The diurnal cycle and African easterly waves: A land surface perspective

Abstract

AbstractFluxes from the land surface into the atmosphere exhibit strong variability on diurnal and daily time‐scales during the Sahelian wet season. The influence of this variability on the atmosphere is examined here using a general circulation model. A land‐surface scheme with an improved description of the sparse vegetation which characterizes the Sahel is introduced, and compared with simulations using existing parameters. With smaller evaporation rates, the sparse vegetation results in a warmer and deeper planetary boundary layer (PBL) in better agreement with observations. The diurnal cycle in simulated rainfall is much weaker, due to less frequent triggering of daytime convection. The damping of diurnal variability is accompanied by enhanced daily variability, with a deep moist PBL developing over several days. Above the sparsely vegetated surface, there is also greater African easterly wave (AEW) activity, and more long‐lived rain events associated with it.A mechanism is proposed to explain the increased frequency of AEWs above sparser vegetation, based on wave composites in the model. Surface heating of the lower atmosphere is modulated strongly by AEWs. This occurs via a surface‐convection‐radiation feedback. Estimates of the boundary‐layer heat budget indicate that variations in surface heating can, to a large extent, account for the low‐level temperature signal characteristic of AEWs. By reinforcing the low‐level temperature anomalies, the surface may influence the meridional wind signature of the wave above the PBL. Further evidence of the close coupling between the land surface and atmosphere is found by examining the modulation of precipitation by AEWs. Surface heat and moisture fluxes influence the build‐up of convective available potential energy ahead of the wave trough. In this way, the properties of the surface can affect the timing and intensity of rainfall associated with, an AEW.