The influence of soil wetness distribution on short-range rainfall forecasting in 
the West African Sahel 

D. P. ROWELL 
Department of Meteorology, University of Reading 
C. BLONDIN 
European Centre for Medium Range Weather Forecasts, Reading 

SUMMARY 
Surface moisture supply is known to be a factor which can be important in triggering 
convection. This paper aims to investigate its impact on short-range numerical 
forecasts of tropical rainfall. The ECMWF (European Centre for Medium Range Weather 
Forecasts) operational forecasting model was integrated for 5 days with realistic 
positive and negative anomalies of the initial surface moisture distribution in the 
West African Sahel. The large-scale flow is found to be relatively unaffected, but 
significant differences occur in the rainfall forecasts for about the first 4 days. 
Rainfall is greatest with a moister surface, except on days 3 and 4 in the south 
Sahel/Savanna region, for which a decrease of rainfall occurs. The opposite result 
is seen when an initially drier surface is introduced. The mechanisms involved are 
investigated by carrying out a detailed hydrological budget, and by studying the 
evolution of the atmospheric profiles of moisture and temperature. We conclude that 
an improved surface moisture analysis is likely to result in some improvement of 
short-range rainfall forecasts in the Sahel, and presumably also in other tropical 
regions. In the coming years it should be possible to achieve this using 
satellite-derived soil wetness maps.