9405431 Xue The climate in the Sahel area of Africa has been dominated by drought since the late 1960's. A number of studies have shown that the land surface-atmosphere interaction in the sub-Sahara region is very strong and that the land surface feedback may have played a role in the Sahel drought. However, in most of these studies only simple surface models were used and the results were never verified against observational data. Two comprehensive surface field measurements (HAPEX-Sahel and SEBEX) were conducted in the Sahel during the past five years. These data sets will be used to validate a simplified version of the Simple Biosphere Model (SSiB) and to create vegetation data sets for GCM boundary conditions. In the calibration study, special attention will be given to the realistic simulation of co-existing vegetation covers, which is one of the manifestations of surface heterogeneity. The sensitivity of the land-surface scheme to vegetation and soil parameter values will be investigated. This calibration will help us to improve the simulation of the hydrological cycle and the surface energy budget. The coupled biosphere-atmosphere model will be used to study the roles of the surface processes and atmospheric dynamics in determining the variation in the climate and hydrology of the Sahel. To understand the mechanisms of land surface feedback, we will divide the vegetation and soil parameters into several groups. The effects of each group and their combination on the simulated climate anomaly in the Sahel will be examined. The effect of heterogeneity will also be investigated. Based on the results of the above proposed studies, two multi-year numerical integrations will be conducted. The long term integration will allow more stringent statistical significance testing of the results. The results from the control run will be used to analyze The simulation of the annual cycle of water and energy balances in the sub-Sahara area. the anomaly run, in which the land surface in the Sahel is assigned as a desertification area, will be used to assess the effects of the desertification on the regional and global climate and hydrology during all seasons. Since this July 25, 1994 sensitivity study is based upon comprehensive observational data, its successful completion will help us in realistically assessing the consequences of the desertification in the regional as well as the global climate, in particular the Sahel drought.
