Biosphere-atmosphere interactions over semi-arid regions : modeling the role of mineral aerosols and irrigation in the regional climate system

Abstract

This dissertation describes the role of land surface processes in shaping semi-arid climates, namely those of Southwest Asia and Northwest Africa. The interactions between dust emissions, irrigation, and climate processes are studied and quantified using a regional climate model to perform a series of carefully designed numerical experiments. The performance of the numerical model is tested by comparing simulation results against observations from satellites and other standard platforms. Modeling results indicate significant improvements in simulating mineral aerosols over Southwest Asia. Results suggest that including representations for sub-grid scale wind gustiness as well as mineral aerosols at the boundaries, improve the model skill in simulating the spatial distribution and magnitude of suspended dust. Over Southwest Asia, a large bias in original simulations of surface temperature is eliminated by improving surface albedo, and including mineral aerosols and irrigation. These modifications reduced other biases associated with simulated surface shortwave incident radiation, surface absorbed radiation, and surface vapor pressure. As a result of these improvements, the model now successfully reproduces the climate of Southwest Asia. Another set of numerical experiments is performed over West Africa focusing on the same processes of dust emissions and irrigation. Over the Sahel region, it is found that both mineral aerosols and irrigation have similar effects on the surrounding climate: cooling of surface temperature, increased surface humidity, but no change in rainfall. With dust, a shallower boundary layer redistributes moisture closer to the surface thus offsetting negative temperature effects on the boundary layer moist static energy. With irrigation, a large reduction of the boundary layer height results in less triggering of convective activity and hence mitigates any increase in convective rainfall efficiency due to irrigation. Lastly, a numerical simulation over West Africa that includes simultaneous representations of dust emissions and irrigation is analyzed. Increased soil moisture, vegetation coverage, and dry deposition due to irrigation result in decreased emissions and suspension of dust. This experiment revealed an additional feedback due to irrigation: warming of the surface temperature due to a reduction in mineral aerosols concentration.