Control of the Dust Vertical Distribution over Western Africa by Convection and Scavenging

Abstract

Saharan dust represents more than 50% of the total desert dust emitted around the globe and its radiative effect significantly affects the atmospheric circulation at a continental scale. Previous studies on dust vertical distribution and the Saharan Air Layer (SAL) showed some shortcomings that could be attributed to imperfect representation of the effects of deep convection and scavenging. The authors investigate here the role of deep convective transport and scavenging on the vertical distribution of mineral dust over Western Africa. Using multi-year (2006–2010) simulations performed with the variable-resolution (zoomed) version of the LMDZ climate model. Simulations are compared with aerosol amounts recorded by the Aerosol Robotic Network (AERONET) and with vertical profiles of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements. LMDZ allows a thorough examination of the respective roles of deep convective transport, convective and stratiform scavenging, boundary layer transport, and advection processes on the vertical mineral dust distribution over Western Africa. The comparison of simulated dust Aerosol Optical Depth (AOD) and distribution with measurements suggest that scavenging in deep convection and subsequent re-evaporation of dusty rainfall in the lower troposphere are critical processes for explaining the vertical distribution of desert dust. These processes play a key role in maintaining a well-defined dust layer with a sharp transition at the top of the SAL and in establishing the seasonal cycle of dust distribution. This vertical distribution is further reshaped offshore in the Inter-Tropical Convergence Zone (ITCZ) over the Atlantic Ocean by marine boundary layer turbulent and convective transport and wet deposition at the surface.