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Y. Mintz and G. K. Walker

Abstract

The global fields of normal monthly soil moisture and land surface evapotranspiration are derived with a simple water budget model that has precipitation and potential evapotranspiration as inputs. The precipitation is observed and the potential evapotranspiration is derived from the observed surface air temperature with the empirical regression equation of Thornthwaite. It is shown that at locations where the net surface radiation flux has been measured, the potential evapotranspiration given by the Thornthwaite equation is in good agreement with those obtained with the radiation-based formulations of Priestley and Taylor, Penman, and Budyko, and this provides the justification for the use of the Thornthwaite equation.

After deriving the global fields of soil moisture and evapotranspiration, the assumption is made that the potential evapotranspiration given by the Thornthwaite equation and by the Priestley–Taylor equation will everywhere be about the same; and the inverse of the Priestley–Taylor equation is used to obtain the normal monthly global fields of net surface radiation flux minus ground heat storage. This and the derived evapotranspiration are then used in the equation for energy conservation at the surface of the earth to obtain the global fields of normal monthly sensible heat flux from the land surface to the atmosphere.

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Y. C. Sud and G. K. Walker

Abstract

A rain evaporation and downdraft parameterization is designed to complement the cumulus convection scheme of the Goddard Laboratory for Atmospheres General Circulation Model (GLA GCM). The scheme invokes (i) a diagnostic determination of the commencement level of rain-evaporation-induced downdrafts, (ii) a method for calculating downdraft mass fluxes emanating from different levels of the atmosphere, and (iii) an explicitly prescribed overall fraction of rain evaporation within the downdraft.

The parameterization was tested with the GATE [GARP (Global Atmospheric Research Program) Atlantic Tropical Experiment] phase III data in a fully prognostic mode and with the entire atmospheric and surface forcings prescribed with data. It was found that the near-surface downdraft cooling largely mitigates the observed surface sensible heating. In the absence of this cooling, the boundary layer must get rid of the surface heat flux by spurious turbulent transport, which becomes significant in simulations that ignore both the rain evaporation and downdrafts. The time mean as well as root-mean-square errors in the vertical temperature profiles are somewhat larger for simulations without downdrafts and are much larger for simulations without both downdrafts and rain evaporation. The downdrafts are found to produce a useful correction in the simulated near-surface temperature and humidity in GCM simulations, and in that way, the parameterization improves the simulation of tropospheric temperature and humidity. In a one-year comparison of GLA GCM simulations with and without downdrafts, the former produced better precipitation climatology and surface temperatures.

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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 G. K. Walker

Abstract

In all the atmospheric general circulation models (GCMs) at the Goddard Laboratory for Atmospheres (GLA), the influence of oceanic salinity on the saturation vapor pressure of seawater is ignored. Since the relative humidity in the oceanic boundary layer is generally high while the saturation vapor pressure of seawater is lowered by salinity, its neglect could have a nontrivial influence on the near-surface specific humidity gradient, a primary determinant of oceanic evaporation. Such an approximation might effect the simulated circulation and rainfall systematically. To evaluate this idea, we carried out a 5-yr-long salinity simulation (S) with the GLA GCM in which the influence of salinity on the saturation vapor pressure of seawater was included. Corresponding to this, a control simulation (C) with the GLA GCM in which the salinity effect was ignored was also available. Analyses of S-minus-C fields have shown some evidence of discernible systematic errors in the global evaporation, boundary layer specific humidity, and several key parameters that affect the onset of moist convection, for example, convective available potential energy. Several other systematic effects are also evident even though they remain small compared to the interannual variability of climate. The systematic interactions caused by the neglect of salinity are evidently spurious, and even though the final outcome is less dramatic than anticipated originally, several persistent systematic errors can be noted in the 5-yr mean fields. Based on these results, we infer that coupled ocean–atmosphere models that ignore the influence of salinity on ocean evaporation might also benefit from the salinity correction. Indeed, the correction is so trivial to include, its neglect in the modern state-of-the-art GCMs is unwarranted.

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Y. C. Sud and G. K. Walker

Abstract

A prognostic cloud scheme named McRAS (Microphysics of Clouds with Relaxed Arakawa–Schubert Scheme) has been designed and developed with the aim of improving moist processes, microphysics of clouds, and cloud–radiation interactions in GCMs. McRAS distinguishes three types of clouds: convective, stratiform, and boundary layer. The convective clouds transform and merge into stratiform clouds on an hourly timescale, while the boundary layer clouds merge into the stratiform clouds instantly. The cloud condensate converts into precipitation following the autoconversion equations of Sundqvist that contain a parametric adaptation for the Bergeron–Findeisen process of ice crystal growth and collection of cloud condensate by precipitation. All clouds convect, advect, as well as diffuse both horizontally and vertically with a fully interactive cloud microphysics throughout the life cycle of the cloud, while the optical properties of clouds are derived from the statistical distribution of hydrometeors and idealized cloud geometry.

An evaluation of McRAS in a single-column model (SCM) with the Global Atmospheric Research Program Atlantic Tropical Experiment (GATE) Phase III data has shown that, together with the rest of the model physics, McRAS can simulate the observed temperature, humidity, and precipitation without discernible systematic errors. The time history and time mean in-cloud water and ice distribution, fractional cloudiness, cloud optical thickness, origin of precipitation in the convective anvils and towers, and the convective updraft and downdraft velocities and mass fluxes all simulate a realistic behavior. Some of these diagnostics are not verifiable with data on hand. These SCM sensitivity tests show that (i) without clouds the simulated GATE-SCM atmosphere is cooler than observed; (ii) the model’s convective scheme, RAS, is an important subparameterization of McRAS; and (iii) advection of cloud water substance is helpful in simulating better cloud distribution and cloud–radiation interaction. An evaluation of the performance of McRAS in the Goddard Earth Observing System II GCM is given in a companion paper (Part II).

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R. D. Koster and G. K. Walker

Abstract

The time scales that characterize the variations of vegetation phenology are generally much longer than those that characterize atmospheric processes. The explicit modeling of phenological processes in an atmospheric forecast system thus has the potential to provide skill to subseasonal or seasonal forecasts. We examine this possibility here using a forecast system fitted with a dynamic vegetation phenology model. We perform three experiments, each consisting of 128 independent warm-season monthly forecasts: 1) an experiment in which both soil moisture states and carbon states (e.g., those determining leaf area index) are initialized realistically, 2) an experiment in which the carbon states are prescribed to climatology throughout the forecasts, and 3) an experiment in which both the carbon and soil moisture states are prescribed to climatology throughout the forecasts. Evaluating the monthly forecasts of air temperature in each ensemble against observations—as well as quantifying the inherent predictability of temperature within each ensemble—shows that dynamic phenology can indeed contribute positively to subseasonal forecasts, though only to a small extent, with an impact dwarfed by that of soil moisture.

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Y. C. Sud, Winston C. Chao, and G. K. Walker

Abstract

Several integrations were made with a coarse (4° × 5° nine-sigma level) version of the GLA GCM, which has the Arakawa–Schubert cumulus parameterization, predicted fractional cloud cover, and a parameterization of evaporation of falling rainfall. All model simulation experiments started from the ECMWF analysis for 15 December 1982 and were integrated until 31 January 1983 using climatological boundary conditions. The first ten days of model integrations show that the model-simulated tropics dries and warms as a result of excessive precipitation.

Three types of model development-cum-analysis studies were made with the cumulus scheme. First, the Critical Cloud Work Function (CCWF) dataset for different sigma layers were reworked using the Cloud Work Function (CWF) database of Lord et al. as representative of time-average CWF and not the actual CCWF values as in the Arakawa–Schubert implementation of cumulus convection. The experiments with the new CCWF dataset helped to delineate the influence of changing CCWF on model simulations. Larger values of CCWF partially alleviated the problem of excessive heating and drying during spinup and sharpened the tropical ITCZ (Intertropical Convergence Zone). Second, by comparing two simulations, one with and one without cumulus convection, the role of cumulus convection in maintaining the observed tropical rainfall and 850 mb easterly winds is clarified. Third, by using Simpson's relations between cloud radii and cumulus entrainment parameter, λ, in the Arakawa–Schubert cumulus scheme, realistic upper and lower bounds on λ were obtained. This improvement had a significant impact on the time evolution of tropical temperature and humidity simulation. It also significantly suppressed the excessive rainfall during spinup. Finally, by invoking λ min = 0.0002 m−1 (R max = 1.00 km) another simulation was made. In this simulation, not only the excessive initial rainfall was virtually eliminated, but a more realistic vertical distribution of specific humidity in the tropics was produced. Despite the conceptual simplicity of the latter, it has made some very significant improvement to the monthly simulation in the tropics.

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Y. C. Sud, G. K. Walker, and W. E. Smith

Abstract

An ensemble of three sets of simulations is produced with the GLA (Goddard Laboratory for Atmospheres) GCM to assess the effect of the El Niño event of 1982–83 on winter climate. The three sets of runs are started from the analyzed initial states of the atmosphere for the 14, 15, and 16 December 1982, respectively, and are integrated through the end of February 1983. Each set consists of a control run, which was forced with climatological SSTs, and a corresponding anomaly run, which was forced with the observed SSTs. The ensemble mean of the model-simulated atmospheric circulation and rainfall anomalies is compared with the corresponding analyses of observations.

Most of the observed circulation anomaly features in the tropics are simulated rather well by the model. The tropical sea level pressure anomalies show a typical ENSO pattern: 850 mb wind anomalies show westerly winds over the equatorial Pacific Ocean; 200 mb anomalous winds show anticyclonic vortices straddling the equator; and the 200 mb height anomalies agree well with the corresponding observations. The regions of statistically significant anomaly patterns in the tropics are also in good agreement with observations. The model simulated rainfall anomalies also compare well with the rainfall analysis based on satellite-derived water vapor in the atmosphere from 37 GHz SMMR data. The model's OLR anomaly patterns show close correspondence with the anomalies in satellite observations of OLR. However, there is about a 5–10 degree eastward shift in the major simulated anomalies. This shift is also evident directly over the warm water of the equatorial eastern Pacific, with an even larger shift in the associated patterns in the extratropics. This discrepancy in the simulations leads to some very poor anomaly correlations in the extratropics.

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Michael G. Bosilovich, Siegfried D. Schubert, and Gregory K. Walker

Abstract

In this study, numerical simulations of the twentieth-century climate are evaluated, focusing on the changes in the intensity of the global water cycle. A new model diagnostic of atmospheric water vapor cycling rate is developed and employed that relies on constituent tracers predicted at the model time step. This diagnostic is compared to a simplified traditional calculation of cycling rate, based on monthly averages of precipitation and total water content. The mean sensitivity of both diagnostics to variations in climate forcing is comparable. However, the new diagnostic produces systematically larger values with more variability.

Climate simulations were performed using SSTs of the early (1902–21) and late (1979–98) twentieth century along with the appropriate CO2 forcing. In general, the increase of global precipitation with the increases in SST that occurred between the early and late twentieth century is small. However, an increase of atmospheric temperature leads to a systematic increase in total precipitable water. As a result, the residence time of water in the atmosphere increased, indicating a reduction of the global cycling rate. This result was explored further using a number of 50-yr climate simulations from different models forced with observed SST. The anomalies and trends in the cycling rate and hydrologic variables of different GCMs are remarkably similar. The global annual anomalies of precipitation show a significant upward trend related to the upward trend of surface temperature, during the latter half of the twentieth century. While this implies an increase in the simulated hydrologic cycle intensity, a concomitant increase of total precipitable water again leads to a decrease in the calculated global cycling rate. An analysis of the land/sea differences shows that the simulated precipitation over land has a decreasing trend, while the oceanic precipitation has an upward trend consistent with previous studies and the available observations. The decreasing continental trend in precipitation is located primarily over tropical land regions, with some other regions, such as North America, experiencing an increasing trend. Precipitation trends are diagnosed further using the water tracers to delineate the precipitation that occurs because of continental evaporation, as opposed to oceanic evaporation. These model diagnostics show that over global land areas, the recycling of continental moisture is decreasing in time. However, the recycling changes are not spatially uniform so that some regions, most notably over the United States, experience continental recycling of water that increases in time.

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Y. C. Sud, Winston C. Chao, and G. K. Walker

Abstract

A coarse (4° &times 5° × 9-sigma level) version of the Goddard Laboratory for Atmospheres (GLA) General Circulation Model (GCM) was used to investigate the influence of a cumulus convection scheme on the simulated atmospheric circulation and hydrologic cycle. Two sets of integrations, each containing an ensemble of three summer (June, July, and August) simulations, were produced. The first set, containing control cases, included a state-of-the-art cumulus parameterization scheme in the GCM; whereas the second set, containing experiment cases, used the same GCM but without the cumulus parameterization. All simulations started from initial conditions that were taken from analysis of observations for three consecutive initial times that wore only 12 h apart beginning with 0000 UTC 19 May 1988. The climatological boundary conditions—sea surface temperature, snow, ice, and vegetation cover-were kept exactly the same for all the integrations. The ensemble sets of control and experiment simulations are control and differentially analyzed to determine the influence of a cumulus convection scheme on the simulated circulation and hydrologic cycle.

The results show that cumulus parameterization has a very significant influence on the simulated circulation and precipitation. The influence is conspicuous in tropical regions, interior of continents in the Northern Hemisphere, and some oceanic regions. The upper-level condensation heating over the intertropical convergence zone (ITCZ) is much smaller for the experiment simulations as compared to the control simulations; correspondingly, the Hadley and Walker cells for the control simulations are also weaker and are accompanied by a weaker Ferrel cell in the Southern Hemisphere. The rainfall under the rising branch of the southern Ferrel cell (at about 50°S) does not increase very much because boundary-layer convergence poleward reduces the local evaporation. Overall, the difference fields show that experiment simulations (without cumulus convection) produce a cooler and less energetic atmosphere. The vertical profile of the zonally averaged diabatic heating also shows large differences in the tropics that are physically consistent with accompanying differences in circulation. Despite producing a warmer and wetter planetary boundary layer (PBL) in the tropics (20°S–20°N), the control simulations also produce a warmer but drier 400-mb level. The moisture transport convergence fields show that while only the stationary circulation is affected significantly in the PBI, both the stationary and eddy moisture transports are altered significantly in the atmosphere above the PBL. These differences no only reaffirm the important role of cumulus convection in maintaining the global circulation, but also show the way in which the presence or absence of a cumulus parameterization scheme can affect the circulation and rainfall climatology of a GCM.

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