The interaction between moist diabatic processes and the atmospheric circulation in African Easterly Wave propagation

An objective tracking algorithm is used to characterise the three-dimensional structure of African EasterlyWaves (AEWs) in ERA-Interim reanalysis and a Met Office Unified Model (UM) simulation. A special focus is dedicated to the coupling of dynamical aspects of the wave and moist convection. The relation between baroclinic features of the wave and latent heating is explored. Latent heating at and slightly ahead of the wave trough is found to reinforce and sustain the anomalous wave circulation through potential vorticity (PV) generation and vortex stretching. The coupling of moist processes and the circulation takes place mainly through moisture convergence at lower mid-tropospheric levels, between 850 hPa and 500 hPa. These findings are confirmed and examined in more detail in a case study of a strong AEW based on high-resolution UM simulations. PV tracers are used to investigate how different moist diabatic processes invigorate the wave. Again moisture anomalies are found to be the main contributors to generating small-scale convergence centres and updrafts ahead of the trough at mid-tropospheric levels. Although buoyancy effects are ultimately

responsible for the convective uplift, the results suggest that mesoscale circulations associated with the AEW dynamics are crucial in creating the small-scale moist static instabilities and vortices which are essential for the AEW maintenance. Boundary layer mixing and advection from the northern Sahel may create pockets of high-PV air around the trough in some instances, but this mechanism of wave sustainment needs further investigation.