Soil Moisture Influences on Warm-Season Convective Precipitation for the Midwestern U.S. Corn Belt

Abstract:

Recent climatic studies for the dominantly rain-fed agricultural Midwest U.S. Corn Belt suggest an influence of land use/land cover (LULC) spatial differences on convective processes set within the synoptic atmospheric conditions. However, the potential role of soil moisture (SM) content in the LULC association with atmospheric humidity, horizontal wind and convective precipitation (CVP) has received more limited attention, mostly as modelling studies or empirical analyses for regions non-analogous to the Corn Belt. Accordingly, we determine the associations between SM and the near-surface atmospheric humidity (q), and horizontal wind (V) at four Corn Belt locations having high resolution SM data for the nine warm-seasons (May-September) of 2011-2019. To identify recurring configurations of SM - q - V conducive to CVP, statistical analyses are undertaken for the warm-season as a whole, and also stratified into three phenologically distinct sub-seasons (early-, mid-, and late-season).

We show that Corn Belt CVP occurs preferentially with high humidity and strong, southerly winds frequently comprising a low-level jet (LLJ), particularly on early-season days having low SM and late-season days having high SM. These results likely owe to enhanced sensible heat fluxes for the early-season, and high latent heat fluxes given sufficient SM and atmospheric moisture for late-season days (i.e., weak static stability and associated high values of Convective Available Potential Energy, CAPE). Additionally, CVP days during the mid-season that have weaker winds (i.e., non-LLJ) tend to be associated with medium SM content and high humidity. The results for the three sub-seasons are likely explained by the sensible and latent heat fluxes varying according to LULC type; in particular, strong sensible heating over croplands during the early-season promotes instability (high CAPE and upward vertical motion of air). These empirical results provide a basis for mesoscale modeling studies of Corn Belt SM and CVP interactions to further clarify the associated physical processes.