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Keywords:

  • West African monsoon;
  • squall line;
  • soil moisture–atmosphere interaction;
  • mesoscale circulation;
  • mesoscale numerical model;
  • African Monsoon Multidisciplinary Analysis (AMMA)

Abstract

A sensitivity analysis of a moist convection case-study of the 2005 West African monsoon has been performed with the limited-area model PROMES. In the control simulation, soil moisture was initialized based on European Centre for Medium-Range Weather Forecasts (ECMWF) soil moisture. A relatively wet patch appears over an area that was affected later by the passage of a squall line. The control simulation is able to reproduce a squall line that crosses over this wet surface, which makes it possible to analyse the sensitivity of the modelled convective system to a soil moisture increase up to saturation in the cited patch. The wetter land surface in the sensitivity run generates a cooler and moister area in the near-surface atmospheric fields compared to the control run, and the surface wind differences between both runs show a divergent pattern. The humidity and temperature differences follow the diurnal cycle of the monsoon flow, as the cool and moist atmospheric perturbation remains over the wet patch during the afternoon, and moves towards the northeast during the night. There is a clear interaction between the soil moisture perturbation and an approximating African Easterly Wave. This interaction generates larger differences between the two runs on the second simulation day than on the first. The precipitation is reduced over the wet patch in the sensitivity run, but over a larger area a precipitation increase is obtained, reflecting a complex soil moisture-precipitation feedback. Copyright © 2009 Royal Meteorological Society