The impact of soil moisture inhomogeneities on the modification of a mesoscale convective system: An idealised model study

摘要

In order to investigate the sensitivity of a mesoscale convective system (MCS) to soil moisture inhomogeneities in West Africa, convection-permitting simulations with the COSMO model are performed. Three scenarios are investigated in detail: a homogeneous soil texture and soil moisture distribution and a homogeneous soil texture field with a north-south oriented band of two degrees width with reduced and increased soil moisture, respectively. In all experiments an MCS develops in the late afternoon some hundred kilometres east of the band with modified soil moisture. About 100 km east of the band, significant differences in precipitation occur: when the MCS approaches the band with lower soil moisture, precipitation decreases because of higher convective inhibition (CIN) and higher saturation deficit above the lifting condensation level ahead of the band. Reaching the dry band, precipitation restarts. The moist band causes an increase of precipitation ahead of the band and a decrease over it due to very high CIN and saturation deficit values above the boundary layer. In both cases, the soil moisture patterns induce a secondary circulation which modifies the conditions in the lower troposphere. These altered conditions are responsible for the changes of the convection-related parameters and for the modification of the MCS. Additionally, the dry band causes the evolution of precipitating cells over its western part in the late afternoon. The cells are triggered by superimposed convergences, one generated by the thermally-forced circulation of the dry band and the other one caused by different boundary-layer depths over the dry band and its surroundings. The very deep boundary layer over the dry band is accompanied by downward mixing of momentum from the African Easterly Jet, resulting in a reduced westerly monsoon flow so that convergence and divergence zones develop at the transitions from the deeper to the lower boundary layers.