The impact of mesoscale convective systems on the surface and boundary-layer structure in West Africa: Case-studies from the AMMA campaign 2006

Within the framework of the AMMA project, atmospheric, surface and soil observations were performed during the pre-onset phase of the monsoon (Special Observing Period SOP 1) and during the summer monsoon (SOP 2) in 2006. Based on several case-studies the paper investigates the impact of mesoscale convective systems (MCSs) on the surface and boundary layer (PBL), the recovery time of the surface and the PBL after MCS passages, and the differences between the two monsoon phases. In the pre-MCS environment the mean conditional and convective instabilities were higher in SOP 1 than in SOP 2 (e.g. CAPE = 1815 J kg(-1) and CAPE = 1295 J kg(-1), respectively). In both monsoon phases these instabilities, which were strongly reduced by MCSs, recovered within less than 2 days. Precipitation of the MCSs and the resulting soil moisture increase caused a significant decrease in the surface temperature by up to 10 K and an increase in evapotranspiration by up to 2.5 mm d(-1). In both phases of the monsoon, these surface anomalies and, hence, the conditions favourable for triggering MCSs by thermally induced circulation systems, diminished largely within 2 to 3 days. Due to the repeated passage of the first MCSs at intervals of a few days during SOP 1, the surface properties exhibited trends towards higher soil moisture, evapotranspiration and humidity, and lower albedo, temperature and Bowen ratio. After two weeks only, the mean conditions present in the summer monsoon were reached. In SOP 2 no significant trends could be detected. Copyright (C) 2010 Royal Meteorological Society