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

This work aims to identify a mechanism of interaction between soil moisture (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 US Great Plains. A uniformly dry SM patch at a 5° × 5° scale is centered at the point of a documented MCS initiation to observe spatiotemporal changes of the simulated MCS events, totaling 97 cases between 2004 and 2017. A storm-centered composite analysis of SM at the location of simulated MCS events depicted SM heterogeneity [O(100) km] structured as significantly drier soils to the southwest (SW) transitioning to wetter soils northeast (NE) of the mean simulated initiation. Further analyses showed that this SM configuration influenced near-surface fluxes, which created a gradient of 2m-temperature and 2m-humidity, also aligned SW-to-NE, which affected the growth of the planetary boundary layer to trigger MCS initiations earlier in time (∼1-2 hr on average) compared to Control simulations. The implementation of the dry SM perturbation introduced drier-to-wetter SM gradients along the edges of the perturbed area, and MCS initiations were subsequently preferred on the drier side of those transition zones, with the most common orientation of simulated MCS events embedded within southwesterly flow. These results emphasize the importance of the low-level wind field alignment to organized SM gradients, which suggests that SM heterogeneity can drive MCS initiation related to near-surface atmospheric variable fluctuations as the main mechanism of interaction in weakly forced synoptic environments.