The effect of soil moisture on circulation and rainfall in a tropical model 

JULIA WALKER and P. R. ROWNTREE
Meteorological Office, Bracknell 

SUMMARY 
The sensitivity of a tropical model to soil moisture content has been investigated 
using a simplified version of the UK Meteorological Office 11-layer model . The model 
area represented a zonally-symmetric version of West Africa. The results of one 
experiment (D). with a desert in the same latitudes as the Sahara were compared with 
those of another experiment (W) in which the desert was replaced by moist land. The 
initial fields contained an idealized easterly wave which developed during the 
experiments to form depressions whose movement, structure and associated rainfall 
were strongly influenced by the dryness of the underlying surface. The energetic 
of these disturbances were analyzed and two distinct mechanisms for maintaining their 
eddy kinetic energy are proposed, depending on the soil moisture. Results from W 
suggest that once the land was moist it maintained itself in this state for at least 
several weeks. In D, however, the initial aridity north of 14'N was sustained, 
suggesting that ground dryness alone can cause deserts to persist. An analysis of 
observed rainfall data for the northern Sahel provides much stronger evidence for 
the persistence of rainfall anomalies from year to year than do similar analyses for 
the southern Sahel. The possibility that soil moisture anomalies could contribute 
to this persistence is briefly discussed.