U.S. Dept. of Commerce / NOAA / OAR / ERL / GFDL *Disclaimer  

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In this section, some research highlights are listed that may be of interest to those persons less concerned with the intricate details of GFDL research. Selected are items that may be of special significance or interest to this wider audience.

Many of the items in this section have been ordered according to the current NOAA Strategic Plan Elements:

Advance Short-Term Forecasts and Warnings

Seasonal to Interannual Climate Forecasts

Predict and Assess Decadal to Centennial Changes

While the categorization of scientific research into these three elements based on time-scale may make some sense for certain topics (so long as one accepts a certain overlap at the boundaries), it ignores the fact that much scientific progress has application to phenomena at a wide variety of scales. We have therefore defined a new category which cuts across the time scales represented by the previous elements:

Basic Geophysical Processes

This new category not only avoids an awkward force-fit of certain topics into a particular time-scale, it also highlights the fundamental role that these topics play as building blocks for progress in other research areas.

Note that the categories described above are organized rather differently than the GFDL research project areas presented in the main body of the report. This is but another reflection of the variety and interplay of activities within such a fertile research environment. As an aid in cross-referencing, the number in parentheses following each highlight refers to sections in the main body of the report.

 

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The need for short-term warning and forecast products covers a broad spectrum of environmental events which have lifetimes ranging from several minutes up to a month or so. Some examples of these events are tornadoes, hurricanes, tsunamis, and coastal storms, as well as "spells" of unusual weather (warm, cold, wet, or dry). Benefits of these products can be measured in terms of lives saved, injuries averted, and expenses spared. NOAA's vision for improvement in this area involves operational modernization and restructuring, strengthening of observing and prediction systems, and improved applications and dissemination of products and services.

Efforts at GFDL are centered around the development of numerical models which may be used in the prediction of "short-term" atmospheric and oceanic phenomena. Simulations from these models are studied and compared with observed data to aid in the understanding of the processes which govern the behavior of the various phenomena.

With regard to tropical weather systems, efforts are aimed at the genesis, growth, and decay of tropical storms and hurricanes. In extratropical regions, interest includes the development of severe weather systems, the interaction of medium-scale atmospheric flow with that on larger scales, and the influence of the underlying topographic features. Experimental prediction of regional-scale weather parameters weeks to months in advance is being pursued; included in this context is the study of "ensemble forecasting." With regard to the marine environment, forecasts of coastal conditions on a day-to-day basis can be made by coupling of ocean and atmosphere models. Ocean models are also used to simulate coastal bays and estuaries, the response of coastal zones to transient atmospheric storms, and Gulf Stream meanders and rings.

ACCOMPLISHMENTS FY97

Hourly observations of "water vapor" radiances from geostationary satellites were used in conjunction with an objective pattern-tracking algorithm to trace upper tropospheric water vapor features from time-lapse satellite imagery. A close correlation is observed between upper tropospheric relative humidity changes and displacements in the water vapor pattern, reflecting the strong dependence of relative humidity upon the atmospheric circulation. Reevaporation of cloud condensate is also observed to influence the drying rates of the upper troposphere (2.3.1).

The Coastal Ocean Forecast System (COFS), which provides daily predictions for the U.S. east coast, has been upgraded to include tidal forcing and has been extended into the Gulf of Mexico and the Caribbean Sea (5.3.3).

An extensive multiyear dataset with a near-global coverage of various cloud properties at high temporal and spatial resolutions (3 hours and 30 km, respectively) has been acquired for diagnosis of the mesoscale characteristics of prevailing weather systems in different geographical locations. For selected synoptic cases, superposition of the cloud fields on the concurrent NCEP reanalyses of the wind, pressure, temperature and water vapor distributions reveal the detailed organization of cloud cover associated with midlatitude frontal bands and tropical convective disturbances (6.1.2, 6.2.1).

The GFDL Hurricane Prediction System continued its excellent performance as the official operational model of the National Weather Service during the 1996 hurricane season. The system made 164 forecasts in the Atlantic and 74 forecasts in the Eastern pacific. It was the best performer in the Atlantic beyond 12h among all operational models, showing more than 22% higher skill compared with the next best performer at 24h and beyond (7.1.1).

Results from the extended range track prediction studies indicate that both the GFDL hurricane model and operational global models can provide meaningful operational guidance of four and five day tropical cyclone track, while the GFDL hurricane model is superior in tracking storms for even longer periods (7.2.1).

A coupled model consisting of the GFDL hurricane model and the Princeton Ocean Model was used to investigate the impact of the hurricane-ocean interaction on the intensity changes of four real storms. In all cases, inclusion of the interaction effects in the model produced more realistic changes in storm intensity. In the case of Hurricane Fran, the presence of a cold wake generated by a preceding storm (Edouard) also significantly contributed to the forecast improvement, indicating that accurate initial SST fields are important for intensity forecasts (7.3.1).

A combination of high-resolution numerical and analytical models have been used to estimate the total mountain drag exerted by the earth's terrain down to horizontal scales of 20 km. With some simple assumptions, the analytical model requires only the topographic profile and the low-level buoyancy frequency to produce estimates of total drag. These estimates have been confirmed by fully compressible nonhydrostatic numerical simulations (8.3).

The hydrostatic, terrain-following Zeta model has been generalized to include the anelastic approximation for more realistic atmospheric density profiles. The nonhydrostatic Zeta model now includes a radiative upper boundary condition designed to make the upper boundary transparent to upward propagating internal gravity waves (8.4).

PLANS FY98

A high-resolution satellite dataset of cloud parameters will be analyzed in conjunction with coincident atmospheric circulation fields in order to study the space-time relationships between mesoscale cloud features and the ambient flow pattern. Emphasis will be placed on cloud organization in recurrent weather systems such as midlatitude cyclone waves and tropical convective disturbances.

Developmental work on hurricane vortex specification will continue.

The linear model of total mountain drag will be tested more extensively against numerical simulations using realistic temporal and spatial variations of the basic flow. The feedback between the mesoscale topographic forcing and synoptic-scale disturbances will also be examined.

The development of the zeta limited area models will emphasize the incorporation of moist physics and boundary layer processes into the new, high resolution anelastic hydrostatic and non-hydrostatic models expressed in spherical coordinates.

 

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Seasonal to interannual climate fluctuations have far-reaching consequences for agriculture, fishing, water resources, transportation, energy consumption, and commerce, among others. Short-term climate anomalies which persist from a season to several years affect rainfall distributions, surface temperatures, and atmospheric and oceanic circulation patterns. Reliable climate forecasts may be used to reduce the disruption, economic losses, and human suffering that occur in connection with these anomalies. NOAA's vision for improvement in this area is based on better predictive capability, enhanced observations, greater understanding of climate fluctuations, and assessment of impacts.

The study of seasonal to interannual climate fluctuations at GFDL is based on both theoretical and observational studies. Available observations are analyzed to determine the physical processes governing the behavior of the oceans and atmosphere. Mathematical models are constructed to study, simulate, and predict the coupled ocean-atmosphere, land-surface, sea-ice system.

Simulations based on the numerical models maintained at GFDL, in conjunction with observations, are used to study climate variations on seasonal and longer time scales. Processes under study include large-scale wave disturbances and their role in the general circulation, the effects of boundary conditions such as sea surface temperature and soil moisture, influence of clouds, radiation, and atmospheric convection, and the "teleconnection" of atmospheric anomalies across the global atmosphere. Furthermore, experimental model forecasts are used to evaluate atmospheric predictability and to assess skill in forecasting atmospheric and oceanic climate anomalies, both in general and in connection with the El Nio-Southern Oscillation phenomenon. Also, a more accurate representation of the state of the global ocean is being studied through data assimilation for better initialization of seasonal-interannual forecasts.

ACCOMPLISHMENTS FY97

A simple methodology for converting climate-model runoff to river discharge was developed and applied, facilitating estimates of the statistical characteristics of river-flow extremes (droughts and floods) from climate-model outputs (1.2.1).

An ensemble forecast methodology was developed and applied to examine the value of soil-moisture information for monthly to seasonal climate forecasting. For midlatitude summer conditions, soil moisture itself is predictable on a time scale of one month. However, associated predictability of near-surface air temperature is weak (but significant), and precipitation forecasts appear independent of initial soil moisture (1.2.3).

Development of a system of flexible tools for constructing models of the coupled ocean-atmosphere system has progressed to the point where a number of different model configurations have been tested. Two atmospheric dynamical cores, a large number of atmospheric physical parameterizations, and preliminary versions of tools for coupling model components have been completed and tested (4.1).

A major restructuring of the GFDL ocean model physical parameterizations has been undertaken, resulting in significant improvements in the abilities of the ocean model for seasonal/interannual prediction (4.2.2).

Improved versions of the seasonal/interannual prediction coupled model components have been developed and are being used to produce a large ensemble of atmosphere-only integrations and coupled model forecasts with completely consistent models. These model results should give new insight into the predictability of the coupled ocean-atmosphere system (4.3.1).

Coupled dynamics have been found to play a crucial role in the maintenance of tropical intraseasonal oscillations in numerical models, indicating that fully coupled models may be essential for prediction of these phenomena (4.3.4).

The interannual variability of tropical storm intensity, frequency, and tracks have been demonstrated to be potentially predictable in ensembles of atmosphere-only simulations (4.3.6).

Data assimilation experiments have indicated the need for accurate measurements of salinity in the Pacific warm pool in order to accurately assess the state of the tropical Pacific ocean (4.4.1).

A fully nonlinear data assimilation technique has been developed to provide initial conditions for ensemble forecasts in low-order dynamical systems. The method has also been used to produce ensemble initial conditions for forecasts in a simple global atmospheric model (4.4.2).

Experimentation with a medium-resolution climate model (R30L14) yields more realistic atmospheric responses to El Nio-related sea surface temperature anomalies in the tropical Pacific as compared to lower-resolution (R15L9) simulations. Improvements are seen in the intensity of tropical precipitation anomalies, and in the amplitudes of quasi-stationary wavetrains and synoptic-scale disturbances in midlatitudes (6.3.1, 6.3.2).

A recently completed study of cyclone wave activity confirms its importance in shaping the large-scale quasi-stationary circulation. It also provides a clear picture of the month-to-month evolution of the storm track during the transition from fall to winter. Consistent with these results, it also describes the relationship between interannual variability and the ENSO cycle. This may have important implications for the trajectory of winter storms entering the North American continent and improving forecast skill during these events (8.1).

PLANS FY98

New land surface models for seasonal/interannual prediction will be developed as completely independent components of a coupled prediction system. A set of flexible coupling tools that can combine ocean, atmosphere, sea ice, and land surface model components will be used to generate coupled models suitable for climate and seasonal/interannual prediction purposes. Models based on the new flexible/modular modeling system will become the primary research tools for seasonal/interannual prediction.

The prognostic cloud water scheme will be enhanced and evaluated using the flexible/modular modeling system. The interaction of these prognostic clouds with other atmospheric parameterizations will be explored.

The effects of subgrid-scale parameterizations on the upper ocean thermodynamic balances will be a focus of ocean model development.

Runs of the coupled model ensemble prediction experiment, including both atmosphere-only and coupled model integrations, will be completed and analyzed to evaluate potential predictability, as well as prediction skill for seasonal forecasts. An extended integration of the coupled model will be used to assess the interannual variability of this model.

The impacts of cloud forcing on coupled dynamics in both the tropics and extratropics will be examined and used to evaluate improved parameterizations for seasonal prediction.

The ocean data assimilation system will be used to quantify systematic errors in the parameterizations of the ocean model. Results from this study will be used to guide ocean model development in an attempt to reduce any systematic error.

The nonlinear filter for data assimilation will be applied to realistic forecast models. Heuristic extensions to this method will be examined to determine if it could be used efficiently in an operational setting.

A new suite of multidecadal experiments will be performed using a medium-resolution climate model (R30L14) to study the interactions of El Nio episodes in the tropical Pacific with the global atmospheric circulation and the surface temperatures of the other ocean basins. Diagnosis of these model runs will improve understanding of the variability of the observed coupled ocean-atmosphere system in recent decades.

The ongoing investigation into the interaction between cyclone wave activity and the large-scale quasi-stationary circulation will be extended to the Southern Hemisphere. Variations in cyclone development over the entrance of the Atlantic-Indian Ocean storm track during the warm and cold phases of the ENSO cycle will be examined. Extended numerical simulations of storm tracks will focus on the mechanisms responsible for the growth, maintenance, and dissipation of the quasi-stationary features in idealized storm tracks.

 

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PREDICT AND ASSESS DECADAL TO CENTENNIAL CHANGES

Events such as the Sahel drought, the dust bowls in the Midwest, the Little Ice Age, stratospheric ozone depletion, and global warming may define eras in history. Events such as these have lifetimes of decades to centuries and their causes may be either natural or anthropogenic. An ability to predict such changes and to assess the causes is essential in long-range policy making. Adapting to these changes and reducing the effects of human activities will require enhanced predictive capability. NOAA's vision for improvement in this area is based on a commitment to research in climate and air quality, as well as to insure long-term climate and chemical records.

The related research efforts at GFDL require judicious combinations of theoretical models and specialized observations. The modeling efforts draw on principles from the atmospheric, oceanic, chemical, and biological sciences. One area of focus is long-term climate variability and secular change associated with the atmosphere and oceans. This area encompasses a number of topics, including the effects of changes in the concentration of atmospheric gases such as carbon dioxide, the simulation of past climates, and the variability of the oceanic thermohaline circulation. Another area of focus is the formation, transport, and chemistry of atmospheric trace constituents. This area addresses problems such as: the transport of quasi-conservative trace gases; the biogeochemistry of climatically significant long-lived trace gases; the transport, sources, and sinks of aerosols; the chemistry of ozone and its regulative trace species; the effects of clouds and aerosols on chemically important trace gases; and the impact of anthropogenic chlorofluorocarbons on stratospheric ozone amounts. Yet another area of focus relates to the modeling of the marine environment. It includes the dispersion of geochemical tracers in the world oceans, the oceanic carbon cycle and trace metal geochemistry, and ecosystem structures.

ACCOMPLISHMENTS FY97

Experiments using estimates of greenhouse gas and sulfate aerosol forcing have shown that the GFDL global coupled ocean-atmosphere climate model can successfully simulate the global-mean surface air temperature over the past 100 years. These experiments also show that the simulated weakening of the thermohaline circulation and drying over the midlatitude Northern Hemisphere continents in summer differ significantly from the control values early in the twenty-first century (1.1.1).

The contributions due to natural variability and anthropogenic forcing of the tropical Pacific warming trends observed in recent decades have been investigated. According to model simulations using the coupled ocean-atmosphere model, both natural variability and anthropogenic forcing have probably contributed to the recent tropical Pacific warming, although their relative contributions are uncertain (1.1.2).

The observed upward trend in the cold ocean-warm land pattern over the last two decades was unusually large when compared to variations in this pattern simulated by the GFDL coupled atmosphere-ocean model. A similar upward trend occurs in a coupled model simulation forced by past and future increases in CO2 and sulfate aerosol forcing (1.1.3).

The weakening of the thermohaline circulation (THC) associated with global warming depends critically upon the rate of CO2 growth in the atmosphere. The slower the rate of CO2 growth, the larger the weakening of the THC that occurs by the time of CO2 doubling. However, the amount of weakening which occurs after the CO2 doubles and stops increasing is smaller with the slower rates of CO2 growth (1.1.4).

An improved version of the medium resolution coupled model has been integrated for more than 80 years with very little drift (less than 0.1C). The successful development and integration of this model represents a critical step in addressing a wide range of climate research issues, including both the response of the climate system to various forms of radiative forcing and natural climate variability on a wide range of time scales. The higher resolution of both the atmospheric and oceanic components of this model (relative to previous versions) is expected to substantially improve the simulation of many phenomena, including ENSO, the Asian monsoon, and decadal variability (1.1.7).

A statistically significant (95% level) upward trend in river flow was identified in 2 of 9 major world rivers examined. However, anthropogenic influences (including greenhouse warming) in the river basins appear insufficient to explain the magnitude of these trends (1.2.2).

Regional variations in the magnitude of tropical atmospheric cooling appear in an atmosphere-mixed layer ocean simulation of the climate of the last glacial maximum. Similar patterns, but of opposite sign, appear when atmospheric CO2 is doubled. Evidence suggests that the regional variations in tropical temperature change in both experiments may be associated with interhemispheric asymmetries in high latitude temperature change (1.3).

Models of the Jovian circulations that take into account the vertical wind data provided by the recent "Galileo" probe have been used to simulate successfully the genesis and maintenance of the three major sets of vortices that occur in Jupiter's atmosphere, namely, the Great Red Spot, the Large Ovals, and the Small Ovals (1.5).

Using aerosol distributions simulated by chemistry-transport models, the direct radiative forcing due to an external mixture of anthropogenic sulfate and black carbon aerosols is estimated; there is a significant geographical contribution by both species. Tests indicate that, while the forcing per unit mass decreases with increasing altitude for sulfate aerosols (owing to the effects of relative humidity), that for hydrophobic black carbon aerosols increases with height because the aerosols become located above low clouds (2.4.3).

Transient integrations with the coupled atmosphere-ocean model indicate that the sum of the individual climatic effects of increases in greenhouse gases and tropospheric sulfate aerosols over the past century is similar to the response for their combined forcings. This indicates that the modeled climate response to greenhouse gas and tropospheric aerosol forcing is linearly additive (2.4.4).

A study of observed and modeled global-mean middle and upper stratospheric temperatures reveals that the model underestimates the observations slightly. This bias is likely due to underestimates in the solar radiative heating, with uncertainties in the satellite-derived temperatures also a contributing factor (2.4.6).

With the addition of a new emission inventory for Asia to the current global time-dependent fossil fuel source, the development of a realistic global time-dependent lightning source, and the continuing refinements to the global time-dependent soil-biogenic source, the GFDL GCTM now operates with the most advanced "state-of-the-art" NOx emissions sources available to the global modeling community (3.1.4).

A long integration performed using the SKYHI model with an imposed quasi-biennial oscillation (QBO) in the tropical stratosphere was analyzed. One important result was an apparent modulation of the stratospheric stationary wave field at low latitudes by the mean winds, with significant planetary wave penetration across the equator when the mean winds are westerly. This means that, contrary to conventional wisdom, the QBO may be expected to have a significant non-zonally-symmetric component. Some recent satellite wind observations in the tropical stratosphere support this conclusion. These findings suggest that the present observational radiosonde network is not adequate for monitoring the QBO near the equator (3.2.7).

The decadal variability and predictability of the ocean-atmosphere system in the North Atlantic region has been quantified using ensembles of coupled model integrations. Details of the thermohaline circulation are predictable for approximately one decade (4.3.9).

Model development work with both a traditional level-coordinate model and an isopycnal-coordinate model is showing that the ocean's thermohaline circulation can be sustained with very little vertical mixing. This is a significant departure from conventional oceanographic theory which has called upon the downward mixing of heat to initiate the thermohaline circulation by warming the ocean's interior. This research looks to the Antarctic Circumpolar Current to replace vertical mixing in the conventional theory (5.1.1, 5.1.2).

A new representation of frictional bottom boundary layers has been incorporated into the Modular Ocean Model (MOM). This treatment should help MOM represent the downslope flow of thin layers of dense shelf or overflow waters as observed in the formation of bottom water around Antarctica or in the overflow of Norwegian/Greenland Sea water through the Denmark Straits west of Iceland (5.1.3).

A study of the Labrador Sea has examined the sensitivity of the basin's circulation and deep-water formation to episodic inputs of fresh water from the Arctic. The study has been carried out using a series of models with different resolutions ranging from 1 to 1/16. The impact of freshwater inputs is found to be vary dependent on model resolution (5.1.5).

Inverse modeling of atmospheric CO2 concentrations for the decade 1981-1992 suggests that 1 to 2 PgC is being taken up per year by terrestrial vegetation in temperate North America and boreal Asia. These findings are consistent with ideas that forest regrowth, a lengthening of the growing season, and stimulation of forest productivity due to nitrogen deposition and higher CO2 levels have made the terrestrial biosphere a major sink for fossil fuel CO2. The inverse model solution is also consistent with a large oceanic sink (~2 pgC/yr) (3.1.10).

PLANS FY98

Multicentury integrations of the medium-resolution (R30) coupled model will be examined with regard to both internally generated tropical Pacific variability and the tropical Pacific regional response to anthropogenic forcing (greenhouse gases plus aerosols). This should provide more reliable indications of the possible contributions of natural climate variability and anthropogenic forcing to the recent observed trends in the Pacific SSTs.

A parameterization of land-surface water and energy balance will be run in stand-alone mode with reconstructed historical forcing over a global domain. Methods for parameter assignment on the basis of available global land datasets will be explored. Performance will eventually be evaluated using available observations of river discharge and snow cover.

The sensitivity of a climate model to inclusion of non-water-stressed stomatal resistance will be explored, and atmospheric feedbacks will be evaluated. In connection with this study, an estimate of climate sensitivity to stomatal resistance doubling will also be made, providing a partial basis for assessing the direct impact of atmospheric carbon-dioxide increase on the global water cycle.

A detailed analysis of simulated glacial climate over the climatically-sensitive North Atlantic regions will be undertaken.

Climate change in the Southern Hemisphere in a variety of global warming simulations will be used to assess various theories of general circulation dynamics in light of ongoing global change studies.

Calculations will be made in an effort to obtain a more complete simulation of the various jets and vortices in models for Jupiter's circulation.

A new release of the Modular Ocean Model (MOM 3) is planned which will include a new bottom boundary layer parameterization, new free surface options, a new isoneutral diffusion formulation, and options for enhancing performance on parallel computer architectures.

A new coupled ocean-atmosphere model will be set up to take advantage of new ocean physics options in MOM 3. New features in the model will include an explicit representation of freshwater inputs to the ocean, an explicit representation of flow through Bering Strait and the Arctic Ocean, and a bottom boundary layer parameterization to better represent the downslope flow of dense water from shelves and sills.

The study of episodic freshwater inputs to the Labrador Sea will be completed with an examination of the role of resolved eddies in a 1/16 simulation.

 

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A number of the research topics at GFDL cut across the various time scales characteristic of each of the foregoing sections. Progress on these topics impacts many other research areas which depend critically on the successful representation of numerous lower-level processes which are common to problems at all scales. Topics which fall into this category include hydrological processes, radiative transfer (including the effects of aerosols and clouds), cloud prediction/specification, "teleconnection" of atmospheric anomalies across the global atmosphere, satellite data interpretation, transport processes, gravity wave effects and parameterization, model resolution effects, and many other model enhancement efforts. As these processes become better understood and more accurately represented, benefits will accrue to a multitude of other research efforts.

ACCOMPLISHMENTS FY97

Linear stochastic models have proven surprisingly capable of simulating the eddy statistics in the midlatitude storm tracks, given the larger-scale background flow. In particular, they have been shown to be capable of simulating the counter-intuitive minimum in the strength of the Pacific storm track in midwinter. This approach promises to provide improved understanding of the changes in storm tracks during ENSO events and their response to climate change (1.4).

"Benchmark" computations for water clouds reveal that the solar flux reaching the surface beneath overcast skies is independent of the altitude at which the cloud is located, in contrast to the flux absorbed in the atmosphere which is critically dependent on the cloud location (2.1.2).

The transport and transformation of sulfur in convective systems has been studied by combining a three-dimensional cloud-system model and a process-level model of the transformation of sulfur dioxide to sulfate. The combined model will provide a valuable tool for examining the effects of sulfates on the radiative properties of clouds associated with convective systems and on the vertical transport by these cloud systems (2.2.2).

The first three-dimensional simulations of horizontally-homogeneous radiative-convective equilibrium with resolved moist convection and incorporating full cloud-radiative interaction are nearing completion. The model is being used to study moist convective organization, the maintenance of CAPE (convective available potential energy) in the tropics, and cloud feedback mechanisms (2.2.4).

Systematic discrepancies between satellite observations and GCM simulations of the clear-sky absorbed solar radiation have been noted. The discrepancies are most likely related to one or more of the following: 1) deficiencies in our current understanding of water vapor absorption in the solar spectrum; 2) the absence of tropospheric aerosols in the model; 3) the absence of a wind-speed dependent ocean surface albedo in the model (2.3.2).

A cloud prediction scheme, along with improved land surface albedos and new solar and longwave radiation algorithms, has been incorporated into the SKYHI GCM. The first integrations with the 3 resolution SKYHI indicate that the cold, dry tropospheric bias present in earlier versions of the model is essentially removed. In particular, the tropical upper troposphere compares more reasonably with observations than the earlier simulations in which clouds were held fixed in space and time (2.4.1).

The recently developed four-way interpolation tables, with their dependence on NOx, O3, CO, and H2O, provide essentially real-time on-line chemistry in the GFDL GCTM, while requiring much less computer time than existing in situ atmospheric chemistry modules (3.1.2).

Simulations of global tropospheric ozone by the GFDL GCTM, driven by previous simulations of NOx distributions, have been developed which quantitatively capture the observed latitudinal, vertical, and seasonal behavior. Detailed analyses quantify the overall impact of human activity on tropospheric ozone and the relative importance of transport from the stratosphere, chemical production or destruction in the clean background troposphere, and chemical production in the polluted boundary layer (3.1.6).

An integration for the Northern Hemisphere summer period using a version of the SKYHI model at one-third degree latitude resolution was performed. This model was found to produce a simulation of the Southern Hemisphere polar night jet that is very close to observations (3.2.4).

The one-third degree resolution version of SKYHI was found to produce some very intense West Pacific tropical cyclones. This appears to be the most realistic simulation of tropical storm intensity achieved by any global climate model to date (3.2.4).

Westward travelling waves with periods of approximately one month have been detected in high-latitude zones of both the observed and model-simulated atmospheres. The evolution of these phenomena resembles that of blocking events. These high-amplitude fluctuations contribute significantly to the local atmospheric variability on weekly and monthly time scales. The principal dynamical processes operating in these waves have been studied using composite analysis of the prominent episodes (6.4.1).

Numerical experiments were conducted to study how hurricane intensities might be affected by global climate change due to increases in atmospheric CO2. The GFDL hurricane model was initialized from simulations made by the GFDL climate model for control and enhanced-CO2 conditions. Also, experiments using idealized initial conditions were carried out to clarify the mechanism and sensitivity. Results from 51 cases indicate an increase of the maximum wind speed of about 5 m/s for the high CO2 climate (7.3.2).

PLANS FY98

Analyses of trends and natural variability of runoff and river discharge will continue. The ability of the climate model to reproduce natural variability of river discharge will be evaluated. Recently completed climate-change experiments will be used to estimate the effects of global climate change on river discharge and the detectability of those effects. The physical processes leading to runoff changes will also be identified and analyzed.

The use of linear stochastic modeling for studies of eddy statistics and wave-mean flow interactions will continue. The possibility of using such a theory to improve existing models of the stationary eddies in the troposphere, and the deviations of the flow from zonal symmetry, will be examined.

"Benchmark" radiative transfer computations and available observations will be used to diagnose the absorbed solar fluxes in atmosphere containing water and ice clouds, and the solar flux reaching the surface. Effects due to the new radiative parameterizations on the biases in the GCM simulations will be analyzed.

The sensitivity of climate to changes in the concentrations of well-mixed greenhouse gases, ozone, tropospheric and stratospheric aerosols, and to changes in the physical properties of clouds, will be investigated using GCMs. Explicit simulation of the liquid and the solid phases of H2O in GCMs, and studies of aerosol-cloud-radiation interactions in both the general circulation and LAN models will be pursued.

Differences between the simulated and observed components of the hydrologic cycle and radiative energy budgets will be analyzed and their causes determined. Comparisons with satellite data will continue to aid the evaluation and improvement of the GCM simulations.

Satellite data will be used to understand the variations in the hydrologic and radiative parameters on different spatial and temporal scales. The pattern-tracking algorithm based on satellite data will be used to examine the relationship between upper tropospheric water vapor and tropical circulation at a variety of time scales.

A parameterization for deep cumulus convection will be incorporated in the SKYHI GCM. This parameterization has been designed to interact with the upper-tropospheric cloud field by treating cumulus-scale vertical velocities and microphysics in a consistent manner. This parameterization also forces the large-scale fields of temperature and water vapor over a greater vertical extent than SKYHI's current convective parameterization.

The three-dimensional simulations of horizontally homogeneous radiative-convective equilibrium will be analyzed in detail. Experimentation will begin with inhomogeneous lower boundary conditions aimed at simulating an idealized Walker cell in a small domain.

Having separated tropospheric O3 into three tracers (that supplied by the stratosphere, produced from transported precursors in the free troposphere, and produced in the polluted boundary layer), efforts will focus on quantifying the relative contributions of transport from the stratosphere and chemical production in the troposphere and further divide the chemical contribution into direct transport of O3 and the in situ chemical production of O3.

Regional studies employing analysis tools developed over the last 10 years will focus on the North Atlantic (in association with AEROCE and NARE observations), the eastern South Pacific off the west coast of South America, the Indian Ocean (in association with the INDOEX campaign), and the South Pacific Ocean (in association with Samoa and PEM-tropics observations). Both seasonal behavior and major synoptic transport events will be examined.

Analysis of the tropical cyclones in the very high horizontal resolution version of SKYHI will continue. The analysis will focus on both the ability of the model to simulate realistic tropical storm structure and climatology, and the role of tropical cyclones in generating stratospheric gravity waves.

The limited-area model simulations of stratospheric gravity waves forced by tropospheric moist convection will continue. The relation of gravity wave fluxes to the intensity, suddenness, and mesoscale organization of the convection will be studied.

Studies of the impact of climate change on hurricane intensity will continue. Studies of the hurricane-ocean interaction will also continue in the course of a semi-operational test of the coupled model as well as in the context of the climate impact problems.


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