Sensitivity of the soil moisture initialization in the genesis of two simulated mesoscale convective systems

This study examines the sensitivity of the horizontal heterogeneities of the soil moisture initialization (SMI) in the cloud-resolving grid of two real-data mesoscale convective system (MCS) simulations during their genesis phase. We used a nested grid setup similar to some of the current realtime forecast models. Both systems were quasi-stationary. One system (Case 980726) formed in the Texas/Oklahoma border with a lifetime of 9h. The second system (Case 990802), with origins in western Oklahoma, also had a lifetime of 9h.; Soil moisture for the cloud-resolving grid was derived from the Antecedent Precipitation Index (API) using 4-km grid spacing precipitation data for a three-month period. In order to test the sensitivity of the heterogeneities of the SMI in the cloud-resolving grid, (i) Barnes objective analysis was used to alter the resolution of the SMI, (ii) the amplitude of the soil moisture field was reduced by 50%, (iii) the position of a soil moisture anomaly was altered, and (iv) two experiments with homogeneous soil moisture (31% and 50% saturation) were performed.; Large-scale forcing provided the favorable environment for convection to develop, but the distribution of the soil moisture determined where convection was likely to occur. The soil moisture anomalies generated physiographic-induced mesoscale systems (PIMSs), analogous to sea breeze, due to differential surface heating, and they assisted in organizing the convection as the MCS was developing. The larger soil moisture anomalies were more influential in initiating and/or interacting with convection. As the initial soil moisture was smoothed, the PIMSs associated with the larger soil moisture anomalies started to strengthen, but as the smoothing reached a cutoff wavelength of 80 km, the PIMSs began to weaken. Although the effects of the smaller soil moisture anomalies were not negligible in initiating and/or enhancing convective precipitation, they tended to lose their signatures with the smoothing operation. In the experiments, a negative feedback existed between wet soil and convective precipitation which tended to suppress convection over wet soil but favored convection at the periphery of the wet soil.