The interaction between moist diabatic processes and the atmospheric circulation in African Easterly Wave propagation. (29th November 2017)

• Record Type: Journal Article
• Authors: Tomassini, Lorenzo; Parker, Douglas J.; Stirling, Alison; Bain, Caroline; Senior, Catherine; Milton, Sean
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 143:Number 709(2017)
• Page Start: 3207
• Page End: 3227
• Publication Date: 2017-11-29
• Subjects: African easterly waves -- interaction between moist convection and circulation -- convective organization; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3173
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract : An objective tracking algorithm is used to characterize the three‐dimensional structure of African Easterly Waves (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 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 updraughts 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. Boundarylayer 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. Abstract : The interaction between moist diabatic processes and the atmospheric circulation in African Easterly Waves (AEWs) is investigated in detail. 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, which generates small‐scale convergence centres and moist instabilities ahead of the trough.