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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
,
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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N. Wolfson
,
R. Atlas
, and
Y. C. Sud

Abstract

A series of numerical forecast experiments has been conducted with the Goddard Laboratory for Atmospheres (GLA) fourth-order general circulation model in order to study the role of (i) sea-surface temperature (SST) anomalies in the North Pacific, (ii) soil moisture anomalies over the continental United States, as derived from observations of precipitation and surface temperature, and (iii) solar radiational forcing in the maintenance and breakdown of the severe heat wave over the United States in the summer of 1980.

Results from these experiments show opposite effects of the SST and soil moisture anomalies on the model's medium-range (ten-day) simulations of the maintenance of the heat wave. The lower than normal soil moisture over the United States during the summer of 1980 resulted in reduced surface evaporation, higher ground temperature, increased sensible heat flux from ground to air, higher surface temperature, lower sea-level pressure and higher 500 mb height over much of the Great Plains in the model forecasts. In contrast, the SST anomalies in the North Pacific during the same period resulted in an enhanced northerly flow of cooler and dryer air, increased evaporation, decreased ground and air temperature and reduced 500-mb heights over the Great Plains. These results suggest that once established, soil moisture deficits contribute to maintaining warm, dry conditions. Although long-term effects of North Pacific SST anomalies may be to create or enhance the heat wave, ten-day simulations showed that such anomalies contributed to lower temperatures over shorter time scales.

A limited number of experiments, with modified solar radiational forcing, showed a dramatic weakening of the heat wave pattern in the model forecasts and indicated that the interaction of changing solar declination with the prevailing synoptic situation was probably responsible for the breakdown of the heat wave in September 1980.

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P. J. Sellers
,
Y. Mintz
,
Y. C. Sud
, and
A. Dalcher

Abstract

A simple but realistic biosphere model has been developed for calculating the transfer of energy, mass and momentum between the atmosphere and the vegetated surface of the earth. The model is designed for use in atmospheric general circulation models.

The vegetation in each terrestrial model grid area is represented by two distinct layers, either or both of which may be present or absent at any given location and time. The upper vegetation layer represents the perennial canopy of trees or shrubs, while the lower layer represents the annual ground cover of grasses and other herbaceous species. The local coverage of each vegetation layer may be fractional or complete but as the individual vegetation elements are considered to be evenly spaced, their root systems are assumed to extend uniformly throughout the entire grid area. Besides the vegetation morphology, the physical and physiological properties of the vegetation layers are also prescribed. These properties determine (i) the reflection, transmission, absorption and emission of direct and diffuse radiation in the visible, near infrared and thermal wavelength intervals; (ii) the interception of rainfall and its evaporation from the leaf surfaces; (iii) the infiltration, drainage and storage of the residual rainfall in the soil; (iv) the control by the photosynthetically active radiation and the soil moisture potential, inter alia, over the stomatal functioning and thereby over the return transfer of the soil moisture to the atmosphere through the root-stem-leaf system of the vegetation; and (v) the aerodynamic transfer of water vapor, sensible heat and momentum from the vegetation and soil to a reference level within the atmospheric boundary layer.

The Simple Biosphere (SiB) has seven prognostic physical-state variables: two temperatures (one for the canopy and one for the ground cover and soil surface); two interception water stores (one for the canopy and one for the ground cover); and three soil moisture stores (two of which can be reached by the vegetation root systems and one underlying recharge layer into and out of which moisture is transferred only by hydraulic diffusion and gravitational drainage).

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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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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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G. E. Liston
,
Y. C. Sud
, and
E. F. Wood

Abstract

To relate general circulation model (GCM) hydrologic output to readily available river hydrographic data, a runoff routing scheme that routes gridded runoffs through regional- or continental-scale river drainage basins is developed. By following the basin overland flow paths, the routing model generates river discharge hydrographs that can be compared to observed river discharges, thus allowing an analysis of the GCM representation of monthly, seasonal and annual water balances over large regions. The runoff routing model consists of two linear reservoirs a surface reservoir and a groundwater reservoir, which store and transport water. The water transport mechanisms operating within these two reservoirs are differentiated by their time scares, the groundwater reservoir transports water much more slowly than the surface reservoir. The groundwater reservoir feeds the corresponding surface store and the surface stores are connected via the river network.

The routing model is implemented over the GEWEX (Global Energy and Water Cycle Experiment) Continental-Scale International Project Mississippi River basin on a rectangular grid of 2° × 2.5°. Two land surface hydrology parameterizations provide the gridded runoff data required to run the runoff routing scheme: the variable infiltration capacity model, and the soil moisture component of the simple biosphere model. These parameterizations are driven with 4° × 51° gridded climatological potential evapotranspiration and 1979 First GARP (Global Atmospheric Research Program) Global Experiment precipitation. These investigations have quantified the importance of physically realistic soil moisture holding capacities evaporation parameters and runoff mechanisms in land surface hydrology formulations.

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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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Eric P. Salathé Jr.
,
Dennis Chesters
, and
Y. C. Sud

Abstract

TOVS satellite observations are used to evaluate the upper-tropospheric (400–200 mb) moisture distribution simulated by the GLA GCM in a 10-yr (1979–1988) integration produced for the Atmospheric Model Inter-comparison Project, in which several models participated worldwide. The simulated moisture fields show remarkable success in duplicating the large-scale structure and seasonal features in the observations, but they show insufficient contrast between very dry and very moist regions. The simulation generally does well in northern summer (June–July–August) but worse in northern winter. This is consistent with deficiencies in the annual cycle of moist convection so that convective rain stays too close to the equator in northern winter. The related misplacement of convective activity associated with the Asian monsoon produces discrepancies in the moisture over much of the Eastern Hemisphere. The simulation also shows a too weak moisture response to interannual fluctuations in the sea surface temperature, even for the large El Niño episode of 1983. These results indicate that deficiencies in modeling oceanic convection may be in part responsible for errors in the simulated upper-tropospheric moisture patterns.

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G. K. Walker
,
Y. C. Sud
, and
R. Atlas

Numerical simulation experiments were conducted to delineate the influence of in situ deforestation data on episodic rainfall by comparing two ensembles of five 5-day integrations performed with a recent version of the Goddard Laboratory for Atmospheres GCM that has a simple biosphere model (SiB). The first set, called control cases, used the standard SiB vegetation cover (comprising 12 biomes) and assumed a fully forested Amazonia, while the second set, called deforestation cases, distinguished the partially deforested regions of Amazonia as savanna. Except for this difference, all other initial and prescribed boundary conditions were kept identical in both sets of integrations. The differential analyses of these five cases show the following local effects of deforestation.

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