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J. Shukla and Y. Sud

Abstract

The General Circulation Model (GCM) of the Goddard Laboratory for Atmospheric Sciences (GLAS) was integrated for 107 days starting from the initial conditions of 15 May. In this experiment the clouds dynamically generated by the model affect the radiative heating fields continuously. Starting from the initial conditions valid for day 76 of this run, another integration was made for 31 days in which the clouds were specified on certain grid points, remaining fixed during the period of integration. The spatial distribution of the fixed clouds was such that the aggregate cloud frequency for a vertical level and each latitude circle remained the same as in each control run, and the highest cloud frequency grid points were assigned the cloudiness of 100%. The 31-day mean simulation of the second run (fixed clouds) is compared with the last 31-day mean simulation of the first run to study the effects of cloud-radiation feed- back on the mean monthly circulation, atmospheric energy cycle and the hydrological cycle, evaporation and precipitation and the local climate.

Results from these experiments show significant changes in the simulated large-scale dynamical circulation of the global model. Fixed clouds acting as zonally asymmetric radiative heat sources increase the generation of eddy available potential energy (EAPE) and its conversion to eddy kinetic energy. Generation of EAPE by net radiative heating increased by 50%(0.11 W m−2) for the fixed cloud experiment. The increase due to the stationary component was much larger (∼100%) but it was partially compensated by decrease due to the transient component. A substantial increase was found in the variances of the planetary-scale stationary waves and the medium-scale waves (wavenumber 6–10) of 2–7 day period. Although the sea surface temperatures were prescribed identically in both integrations, the changes in evaporation and precipitation were found to be much larger over the oceans compared to those over the land. We suspect that this happens because the ground temperature is determined by the model's beat balance at the earth's surface and therefore internal model feedbacks do not allow the hydrologic cycle over land to be very different between the fixed cloud run and the control run. Based on these calculations, we infer that cloud-radiation feedback is an important mechanism in the general circulation of the model atmosphere. It must be adequately parameterized in numerical experiments designed to simulate the mean climate and/or to examine the sensitivity of GCM's to changes in external boundary conditions or internal atmospheric constituents (such as aerosols and CO2) and their feedback effects.

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Y. Sud and A. Molod

Abstract

The sensitivity of the simulated July circulation to modifications in the parameterization of dry and moist convection, evaporation from failing raindrops, and cloud-radiation interaction is examined with the GLA (Goddard Laboratory for Atmospheres) GCM (general circulation model). Interferences are based on several 47-day summer integrations using the same prescribed boundary forcings. The Arakawa-Schubert cumulus parameterization, together with a more realistic dry convective mixing calculation that allowed moisture, heat and momentum to mix uniformly, yielded a far better intertropical convergence zone (ITCZ) over North Africa than did the previous convection scheme. It also produced a rain-free Sahara desert, which is well-known feature in observations, but is poorly simulated by a number of GCMs. The physical mechanism for the improvement was identified to be the upward mixing of planetary boundary layer (PBL) moisture by vigorous dry convective mixing, which prevented the buildup of moist convective instability in the PBL.

A modified rain-evaporation parameterization which takes into account the raindrop size distribution, the atmospheric relative humidity, and a typical spatial rainfall intensity distribution for convective rain was developed and implemented. It evaporated about 50% of the convective rain while greatly reducing the evaporation of large-scale rain. As compared to the old scheme which produced no evaporation of convective rain and the maximum possible evaporation of large-scale rain, the new scheme led to some major improvements in the monthly mean vertical profiles of relative humidity and temperature, convective and large-scale cloudiness, rainfall distributions, and mean relative humidity in the PBL.

When the convective cloud-radiation interaction was included by assuming that an entire sigma layer of a grid-box at the detrainment level(s) of Arakawa-Schubert clouds of appropriate optical thickness, some major changes in the surface, diabatic heating, and orientation of the ITCZ over equatorial America were simulated. The experiment suggests a strong potential for further improvement of the GCM simulations by including more realistic parameterization of cloud-radiation interaction.

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Y. C. Sud and A. Molod

Abstract

The GLA (Goddard Laboratory for Atmospheres) GCM (general circulation model) was employed to investigate the influence of surface albedo and evapotranspiration anomalies that could result from the hypothetical semiarid vegetation over North Africa (including the Sahara desert) on its July circulation and rainfall. In the first experiment a soil moisture anomaly was prescribed over North Africa, whereas in the second experiment a soil moisture plus surface albedo anomaly was prescribed over North Africa. These two experiments used the first version of the GCM with the old parameterization of evaporation from failing rain drops and were compared with a control run that was made with climatologically normal boundary conditions. The third experiment had the soil moisture and surface albedo anomalies of the second experiment and was run with the second version of the model that included a recently modified parameterization of evaporation of falling rain. It was compared to its control that had climatologically normal boundary conditions.

The results of the first experiment show that the increased soil moisture and its dependent evapotranspiration produces a cooler and moister PBL over North Africa that is able to support enhanced moist convection and rainfall in Sahel and southern Sahara. The results of the second experiment show that the lower surface albedo yields even higher moist static energy in the PBL and further enhances the local moist convection and rainfall. The third experiment, with the modified rain-evaporation parameterization, produces hydrological cycle and accompanying rainfall anomalies that were quite similar to those of the second experiment specifically over the anomaly region; however, some differences between the second and third experiments were evident in distant regions. These differences suggest the importance of a different and/or a better parameterization of falling rain.

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Beth Chertock and Y. C. Sud

Abstract

A global, 7-year satellite-based record of ocean surface solar irradiance (SSI) is used to assess the realism of ocean SSI simulated by the nine-layer Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM). January and July climatologies of net SSI produced by the model are compared with corresponding satellite climatologies for the world oceans between 54°N and 54°S. This comparison of climatologies indicates areas of strengths and weaknesses in the GCM treatment of cloud-radiation interactions, the major source of model uncertainty. Realism of ocean SSI is also important for applications such as incorporating the GLA GCM into a coupled ocean-atmosphere GCM. The results show that the GLA GCM simulates too much SSI in the extratropies and too little in the tropics, especially in the summer hemisphere. These discrepancies reach magnitudes of 60 W m−2 and more. The discrepancies are particularly large in the July case off the western coast of North America. In this region of persistent marine stratus, the GCM climatological values exceed the satellite climatological values by as much as 131 W m−2. Positive and negative discrepancies in SSI are shown to be consistent with discrepancies in planetary albedo.

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Taikan Oki and Y. C. Sud

Abstract

As a first step toward designing a comprehensive model for validating land surface hydrology and river flow in Earth system models, a global river channel network has been prepared at 1° latitude × 1° longitude resolution. The end product is the Total Runoff Integrating Pathways (TRIP) network. The aim of TRIP is to provide information of lateral water movement over land following the paths of river channels. Flow directions were determined from vector data of river channels and river pathways available in two recent atlases; however, an automatic procedure using a digital elevation map of the corresponding horizontal resolution was used as a first guess. In this way, a template to convert the river discharge data into mean runoff per unit area of the basin has been obtained. One hundred eighty major rivers are identified and adequately resolved; they cover 63% of land, excluding Antarctica and Greenland. Most of the river basin sizes are well within a 20% difference of published values, with a root-mean-square error of approximately 10%. Furthermore, drainage areas for more than 400 gauging stations were delineated. Obviously, the stream lengths in TRIP are shorter than the natural lengths published as data. This is caused by the meandering of rivers in the real world. Meandering ratio (r M), the ratio of actual (published) river length to the idealized river length, has been calculated. Averaged globally for all available data, r M is 1.4, although it is 1.3 for rivers with areas larger than 500,000 km2. The r M data will be useful in the design of the Scheme for Total Runoff Integrating Pathways (STRIP). In the current form, TRIP can be used as a template for producing a time series of river flow using a simple version of STRIP.

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Y. C. Sud, J. Shukla, and Y. Mintz

Abstract

The influence of land surface roughness on the large scale atmospheric circulation and rainfall was examined by comparing three sets of simulations made with a general circulation model in which the land surface roughness length, z 0, was reduced from 45 cm to 0.02 cm. The reduced surface roughness produced about a two-fold increase in the boundary layer wind speed and, at the same time, a two-fold decrease in the magnitude of the surface stress. There was almost no change in the surface evaporation and surface sensible heat flux. There was, however, a larger change in the horizontal convergence of the water vapor transport in the boundary layer and a corresponding large change in the rainfall distribution mainly as a consequence of the change in the cut of the surface stress. The result suggests that the height of the earth' vegetation cover, which is the main determinant of the land surface roughness, has a large influence on the boundary layer water vapor transport convergence and the rainfall distribution.

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Martin I. Hoffert and Y. C. Sud

Abstract

A similarity model is developed for the vertical profiles of turbulent flow variables in an entraining turbulent boundary layer of arbitrary buoyant stability. In the general formulation the vertical profiles, internal rotation of the velocity vector, discontinuities or jumps at a capping inversion and bulk aerodynamic coefficients of the boundary layer are given by solutions to a system of ordinary differential equations in the similarity variable η = z/h, where h is the physical height or thickness, where the system includes six parameters associated with surface roughness, buoyant stability of the turbulence near the surface, Coriolis effects, baroclinicity and stability of the air mass above the boundary layer. To close the system a new formulation for buoyantly interactive eddy diffusivity in the boundary layer is introduced which recovers Monin-Obukhov similarity near the surface and incorporates a hypothesis accounting for the observed variation of mixing length throughout the boundary layer.

The model is tested in simplified versions which depend only on roughness, surface buoyancy and Coriolis effects by comparison with Clarke's planetary boundary layer wind and temperature profile observations, Arya's measurements of flat-plate boundary layers in a thermally stratified wind tunnel, and Lenschow's observations of profiles of terms in the turbulent kinetic energy budget of convective planetary boundary layers. On balance, the simplified model reproduced the trend of these various observations and experiments reasonably well, suggesting that the full similarity formulation be pursued further.

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David M. Mocko and Y. C. Sud

Abstract

Refinements to the snow-physics scheme of the Simplified Simple Biosphere Model (SSiB) are described and evaluated. The upgrades include a partial redesign of the conceptual architecture of snowpack to better simulate the diurnal temperature of the snow surface. For a deep snowpack, there are two separate prognostic temperature snow layers: the top layer responds to diurnal fluctuations in the surface forcing, while the deep layer exhibits a slowly varying response. In addition, the use of a very deep soil temperature and a treatment of snow aging with its influence on snow density is parameterized and evaluated. The upgraded snow scheme produces better timing of snowmelt in Global Soil Wetness Project (GSWP)-style simulations using International Satellite Land Surface Climatology Project (ISLSCP) Initiative I data for 1987–88 in the Russian Wheat Belt region.

To simulate more realistic runoff in regions with high orographic variability, additional improvements are made to SSiB's soil hydrology. These improvements include an orography-based surface runoff scheme as well as interaction with a water table below SSiB's three soil layers. The addition of these parameterizations further helps to simulate more realistic runoff and accompanying prognostic soil moisture fields in the GSWP-style simulations.

In intercomparisons of the performance of the new snow-physics SSiB with its earlier versions using an 18-yr single-site dataset from Valdai, Russia, the revised version of SSiB described in this paper again produces the earliest onset of snowmelt. Soil moisture and deep soil temperatures also compare favorably with observations.

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Y. C. Sud and G. K. Walker

Abstract

A prognostic cloud scheme named the Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS) and the Simple Biosphere Model have been implemented in a version of the Goddard Earth Observing System (GEOS) II GCM at a 4° latitude × 5° longitude × 20 sigma-layer resolution. The McRAS GCM was integrated for 50 months. The integration was initialized with the European Centre for Medium-Range Weather Forecasts analysis of observations for 1 January 1987 and was forced with the observed sea surface temperatures and sea-ice distribution; on land, the permanent ice and vegetation properties (biomes and soils) were climatological, while the soil moisture and snow cover were prognostic. The simulation shows that the McRAS GCM yields realistic structures of in-cloud water and ice, and cloud-radiative forcing (CRF) even though the cloudiness has some discernible systematic errors. The simulated intertropical convergence zone (ITCZ) has a realistic time mean structure and seasonal cycle. The simulated CRF is sensitive to vertical distribution of cloud water, which can be affected hugely with the choice of minimum in-cloud water for the onset of autoconversion or critical cloud water amount that regulates the autoconversion itself. The generation of prognostic cloud water is accompanied by reduced global precipitation and interactive CRF. These feedbacks have a profound effect on the ITCZ. Even though somewhat weaker than observed, the McRAS GCM simulation produces robust 30–60-day oscillations in the 200-hPa velocity potential. Comparisons of CRFs and precipitation produced in a parallel simulation with the GEOS II GCM are included.

Several seasonal simulations were performed with the McRAS–GEOS II GCM for the summer (June–July–August) and winter (December–January–February) periods to determine how the simulated clouds and CRFs would be affected by (i) advection of clouds, (ii) cloud-top entrainment instability, (iii) cloud water inhomogeneity correction, and (iv) cloud production and dissipation in different cloud processes. The results show that each of these processes contributes to the simulated cloud fraction and CRF. Because inclusion of these processes helps to improve the simulated CRF, it is inferred that they would be useful to include in other cloud microphysics schemes as well.

Two ensembles of four summer (July–August–September) simulations, one each for 1987 and 1988, were produced with the earlier 17-layer GEOS I GCM with McRAS. The differences show that the model simulates realistic and statistically significant precipitation differences over India, Central America, and tropical Africa. These findings were also confirmed in the new 20-layer GEOS II GCM with McRAS in the 1987 minus 1988 differences.

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Y. C. Sud and W. E. Smith

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Twelve July integrations were made with the GLAS (Goddard Laboratory for Atmospheres) GCM (General Circulation Model) to investigate the influence of changes in the land-surface fluxes over the Indian subcontinent on the monsoon circulation and rainfall. The runs consist of an ensemble of three integrations for each of four separate cases: i) a control, ii) an experiment with increased land-surface albodo, iii) an experiment with increased land-surface albedo and reduced land-surface roughness, and iv) an experiment with increased land-surface albedo, reduced surface roughness and no evapotranspiration. All the prescribed land-surface anomalies were limited to the Indian region.

An intercomparison of the ensemble means of monthly fields produced by the experiments with those of the control showed that the Indian Monsoon was significantly weakened by both the increase of surface albedo and by the reduction in surface roughness. Higher surface albedo reduced the monsoon rainfall in conformity with Charney's hypothesis; the low surface roughness made the horizontal transport of moisture in the PBL (planetary boundary layer) more westerly, which reduced the cross-isobaric moisture convergence and hence the rainfall over northwestern India while correspondingly increasing it over China. The curl of surface stress divided by the Coriolis parameter (k· ∇ × τs)/f represents the boundary layer convergence. There is a remarkable correspondence between changes of this field and rainfall for all the experiments. Since the magnitude of prescribed changes in surface albedo and surface roughness could plausibly be produced by deforestation, the results suggest that major changes in the tall natural vegetation over the Indian subcontinent would have a significant influence on its July rainfall.

The last experiment delineated the role of evapotranspiration over India. It was found that the rainfall was essentially unaltered by the absence of evapotranspiration, because the increased moisture convergence produced by the enhanced sensible heating of the PBL largely compensated for the reduction in evapotranspiration.

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