A GCM Simulation Study of the Influence of Saharan Evapotranspiration and Surface-Albedo Anomalies on July Circulation and Rainfall

Y. C. Sud Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, Maryland

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A. Molod Centel Federal Services Corporation, Goddard Space Flight Center, Greenbelt, Maryland

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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.

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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