Simulations of the effects of surface heat flux anomalies on stratification, convective growth, and vertical transport within the Saharan boundary layer

Abstract

Using two cases based on observations from the Geostationary Earth Radiation Budget Intercomparison of Longwave and Shortwave Radiation field campaign, large eddy model (LEM) simulations have been used to investigate the effects of surface flux anomalies on the growth of the summertime Saharan convective boundary layer (CBL) into the Saharan Residual Layer (SRL) above and transport from the CBL into the SRL. Hot surface anomalies generated updrafts and convergence in the CBL that increased transport from the CBL into the SRL. The induced subsidence in regions away from the anomalies inhibited growth of the CBL there. If the domain‐averaged surface fluxes were kept constant, this led to a shallower, cooler CBL. If fluxes outside the anomalies were kept constant so that stronger anomalies led to increased domain‐averaged fluxes, this gave a warmer, shallower CBL. These effects were larger for wider, stronger anomalies with low winds. The low‐level wind speed variations induced by the anomalies were also shown to affect dust uplift rates. Previous observations have shown that the summertime SRL, which often contains sublayers of varying humidity and dust content, can persist until late in the afternoon. The LEM simulations presented show that mesoscale variations in surface fluxes can contribute both to inhibiting the growth of the Saharan CBL into the SRL and to generating layerings within the SRL.