Identifying the Mechanism of Interaction Between Soil Moisture State and Summertime MCS Initiations in Weakly-Forced Synoptic Environments Using Convective-Permitting Simulations

Mesoscale convective systems (MCS) are known to develop in strongly-forced synoptic environments, but recent work has shown that boreal summer MCS events in the U.S. Great Plains often occur in weakly-forced synoptic environments, and convection in this case may be triggered by anomalous soil moisture (SM) conditions, and the presence of organized SM heterogeneity. The main goal of this work is to identify and understand a mechanism of interaction between SM state and the incidence of weakly-forced synoptic scale MCS events during boreal summer by performing a sensitivity study using the Weather Research and Forecasting (WRF) model over the U.S. Great Plains. A uniformly dry SM patch (set to a near-wilting point) at a 5° X 5° scale is centered in the model domain at the point of an observational MCS initiation to observe the effect of the SM perturbation on the location and timing of the simulated MCS events comparatively to a corresponding control simulation, for a total of 97 separate cases. A storm-centered composite analysis of a uniformly dry SM perturbation at the 5° X 5° scale showed that the perturbation tended to accentuate the commonly occurring SM heterogeneity [O(100)km] structured as significantly drier soils to the southwest (SW) of the mean initiation, transitioning to wetter soils northeast (NE) of the mean initiation. Further analyses showed this configuration of SM influenced the values of near-surface atmospheric variables, such that a gradient of 2m-Temperature and 2m-Humidity, aligned SW-to-NE coinciding with the SM configuration, affected the growth of the planetary boundary layer to trigger MCS initiations. Consequently, the tracked events occurred earlier in time (~1-2 hours on average) compared to Control simulations. The implementation of the dry SM perturbation in the model domain introduced drier-to-wetter SM gradients along the edges of the perturbed area, and MCS initiations were subsequently preferred on the dry side of those transition zones depending on the direction of the mean near-surface wind field, with the most common orientation of MCS events on average initiating in the northeastern quadrant of the SM perturbation embedded within southwesterly flow. These findings are similar to previous results obtained in the Sahel that suggest SM heterogeneity can drive MCS initiation, with meso-β circulations related to near-surface variable fluctuations as the main mechanism of interaction.