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

Abstract

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 (from 2200 UTC, 26 July to 0700 UTC, 27 July 1998). The other system (Case 990802), also with a lifetime of 9 h, initiated in western Oklahoma around 1945 UTC, 2 August 1999 and dissipated around 0445 UTC, 3 August 1999. 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 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. All of the experiments in Case 980726 with heterogeneous SMI produced a MCS with a quasi-circular cloud shield, similar to the observed timing, size and location. Even the experiment with a homogeneous SMI at 31% saturation produced a MCS with a quasi-circular cloud shield. However, convection was delayed in the experiment with a homogeneous SMI at 50% saturation, and the evolution of the convective system differed substantially from the experiments with heterogeneous SMI or homogeneous SMI at 31%. The experiments in Case 990802 did not perform as well as Case 980726, but the model was able to reproduce some aspects of the observed system. 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. Long-lived convective cells tended to track around the wet soil and to develop at the periphery of the wet soil.