Eddy diffusion in the lower thermosphere.
Atmospheric circulation and eddy diffusion based on thermal structure, oxygen transport continuity and sodium trail spreading in thermosphere
ATMOSPHERIC CIRCULATION↗
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Stationary Planetary Waves in Northern Late Winter: MRO/MARCI Observations and Mars Climate Model Simulations
Mars reveals similar, yet also rather different, atmospheric circulation patterns compared to those on Earth. In both atmospheres, solar differential heating drives global Hadley circulation cells. However during solstice on Mars, its Hadley cells are hemispherically asymmetric: an intense, deep, cross-hemisphere single cell dominates with rising motion in the summer hemisphere and sinking motion in the winter hemisphere. Both planets also exhibit thermally indirect (i.e., eddy-driven) Ferrel circulation cells in middle and high latitudes. In addition, Earth and Mars exhibit distinctive large-scale orography and, in a broadly defined context, continentality. For Mars’ northern midlatitudes, Tharsis in the western hemisphere, and Arabia Terra and Elysium in the eastern hemisphere, are the primary large-scale topographic features. In the southern-midlatitudes, Tharsis and Argyre in the western hemisphere, and Hellas in the eastern hemisphere are the key topographic features which can influence large-scale circulation patterns. Such underlying orographic complexes not only cause significant latitudinal excursions of the seasonal mean westerly circumnavigating polar vortex but also significantly modulate the intensity and preferred geographic regions of traveling baroclinic weather systems.
Hollingsworth, J. L.↗
Unprecedented Atmospheric Conditions (1948–2019) Drive the 2019 Exceptional Melting Season Over the Greenland Ice Sheet
Understanding the role of atmospheric circulation anomalies on the surface mass balance of the Greenland ice sheet (GrIS) is fundamental for improving estimates of its current and future contributions to sea level rise. Here, we show, using a combination of remote sensing observations, regional climate model outputs, reanalysis data, and artificial neural networks, that unprecedented atmospheric conditions (1948–2019) occurring in the summer of 2019 over Greenland promoted new record or close-to-record values of surface mass balance (SMB), runoff, and snowfall. Specifically, runoff in 2019 ranked second within the 1948–2019 period (after 2012) and first in terms of surface mass balance negative anomaly for the hydrological year 1 September 2018–31 August 2019. The summer of 2019 was characterized by an exceptional persistence of anticyclonic conditions that, in conjunction with low albedo associated with reduced snowfall in summer, enhanced the melt–albedo feedback by promoting the absorption of solar radiation and favored advection of warm, moist air along the western portion of the ice sheet towards the north, where the surface melt has been the highest since 1948. The analysis of the frequency of daily 500 hPa geopotential heights obtained from artificial neural networks shows that the total number of days with the five most frequent atmospheric patterns that characterized the summer of 2019 was 5 standard deviations above the 1981–2010 mean, confirming the exceptional nature of the 2019 season over Greenland.
Greenland ice sheet↗
Analysis of Small-Scale Atmospheric Gravity Waves Using UARS MLS Radiance Measurements
Gravity waves play an important role in determining atmospheric circulation and small-scale mixing. Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) 63-GHz radiances can be used to calculate small-scale wave variances at 30-80 km altitudes. The major results from this new data set are summarized in the following: (1) MLS radiance fluctuations are contributed mostly by gravity waves of small (about 100 km) horizontal and large (>10 km) vertical scales. (2) MLS observations show that variance enhancements are strongly correlated with the stratospheric polar vortices, tropospheric deep convection zones, and surface topography. (3) As expected for gravity wave propagation, the normalized wave variances grow exponentially with height in the stratosphere but saturate in the mesosphere. (4) The long-term variations of the wave variance are dominated by an annual cycle in the stratosphere and a semiannual cycle in the mesosphere. (5) Separate analyses of the ascending and descending measurements show that the variances are sensitive to wave propagation directions. The subtropical variances, which are associated with deep convection, are likely caused by the gravity waves that propagate upward and eastward in the westward background wind. Additional information contained in the original.
Wu, Dong L.↗
The Cloud Feedback Model Intercomparison Project (CFMIP) contribution to CMIP6.
The primary objective of CFMIP is to inform future assessments of cloud feedbacks through improved understanding of cloud-climate feedback mechanisms and better evaluation of cloud processes and cloud feedbacks in climate models. However, the CFMIP approach is also increasingly being used to understand other aspects of climate change, and so a second objective has now been introduced, to improve understanding of circulation, regional-scale precipitation, and non-linear changes. CFMIP is supporting ongoing model inter-comparison activities by coordinating a hierarchy of targeted experiments for CMIP6, along with a set of cloud-related output diagnostics. CFMIP contributes primarily to addressing the CMIP6 questions 'How does the Earth system respond to forcing?' and 'What are the origins and consequences of systematic model biases?' and supports the activities of the WCRP Grand Challenge on Clouds, Circulation and Climate Sensitivity. A compact set of Tier 1 experiments is proposed for CMIP6 to address this question: (1) what are the physical mechanisms underlying the range of cloud feedbacks and cloud adjustments predicted by climate models, and which models have the most credible cloud feedbacks? Additional Tier 2 experiments are proposed to address the following questions. (2) Are cloud feedbacks consistent for climate cooling and warming, and if not, why? (3) How do cloud-radiative effects impact the structure, the strength and the variability of the general atmospheric circulation in present and future climates? (4) How do responses in the climate system due to changes in solar forcing differ from changes due to CO2, and is the response sensitive to the sign of the forcing? (5) To what extent is regional climate change per CO2 doubling state-dependent (non-linear), and why? (6) Are climate feedbacks during the 20th century different to those acting on long-term climate change and climate sensitivity? (7) How do regional climate responses (e.g. in precipitation) and their uncertainties in coupled models arise from the combination of different aspects of CO2 forcing and sea surface warming? CFMIP also proposes a number of additional model outputs in the CMIP DECK, CMIP6 Historical and CMIP6 CFMIP experiments, including COSP simulator outputs and process diagnostics to address the following questions. 1. How well do clouds and other relevant variables simulated by models agree with observations? 2. What physical processes and mechanisms are important for a credible simulation of clouds, cloud feedbacks and cloud adjustments in climate models? 3. Which models have the most credible representations of processes relevant to the simulation of clouds? 4. How do clouds and their changes interact with other elements of the climate system?
carbon dioxide↗
Simulations of the general circulation of the Martian atmosphere. I - Polar processes
Numerical simulations of the Martian atmosphere general circulation are carried out for 50 simulated days, using a three-dimensional model, based on the primitive equations of meteorology, which incorporated the radiative effects of atmospheric dust on solar and thermal radiation. A large number of numerical experiments were conducted for alternative choices of seasonal date and dust optical depth. It was found that, as the dust content of the winter polar region increased, the rate of atmospheric CO2 condensation increased sharply. It is shown that the strong seasonal variation in the atmospheric dust content observed might cause a number of hemispheric asymmetries. These asymmetries include the greater prevalence of polar hoods in the northern polar region during winter, the lower albedo of the northern polar cap during spring, and the total dissipation of the northern CO2 ice cap during the warmer seasons.
Pollack, James B.↗
Midlatitude Cloud Shifts, Their Primary Link to the Hadley Cell, and Their Diverse Radiative Effects
We investigate the interannual relationship among clouds, their radiative effects, and two key indices of the atmospheric circulation: the latitudinal positions of the Hadley cell edge and the midlatitude jet. From reanalysis data and satellite observations, we find a clear and consistent relationship between the width of the Hadley cell and the high cloud field, statistically significant in nearly all regions and seasons. In contrast, shifts of the midlatitude jet correlate significantly with high cloud shifts only in the North Atlantic region during the winter season. While in that region and season poleward high cloud shifts are associated with shortwave radiative warming, over the Southern Oceans during all seasons they are associated with shortwave radiative cooling. Finally, a trend analysis reveals that poleward high cloud shifts observed over the 1983-2009 period are more likely related to Hadley cell expansion, rather than poleward shifts of the midlatitude jets.
Satellite observatioin↗
Thermal radiation effect on the general circulation of the Venus atmosphere
Grey gas approximation of thermal radiation effect on Venus atmospheric model circulation
Yeh, T. T. J.↗
Micro-Pressure Sensors for Future Mars Missions
The joint research interchange effort was directed at the following principal areas: u further development of NASA-Ames' Mars Micro-meteorology mission concept as a viable NASA space mission especially with regard to the science and instrument specifications u interaction with the flight team from NASA's New Millennium 'Deep-Space 2' (DS-2) mission with regard to selection and design of micro-pressure sensors for Mars u further development of micro-pressure sensors suitable for Mars The research work undertaken in the course of the Joint Research Interchange should be placed in the context of an ongoing planetary exploration objective to characterize the climate system on Mars. In particular, a network of small probes globally-distributed on the surface of the planet has often been cited as the only way to address this particular science goal. A team from NASA Ames has proposed such a mission called the Micrometeorology mission, or 'Micro-met' for short. Surface pressure data are all that are required, in principle, to calculate the Martian atmospheric circulation, provided that simultaneous orbital measurements of the atmosphere are also obtained. Consequently, in the proposed Micro-met mission a large number of landers would measure barometric pressure at various locations around Mars, each equipped with a micro-pressure sensor. Much of the time on the JRI was therefore spent working with the engineers and scientists concerned with Micro-met to develop this particular mission concept into a more realistic proposition.
Catling, David C.↗
Choosing Meteorological Input for the Global Modeling Initiative Assessment of High Speed Aircraft
The Global Modeling Initiative (GMI) science team is developing a three dimensional chemistry and transport model (CTM) to be used in assessment of the atmospheric effects of aviation. Requirements are that this model be documented, be validated against observations, use a realistic atmospheric circulation, and contain numerical transport and photochemical modules representing atmospheric processes. The model must also retain computational efficiency to be tractable to use for multiple scenarios and sensitivity studies. To meet these requirements, a facility model concept was developed in which the different components of the CTM are evaluated separately. The first use of the GMI model will be to evaluate the impact of the exhaust of supersonic aircraft on the stratosphere. The assessment calculations will depend strongly on the wind and temperature fields used by the CTM. Three meteorological data sets for the stratosphere are available to GMI: the National Center for Atmospheric Research Community Climate Model (CCM2), the Goddard Earth Observing System Data Assimilation System (GEOS DAS), and the Goddard Institute for Space Studies general circulation model (GISS). Objective criteria were established by the GMI team to identify the data set which provides the best representation of the stratosphere. Simulations of gases with simple chemical control were chosen to test various aspects of model transport. The three meteorological data sets were evaluated and graded based on their ability to simulate these aspects of stratospheric measurements. This paper describes the criteria used in grading the meteorological fields. The meteorological data set which has the highest score and therefore was selected for GMI is CCM2. This type of objective model evaluation establishes a physical basis for interpretation of differences between models and observations. Further, the method provides a quantitative basis for defining model errors, for discriminating between different models, and for ready re-evaluation of improved models. These in turn will lead to a higher level of confidence in assessment calculations.
Douglas, A. R.↗
Choosing Meteorological Input for the Global Modeling Initiative Assessment of High Speed Aircraft
The Global Modeling Initiative (GMI) science team is developing a three dimensional chemistry and transport model (CTM) to be used in assessment of the atmospheric effects of aviation. Requirements are that this model be documented be validated against observations, use a realistic atmospheric circulation, and contain numerical transport and photochemical modules representing atmospheric processes. The model must also retain computational efficiency to be tractable to use for multiple scenarios and sensitivity studies. To meet these requirements, a facility model concept was developed in which the different components of the CTM are evaluated separately. The first use of the GMI model will be to evaluate the impact of the exhaust of supersonic aircraft on the stratosphere. The assessment calculations will depend strongly on the wind and temperature fields used by the CTM. Three meteorological data sets for the stratosphere are available to GMI: the National Center for Atmospheric Research Community Climate Model (CCM2), the Goddard Earth Observing System Data Assimilation System (GEOS DAS), and the Goddard Institute for Space Studies general circulation model (GISS). Objective criteria were established by the GMI team to identify the data set which provides the best representation of the stratosphere. Simulations of gases with simple chemical control were chosen to test various aspects of model transport. The three meteorological data sets were evaluated and graded based on their ability to simulate these aspects of stratospheric measurements. This paper describes the criteria used in grading the meteorological fields. The meteorological data set which has the highest score and therefore was selected for GMI is CCM2. This type of objective model evaluation establishes a physical basis for interpretation of differences between models and observations. Further, the method provides a quantitative basis for defining model errors, for discriminating between different models, and for ready re-evaluation of improved models. These in turn will lead to a higher level of confidence in assessment calculations.
Douglass, A. R.↗
On the Regulation of the Pacific Warm Pool Temperature
In the tropical western Pacific, regions of the highest sea surface temperature (SST) and the largest cloud cover are found to have the largest surface heating, primarily due to the weak evaporative cooling associated with weak winds. This situation is in variance with the suggestions that the temperature in the Pacific warm pool is regulated either by the reduced solar heating due to an enhanced cloudiness or by the enhanced evaporative cooling due to an elevated SST. It is clear that an enhanced surface heating in an enhanced convection region is not sustainable and must be interrupted by variations in large-scale atmospheric circulation. As the deep convective regions shift away from regions of high SST due primarily to seasonal variation and secondarily to interannual variation of the large-scale atmospheric and oceanic circulation, both trade wind and evaporative cooling in the high SST region increase, leading to a reduction in SST. We conclude that the evaporative cooling associated with the seasonal and interannual variations of trade winds in the primary factor that prevent the warm pool SST from increasing to a value much higher than what is observed.
Chou, Ming-Dah↗
Connections Between Upper Tropospheric and Lower Stratospheric Circulation Responses to Increased Co 2
There are a myriad of ways atmospheric circulation responds to increased CO 2 . In the troposphere, the region of the tropical upwelling narrows, the Hadley Cells expand, and the upper level subtropical zonal winds that comprise the subtropical jet strengthen. In the stratosphere, the tropical upwelling narrows and strengthens, enhancing the Brewer-Dobson Circulation. Despite the robustness of these projections, dynamical coupling between the features remains unclear. In this study, we analyze output from the NASA Goddard Institute for Space Studies (GISS) ModelE coupled climate model to examine any connection between the upper tropospheric and lower stratospheric circulation by considering the features’ seasonality, hemispheric asymmetry, scaling, and transient response to a broad range of CO 2 forcings. We find that a narrowing and strengthening of upper tropospheric upwelling occurs with a strengthening of the subtropical jet. There is also a narrowing and strengthening of lower stratospheric upwelling that is related to an equatorward shift in critical latitude for wave breaking and the associated strengthening of the subtropical lower stratosphere’s zonal winds. However, the stratospheric responses display different seasonal, hemispheric, and transient patterns than those in the troposphere, indicating independent circulation changes between the two domains.
Molly E. Menzel↗
The Mars Dust Cycle: Investigating the Effects of Radiatively Active Water Ice Clouds on Surface Stresses and Dust Lifting Potential with the NASA Ames Mars General Circulation Model
The dust cycle is a critically important component of Mars' current climate system. Dust is present in the atmosphere of Mars year-round but the dust loading varies with season in a generally repeatable manner. Dust has a significant influence on the thermal structure of the atmosphere and thus greatly affects atmospheric circulation. The dust cycle is the most difficult of the three climate cycles (CO2, water, and dust) to model realistically with general circulation models. Until recently, numerical modeling investigations of the dust cycle have typically not included the effects of couplings to the water cycle through cloud formation. In the Martian atmosphere, dust particles likely provide the seed nuclei for heterogeneous nucleation of water ice clouds. As ice coats atmospheric dust grains, the newly formed cloud particles exhibit different physical and radiative characteristics. Thus, the coupling between the dust and water cycles likely affects the distributions of dust, water vapor and water ice, and thus atmospheric heating and cooling and the resulting circulations. We use the NASA Ames Mars GCM to investigate the effects of radiatively active water ice clouds on surface stress and the potential for dust lifting. The model includes a state-of-the-art water ice cloud microphysics package and a radiative transfer scheme that accounts for the radiative effects of CO2 gas, dust, and water ice clouds. We focus on simulations that are radiatively forced by a prescribed dust map, and we compare simulations that do and do not include radiatively active clouds. Preliminary results suggest that the magnitude and spatial patterns of surface stress (and thus dust lifting potential) are substantial influenced by the radiative effects of water ice clouds.
Kahre, Melinda A.↗
Causes of slowing‐down seasonal CO2 amplitude at Mauna Loa
Changing amplitude of the seasonal cycle of atmospheric CO2 (SCA) in the northern hemisphere is an emerging carbon cycle property. Mauna Loa (MLO) station (20°N, 156°W), which has the longest continuous northern hemisphere CO2 record, shows an increasing SCA before the 1980s (p < .01), followed by no significant change thereafter. We analyzed the potential driving factors of SCA slowing‐down, with an ensemble of dynamic global vegetation models (DGVMs) coupled with an atmospheric transport model. We found that slowing‐down of SCA at MLO is primarily explained by response of net biome productivity (NBP) to climate change, and by changes in atmospheric circulations. Through NBP, climate change increases SCA at MLO before the 1980s and decreases it afterwards. The effect of climate change on the slowing‐down of SCA at MLO is mainly exerted by intensified drought stress acting to offset the acceleration driven by CO2 fertilization. This challenges the view that CO2 fertilization is the dominant cause of emergent SCA trends at northern sites south of 40°N. The contribution of agricultural intensification on the deceleration of SCA at MLO was elusive according to land–atmosphere CO2 flux estimated by DGVMs and atmospheric inversions. Our results also show the necessity to adequately account for changing circulation patterns in understanding carbon cycle dynamics observed from atmospheric observations and in using these observations to benchmark DGVMs.
Mauna Loa↗
Some observational constraints on the global-scale wind systems of Mars
The present investigation is concerned with the results of a study of wind direction indicators contained in the Viking and Mariner 9 data sets. It is found that wind direction measurements from a variety of indicators, taken over several Mars years at many longitudes, exhibit a high degree of consistency and seasonal reproducibility. It is, therefore, possible to interpret most of the observations in terms of the zonally symmetric circulation of Mars. When combined with models of the atmospheric circulation, observed wind directions can be used to set loose constraints on the polar cap mass fluxes and thermal fields which drive the flows. It is pointed out that polar cap formation seems to control the wind directions at mid and high latitudes in both hemispheres during autumn and early winter.
Kahn, R.↗
The influence of surface roughness of deserts on the July circulation - A numerical study
The effect of the low surface roughness characteristics of deserts on atmospheric circulation in July is examined using numerical simulations with the GCM of the Goddard Laboratory for Atmospheric Science (GLAS). Identical sets of simulations were carried out with the model starting from the initial state of the atmosphere on June 15, for the years 1979 and 1980. The first simulation included a surface roughness factor of 45 cm, and the second set had a surface roughness factor of 0.02 cm for desert regions, and 45 cm for all other land. A comparative analysis of the numerical data was carried out in order to study the variations for the desert regions. It is shown that rainfall in the Sahara desert was reduced significantly in the data set with the nonuniform surface roughness factor in comparison with the other data set. The inter-tropical convergence zone (ITCZ) moved southward to about 15 degrees, which was close to its observed location at about 10 degrees N. In other deserts, the North American Great Plains, Rajputana in India, and the Central Asian desert, no similar changes were observed. Detailed contour maps of the weather conditions in the different desert regions are provided.
Sud, Y. C.↗
On the role of the transient eddies in maintaining the seasonal mean circulation
The role of transient eddies in maintaining the observed local seasonal mean atmospheric circulation was investigated by examining the time-averaged momentum balances and omega equation, using seasonal statistics calculated from daily operational analyses by the European Centre for Medium Range Weather Forecasts. While both the Northern and Southern Hemispheres and several seasons were studied, emphasis was placed upon the Northern Hemisphere during December 1981-February 1982. The results showed that transient eddies played a secondary role in the seasonal mean zonal momentum budget and in the forcing of seasonal mean vertical and a geostrophic motion.
White, G. H.↗