARP Global Experiment's (FGGE) analyzed dataset and a relatively fine scale regional model in combination to investigate the feasibility of numerically simulating tropical disturbances during the FGGE year, 1979. Four different cases were investigated including a cyclone, TC-17, in the Indian Ocean, a developing hurricane, David, and a nondeveloping wave in the Atlantic, and a multi-storm case, Tip and Roger, in the Pacific.
The results were promising when using ECMWF FGGE data in that simulations of genesis or nongenesis were achieved in the three developing cases and in the one nondeveloping case. The accuracy of the intensification rates varied from case to case. For example, in the simulation of TC-17, the maximum low level winds were simulated to be ~45 m s-1 while observations indicated winds of only 22 ms-1. However, in the case of David, the maximum winds increased at a slower rate than observed, while in the case of Tip the slow intensification rate was correctly simulated. An interesting result was the high correlation between model precipitation patterns in the simulations and observed satellite cloud photos. These results indicate that the environment in which an incipient disturbance is embedded plays a major role in the genesis process. An additional striking result was the wide variability of storm development and structure from case to case. Tropical storm David was simulated to be a relatively small scale storm whereas Tip was simulated to be a storm with an enormous area of gale force winds. The model simulations also produced different distributions of the low level wind maximum relative to the moving storm with banding of a number of meteorological fields, including precipitation and vorticity. The formation of storms was related to the presence of an incipient disturbance possessing cyclonic low level vorticity, and ample high relative humidity together with a strong coupling between the disturbance phase speed and the upper level flow field. Most cases including the nondeveloping wave contained upper level anticyclonic conditions. All cases included a weak warm, upper level anomaly including the nondeveloping wave case. Also, it was found that environmental upward motion is not always an accurate indicator of genesis.
A triply-nested. movable mesh model was used to study the behavior of tropical cyclones encountering island mountain ranges. The integration domain consisted of a 37° wide and 45° long channel, with an innermost mesh resolution of 1/6°. The storms used for this study were embedded in easterly flows of ~ 5 and ~ 10 m s-1 initially. Realistic distributions of island topography at 1/6° resolution were inserted into the model domain for the region of the Caribbean, including the islands of Cuba, Hispaniola, and Puerto Rico; the island of Taiwan; and the region of Luzon in the northern Philippines.
It was found that the islands affected the basic flow as well as the wind field directly associated with the storm system. The combination of these effects caused changes in the track and translational speed of the storm. In particular, in the case of the 5 m s-1 easterly flow, the storm accelerated and veered to the north well before reaching Taiwan. For the other island distributions, the northward deflection of the track and the increase of translational speed occurred near and over the islands. After landfall, the surface pressure underwent rapid filling. As the tropical cyclone passed over Hispaniola, the surface low continued to move along with the upper level vortex as it transversed the mountain range, while over Luzon it became obscure before reforming on the lee side slope of the mountain. In case of Taiwan and the 10 m s-1 easterly zonal flow, secondary surface lows developed behind the mountain range. The upper level vortex in this case became detached from the original surface low and eventually coupled with a secondary one.
The intensity changes of the storm near and over the islands were strongly related to the latent energy supply and the vertical coherence of the storm system. Advection of dry air from near or above the mountain tops into the storm area caused significant weakening of all the storm moving with the weaker easterly flow. Storms leaving Hispaniola and moving over open sea quickly reintensified as their vertical structure remained coherent. On the other hand, storms leaving Luzon were disorganized and did not reintensify until several hours later when the vertical coherence of the systems was reestablished
Although these experiments were performed for an idealized experimental design and basic flow, many observed storms have exhibited similar behavior in track deviation and decay. This implies that the effect of detailed topography should be considered if an accurate forecast of the storm direction and behavior is to be made.
Bender, Morris A., Robert E Tuleya, and Yoshio Kurihara, 1985: A numerical study of the effect of a mountain range on a landfalling tropical cyclone In 16th Conference on Hurricanes & Tropical Meteorology, Boston. MA, American Meteorological Society, 146-147.
A triply-nested, movable mesh model was used to study the effects of a mountain range on a landfalling tropical cyclone embedded in an easterly flow of ~ 10 m s-1. The integration domain consisted of a 37 degree wide and 45 degree long channel, with an innermost mesh resolution of 1/6 degrees. An idealized mountain range with maximum height of ~ 958 meters was placed parallel to the shoreline. The mountain range, which spanned 19 degrees in the north-south direction and 5 degrees in the east-west direction, was centered in the middle of the channel. Results obtained were compared with a previous landfall simulation, performed without the effect of the mountain range included. In particular, comparison was made of the total storm rainfall, maximum wind distribution and storm decay rate. It was found that the storm filled much more rapidly in the simulation run with the mountain included. The mountain range affected the decay rate through reduction in the supply of latent and kinetic energy into the storm circulation during, as well as after, passage of the storm over the mountain. It was found that a low-level, warm and dry region was produced where the storm winds descended the mountain slope.
In order to better isolate the effect of the mountain on the basic easterly flow, a supplemental integration was performed for the flow without the storm. It revealed that the mountain range caused a significant change in the basic flow over the mountain as well as up to several hundred kilometers downstream and extending considerably above the mountain top. A low-level southerly jet was observed to the west of the mountain base.
By use of a triply nested, movable mesh model, several ideal simulations of tropical cyclone landfall were performed for a strong zonal flow of ~10 m s-1. The integration domain was a 37 X 45 degree channel with the innermost mesh having a 22 X 22 point resolution of 1/6 degree. General characteristics similar to observed landfalling tropical cyclones are obtained in the primary simulation experiment including an abrupt change in the low level (~68 m) winds at the coastline and a decay of the tropical cyclone as it moves inland. Additional interesting features subject to model and experimental limitations include: little noticeable track change of the model storm when compared to a control experiment with an ocean surface only; a possible temporary displacement of the center of the surface wind circulation from the surface pressure center at landfall; and a distinct decrease in kinetic energy generation and precipitation a few hours after landfall. The sensitivity to the specified land surface conditions was analyzed by performing additional experiments in which the land surface conditions including surface temperature, moisture, and distribution of surface roughness were changed. It was found that a reasonable change in some of these land conditions can make a considerable difference in behavior for a landfalling tropical cyclone. It was also shown that a small, less intense model storm fills less rapidly. This corresponds well with observations that many landfalling hurricanes decay to approximately the same asymptotic value one day after landfall.
Tuleya, Robert E., and Yoshio Kurihara, 1984: The formation of comma vortices in a tropical numerical simulation model In 15th Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 320-324.
A detailed analysis of a numerically simulated tropical disturbance displays a comma-shaped pattern at the mature stage in the low-level vorticity, surface convergence, mid-level upward motion and precipitation fields. This study reveals that the high wind side of the disturbance is the favorable region for the formation of the tail of the comma pattern. The beta effect retards the development of the comma shape in the case of easterly environmental winds and enhances it in the case of westerlies. Analysis of the vorticity field suggests that the initial shape and intensity of the perturbation can influence the wind pattern of an evolving disturbance. Although some indications of band-like features exist in the wind field for dry experiments with no lower-boundary fluxes, surface fluxes of heat, moisture and momentum are found to be vital ingredients for the formative process of the distinct comma shape of the disturbance.
Tuleya, Robert E., Morris A Bender, and Yoshio Kurihara, 1983: A numerical study of simulated hurricane landfall In Sixth Conference on Numerical Weather Prediction, Boston, MA, American Meteorological Society, 323-324.
Kurihara, Yoshio, and Robert E Tuleya, 1982: Influence of environmental conditions on the genesis of a tropical storm In Topics in Atmospheric and Oceanographic Sciences Intense Atmospheric Vortices, Berlin, Germany, Springer-Verlag, 71-79. Abstract
Numerical experiments have been performed in search of the favorable conditions of environmental wind for the genesis of a tropical storm. The low level basic flow has an impact on the latent energy supply which is essential for genesis. The upper level basic flow has to be coupled with the phase speed of the low level incipient disturbance. The low level cyclonic shear of the basic flow is conducive to storm genesis.
Kurihara, Yoshio, and Robert E Tuleya, 1982: On a mechanism of the genesis of tropical storms In MSJ/JMA/WMO/AMS Regional Scientific Conference on Tropical Meteorology, Tsukuba, Japan, 18-22 October 1982, WMO Programme on Research in Tropical Meteorology, World Meteorological Organization, 17-18.
Tuleya, Robert E., and Yoshio Kurihara, 1982: A note on the sea surface temperature sensitivity of a numerical model of tropical storm genesis. Monthly Weather Review, 110(12), 2063-2069. Abstract PDF
In a three-dimensional numerical model of a tropical disturbance, a spectrum of development stages, from a weakening wave to a mature tropical storm, was obtained with a 5 K range (298 to 303 K) sea surface temperature (SST). However, the apparently large SST sensitivity of the model was found to be modulated by other factors including the large-scale environmental temperature and humidity. Through the use of this model, problems concerning a critical value of SST necessary for storm development were discussed.
The genesis of a tropical storm is studied using a numerical simulation model. The model used is an 11-level primitive equation model covering a channel domain of 25 degree span with open lateral boundaries at latitudes 5.5. and 30.5 degrees N. The initial basic flow field is based on the mean condition at 80 degrees W during Phase III of GATE. The superposed wave disturbance is initially confined in the lower troposphere. The time integration of the model is carried out to 96 h, during which a tropical storm develops accompanied by an upper level anticyclone.
The genetic sequence of the disturbance system, from a shallow easterly wave into a tropical depression and further into a tropical storm, is described. The minimum surface pressure of the system deepens from 1008.4 to 1002.6 mb at 96 h. The maximum surface wind at 96 h is above 17 m s-1. The relative vorticity at 950 mb intensifies from 43 x 10-6 s-1 at the initial time to 237 x 10-6 s-1 at 96 h. The surface convergence increases from 24 x 10-6 s-1 to 71 x 10-6 s-1 . The processes involved in the above transformation are extensively discussed. Attention is given to the change in the area of rainfall and cloud from a zonal pattern to a cluster-type, the deepening of the cloud within the system, the appearance of horizontal tilt of the trough axis and the time variation of its vertical tilt, the evolution of the vertical motion field, the thickening of the convergence layer around the depression center, the formation of a warm core at 335 mb and its downward extension, the appearance of a cold core at a higher level, etc. The intensification of the vortex and the growth of a warm core are analyzed by examining budgets of vorticity and heat at the tropical depression stage. The vorticity increase at low levels is due to stretching of the vortex. Relative horizontal advection causes a decrease of vorticity in some outer areas. At upper levels, the upward protrusion of positive vorticity from below and relative horizontal advection cause a positive tendency. Both the effect due to horizontal divergence and the twisting up of a horizontal vortex make negative contributions. The net effect at upper levels is to produce a compact positive vorticity area within a large region of negative vorticity. Upper level warming is largely due to the excess of the condensation-convection heating over the cooling effect associated with the upward motion. The appearance of an upper level disturbance in the present model is caused entirely by the forcing from below. Supplemental experiments confirm that, although the diabatic heating effect of radiation plays an important role, the heating due to the condensation of water vapor is essential for the formation of a tropical storm in the present case.
Tuleya, Robert E., and Yoshio Kurihara, 1981: A numerical study on the effects of environmental flow on tropical storm genesis. Monthly Weather Review, 109(12), 2487-2506. Abstract PDF
The role of the environmental wind in tropical storm genesis is studied using a numerical simulation model. The model used is an 11-level, primitive equation model covering a channel domain of 25 degrees span with open lateral boundaries at 5.5 and 30.5 degrees N. A number of experiments were integrated for 96 h in which the initial zonal mean flow was specified differently. The superposed initial wave disturbances were identical in all experiments. The role of the environmental wind in tropical storm genesis is studied using a numerical simulation model. The model used is an 11-level, primitive equation model covering a channel domain of 25 degrees span with open lateral boundaries at 5.5 and 30.5 degrees N.
The dynamic coupling between the upper-level winds and the low-level movement of the disturbance was found to be an important factor in explaining the role of the environmental wind in storm genesis. Another important factor is the impact of the low-level winds on the latent energy supply. This supply is affected by the relative inflow into a disturbance and by the transfer of momentum from aloft into the boundary layer in a large area surrounding the disturbance.
According to the model results, the storm genesis potential is definitely biased toward easterly vertical shear (easterlies increasing with height) of the environmental flow rather than westerly shear when the mean surface flow is easterly, i.e., -5 ms-1. The initial perturbation developed into a vigorous tropical storm when an easterly vertical shear of 15 ms-1 was specified between 150 and 850 mb. In an experiment with a specified westerly vertical wind shear of 15 ms-1, the perturbation failed to develop beyond a weak tropical depression. In a third experiment with no vertical wind shear but with anticyclonic shear aloft, a tropical storm also developed. In analyzing the structure of the disturbances at the early wave stage it was found that the vertical shear modulated the vertical velocity and rainfall patterns relative to the trough axis.
In studies involving the horizontal wind shear of the basic flow, it was found that cyclonic shear at low levels and, to a lesser extent, anticyclonic shear at upper levels are conducive for storm genesis. The experimental results also indicate a significant change of structure of the disturbance between uniform westerly and easterly flows. Under uniform westerly environmental flow, the initial perturbation developed more and its low-level structure became more characteristic of mid-latitude cyclones.
Kurihara, Yoshio, and Robert E Tuleya, 1978: A scheme of dynamic initialization of the boundary layer in a primitive equation model. Monthly Weather Review, 106(1), 114-123. Abstract PDF
A scheme of dynamic initialization of a primitive equation model is proposed with an emphasis on the dynamic adjustment in the boundary layer. The pre-initialization analysis is important since the restorative method is used in the subsequent dynamic initialization. The first phase of dynamic initialization is designed to establish a reasonable boundary layer structure. For this purpose, a time integration of the primitive equations is performed under a strong constraint such that all meteorological fields except momentum in the boundary layer are frozen. Use of an implicit form for the vertical diffusion term is recommended. The second phase is formulated to reduce the high-frequency noise in the final initialized field. Cyclic integration with a selective damping scheme is carried out under a restorative constraint.
The proposed scheme is applied to a case of simple zonal flow and the evolution of the boundary layer flow is shown. The scheme is also tested for a case of mature tropical cyclone. Starting from the wind data in the free atmosphere only, the initial condition of the model is derived. Subsequent time integration of the model compares favorably with the integration in a control experiment.
A GFDL tropical cyclone model was applied to simulate storm landfall. The numerical model is a three-dimensional, primitive equation model and has 11 vertical levels with four in the planetary boundary layer. The horizontal grid spacing is variable with finest resolution being 20 km near the center. This model was used successfully in the past to investigate the development of tropical cyclones over the ocean.
In the present experiments, a simple situation is assumed where a mature tropical cyclone drifts onto flat land. In such a case, the landfall can be simulated by changing the position of the coastline in the computational domain rather than by moving the storm. As the coastline moves with a specified speed, the surface boundary conditions are altered at the shore from those for the ocean to those for the land by increasing the surface roughness length and also by suppressing the evaporation.
Despite the simplicity and idealization of the experiments, the cyclone's filling rates are quite reasonable and a decay sequence is obtained. Notable asymmetries in the wind, moisture and precipitation fields exist relative to the coastline at the time of landfall. Roughness-induced, quasi-steady convergence and divergence zones are observed where onshore and offshore winds encounter the coastline. Spiral bands propagate and exist over the land area. A comparison of the energy and angular momentum budgets between ocean and land surface boundary conditions indicates a simultaneous broadening and weakening of the storm system in the decay process. The latent energy release through condensational processes is initially augmented over land by greater moisture convergence in the planetary boundary layer which counteracts the lack of evaporation from the land surface.
Supplementary experiments indicate that the suppression of evaporation is the most important factor in the decay of a storm upon landfall. When the evaporation is suppressed, the storm eventually weakens whether the surface roughness is increased or not. An increased surface roughness, which causes increased inflow in the boundary layer, has little immediate negative impact on the storm intensity. Indeed, if the supply of latent energy is sufficient, a storm can deepen when encountering an increase in surface roughness. The decay rate in a later period well after landfall is influenced by the rate with which the water vapor of the storm system is depleted in the earlier period immediately after landfall.
Tuleya, Robert E., and Yoshio Kurihara, 1975: The energy and angular momentum budgets of a three-dimensional tropical cyclone model. Journal of the Atmospheric Sciences, 32(2), 287-301. Abstract PDF
Energy and angular momentum budgets are analyzed for a three-dimensional model hurricane described by Kurihara and Tuleya.
Eddies which developed in the model are maintained in the mature stage by energy supply from both mean kinetic and total potential energy. In the evolution of eddies during the early development stage of the storm, the supply from potential energy is more important.
Eddies export latent, internal, kinetic energy and relative angular momentum from the storm core region. They also contribute to the outward transfer of energy through pressure work. However, the mean flow dominates the transport by importing those quantities into the inner area and exporting potential energy.
The energy and angular momentum budgets are primarily controlled by the mean flow, though the role of eddies is not negligible for the budgets of angular momentum, kinetic and latent energy in the inner region. For the maintenance of mean kinetic energy in the inner area, both generation and advection make positive contributions.
The computed transports and budgets are compared with those available for other three-dimensional models as well as with real data analyses made by other investigators.
Kurihara, Yoshio, and Robert E Tuleya, 1974: Comments "On the importance of precision for short-range forecasting and climate simulation". Journal of Applied Meteorology, 13(5), 601-602. PDF
Kurihara, Yoshio, and Robert E Tuleya, 1974: Structure of a tropical cyclone developed in a three-dimensional numerical simulation model. Journal of the Atmospheric Sciences, 31(4), 893-919. Abstract PDF
A three-dimensional, 11-level, primitive equation model has been constructed for a simulation study of tropical cyclones. The model has four levels in the boundary layer and its 70 x 70 variable grid mesh encloses a 4000-km square domain with a 20-km resolution near the center. Details of the model, including the parameterization scheme for the subgrid-scale diffusion and convection processes, are described.
A weak vortex in the conditionally unstable tropical atmosphere is given as the initial state for a numerical integration from which a tropical cyclone develops in the model. During the integration period of one week, the sea surface temperature is fixed at 302K.
The central surface pressure drops to about 940 mb, while a warm moist core is established. The azimuthal component of mean horizontal wind is maximum at about 60 km from the center at all levels. A strong inflow is observed in the boundary layer. At upper levels, a secondary radial-vertical circulation develops in and around the region of negative mean absolute vorticity. In the same region, the azimuthal perturbation of horizontal wind is pronounced. At the mature stage, the domain within 500 km radius is supplied with kinetic energy for asymmetric flow by both barotropic and baroclinic processes. At 60 km radius, the temperature perturbation field is maintained by condensation-convection heating at upper levels and by adiabatic temperature change due to vertical motion at lower levels. An area having an eye-like feature is found off the pressure center.
Structure of spiral bands in the outer region is extensively analyzed. The phase relationship among the pressure, horizontal motion, vertical motion, temperature and moisture fields is discussed. The spiral band behaves like an internal gravity wave. Once the band is formed in an area surrounding the center, it propagates outward apparently without appreciable further supply of energy, as far as the present case is concerned.
