Influence of evaporation in semi‐arid regions on the July circulation: A numerical study

Abstract

Abstract In this study, two 47‐day simulations were made with the general circulation model of the Goddard Laboratory for Atmospheric Sciences starting from the observed initial conditions for 15 June (00Z) 1979 as produced by NMC analysis. In the first simulation, the soil moisture was normally initialized and then allowed to vary by the model generated surface water balance. In the second simulation, the evaporation was prescribed and maintained zero in four chosen semi‐arid regions: the Sahel, the Thar Desert border in the Indian subcontinent, the Great Plains in North America and north‐east Brazil in South America. Elsewhere the soil moisture was calculated as in the first simulation. The two runs were identical in all other respects. The local climatology simulated by the GCM was analysed in the four anomaly regions. The mean July precipitation increased slightly in the Great Plains and Sahel regions, while it increased substantially (∼70 per cent) in the Thar Desert border region. However, in the north‐east Brazil region it decreased slightly. Relative thermal lows were created in each region, along with enhanced rising motion. There was also an increase in the mean monthly diabatic heating in all four regions. An analysis of the vertical structure of the moisture convergence field revealed that there was an increase in the low level moisture convergence in all of the anomaly regions. This compensated for the local moisture deficit created by the lack of evaporation. These results are based on one set of simulations representing the month of July. Further simulations would be required in order to evaluate their statistical significance.