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Keywords:

  • convective initiation;
  • diurnal cycle;
  • semi-arid conditions;
  • shallow to deep convection transition;
  • West African monsoon

Abstract

A modelling case study designed from observations from the African Monsoon Multidisciplinary Analysis (AMMA) is presented and discussed. It aims at investigating the issue of initiation of convection in a semi-arid environment. This case corresponds to the development of local daytime convection mainly controlled by boundary layer characteristics rather than by atmospheric synoptic scales. A high-resolution three-dimensional simulation is presented and extensively evaluated against the numerous observations available for 10 July 2006 from the AMMA campaign. The simulation, run over a domain of 100 × 100 km, is able to represent main boundary layer structures and processes leading to deep convection initiation as well as the formation of density currents. Sensitivity tests point to the key role of the sensible heat flux, the humidity of low to mid levels, the lapse rate at low levels and of a mesoscale ascent to initiate deep convection in those semi-arid conditions, while evaporation of precipitation is shown to play a minor role. This study thus provides a case to investigate the ability of parametrizations to handle the initiation of convection in a semi-arid environment. Copyright © 2011 Royal Meteorological Society