Sensitivity of a Cloud-Resolving Simulation of the Genesis of a Mesoscale Convective System to Horizontal Heterogeneities in Soil Moisture Initialization

Abstract

This study examines the sensitivity of varying the horizontal heterogeneities of the soil moisture initialization (SMI) in the cloud-resolving grid of a real-data simulation of a midlatitude mesoscale convective system (MCS) during its genesis phase. The quasi-stationary MCS of this study formed in the Texas/Oklahoma panhandle with a lifetime of 9 h (2200 UTC 26 July to 0700 UTC 27 July 1998). Soil moisture for the finest nested grid (the cloud-resolving grid) was derived from the antecedent precipitation index (API) using 4-km-grid-spacing precipitation data for a 3-month period. In order to vary the heterogeneities of the SMI in the cloud-resolving grid, (i) Barnes objective analysis was used to alter the resolution of the soil moisture initialization, (ii) the amplitudes of the soil moisture anomalies were reduced, (iii) the position of a soil moisture anomaly was altered, and (iv) two experiments with homogeneous SMI (31% and 50% saturation) were performed. Because of the severe drought in the Texas/Oklahoma panhandle area, the saturation API value was lowered in order to introduce heterogeneities in the soil moisture for the sensitivity experiments. All of the experiments with heterogeneous SMI (in addition to an experiment with a homogeneous SMI at 31% saturation) produced an MCS with a quasi-circular cloud shield, similar to the observed timing, size, and location. The authors' findings suggest that a soil moisture dataset with approximately 40-km grid spacing may be adequate to initialize a cloud-resolving model for simulating MCSs. For the simulations in this study, the soil moisture distribution determined where convection was likely to occur. Wetter soil tended to suppress convection for this case, and convection preferentially occurred around the peripheries of wet soil moisture anomalies.