Convective Storms over an Idealized Synoptic Scale Soil Moisture Gradient with Horizontal Mean Wind

Abstract:

The Great Plains region typically exhibits an east-west soil moisture gradient with the prevailing westerly winds blowing from the dry, western side to the wet, eastern side. Some of the highest recorded values of convective available potential energy (CAPE) have been observed in this region. This study investigates the causal role of synoptic scale (O(10³ km)) soil moisture gradients on the frequency and intensity of severe storm activity using three-dimensional cloud-permitting model simulations of an idealized representation of the region. Specifically, simulations are conducted using the System for Atmospheric Modeling (SAM) over a homogeneous land surface with long channel geometry. A fixed, synoptic scale soil moisture gradient, and a fixed horizontal mean wind, are both imposed along the domain’s longest axis. A key advantage of these simulations is that they focus on soil moisture effects on precipitation over synoptic scales, which have received much less attention compared with the smaller scales typically studied using cloud-permitting simulations. The soil moisture gradient is varied in three ways, while holding the total domain soil moisture constant: (i) varying the maximum soil moisture; (ii) varying the minimum soil moisture; and (iii) varying the horizontal length of the soil moisture gradient. Sensitivity tests are also conducted to evaluate the effects of changes in the mean wind, horizontal resolution, and microphysics scheme, and the presence or absence of the diurnal cycle. The results are expected to provide insight into the role played by soil moisture gradients—from mesoscales to synoptic scales— in causing extreme CAPE over the Great Plains.