Numerical simulation of the interaction of a mesoscale convective system with the African Easterly Jet

Two sets of dropsonde flights along 2°E between 8°N and 19°N during the JET2000 field experiment documented a dramatic change in the structure of the African Easterly Jet (AEJ) following the passage of a mesoscale convective system (MCS). The AEJ was “split” into two branches with a lowering of the easterlies between the two branches that appear to be associated with convective momentum fluxes. In this study, the Weather Research and Forecasting (WRF) model is used to simulate and understand the modulation of MCS on the AEJ. The WRF model was implemented with 41 vertical levels and a triply nested domain with 81, 27 and 9 km horizontal grid spacing. It simulated a reasonable jet with a strong and confined core before the passage of the MCS, and the simulated African easterly wave on the southern flank of the jet has realistic horizontal structure tilt, vertical structure, and precipitation distribution. This southern wave triggered a MCS to the north of the jet, whose vertical motion, temperature, and moisture profiles resemble a squall system. The simulated meridional circulation indicates that the mesoscale convection weakened the easterly jet to the south and also generated a secondary jet to the north, which leads to a “split” of the jet structure. Momentum budget analysis suggests that the Coriolis force associated with the meridional circulation plays a major role in changing the jet structure. The simulation also shows strong sensitivity to the initial conditions.