Potential Vorticity Generation by West African Squall Lines

Richard H. Johnson, Paul E. Ciesielski
2020 Monthly Weather Review  
TheWest African summer monsoon features multiple, complex interactions between African Easterly Waves (AEWs), moist convection, variable land surface properties, dust aerosols, and the diurnal cycle. One aspect of these interactions, the coupling between convection and AEWs, is explored using observations obtained during the 2006 African Monsoon Multidisciplinary Analyses (AMMA) field campaign. During AMMA, research weather radar operated at Niamey, Niger, where it surveilled 28 squall line
more » ... ems characterized by leading convective lines and trailing stratiform regions. Nieto Ferreira et al. found that the squall lines were linked with the passage of AEWs and classified them into two tracks, northerly and southerly, based on the position of the African Easterly Jet (AEJ). Using AMMA sounding data, we create a composite of northerly squall lines that tracked on the cyclonic shear side of the African Easterly Jet (AEJ). Latent heating within the trailing stratiform regions produced a mid-tropospheric positive potential vorticity (PV) anomaly centered at the melting level, as commonly observed in such systems. However, a unique aspect of these PV anomalies is that they combined with a 400-500 hPa positive PV anomaly extending southward from the Sahara. The latter feature is a consequence of the deep convective boundary layer over the hot Saharan desert. Results provide evidence of a coupling and merging of two PV sources – one associated with the Saharan heat low and another with latent heating – that ends up creating a prominent mid-tropospheric positive PV maximum to the rear of West African squall lines.
doi:10.1175/mwr-d-19-0342.1 fatcat:mgg4rjimrjhwjcm3uydvwtgdsy