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Keywords:

  • soil moisture;
  • Sudanian zone;
  • convection initiation;
  • COSMO model;
  • African Monsoon Multidisciplinary Analyses (AMMA);
  • feedback mechanism

Abstract

Cloud-resolving real-case simulations initialized with European Centre for Medium-range Weather Forecasts (ECMWF) analysis data were performed to investigate the sensitivity of a mesoscale convective system (MCS) to soil properties. Four scenarios were investigated: original ECMWF soil moisture (MOI); ECMWF soil moisture reduced by 35% (CTRL), which better fits with the satellite observations; homogeneous soil type and soil moisture (HOM); and homogeneous soil type with a north–south oriented drier band of two degrees width (BAND).

Initiation of convection was observed in all experiments. The initiation area was characterised by very low convective inhibition (CIN) and high convective available potential energy (CAPE). The simulations showed some evidence that convection was initiated in the vicinity of orography and along soil moisture inhomogeneities. Except for the MOI run, MCSs developed in all experiments. In CTRL, a weakening of the system was observed when approaching an area with reduced total water content and enhanced CIN. In BAND, convection developed almost simultaneously in the original initiation area and inside the dry band, where convergence, generated by thermally-induced circulation and supported by downward mixing of momentum from the African Easterly Jet, triggered convection. When the mature MCS approached the band from the east it weakened, caused by increasing CIN ahead of the band. Triggering of convection on this day was favoured by drier surfaces and/or soil moisture inhomogeneities, while a mature system weakened in the vicinity of a drier surface. Thus a positive feedback between soil moisture and convection existed for a mature system whereas a negative feedback was found for triggering of convection. Copyright © 2009 Royal Meteorological Society