The Effect of Heterogeneous Soil Moisture on a Summer Baroclinic Circulation in the Central United States

Abstract

Thermally induced circulations, similar to sea breezes, may be established in the presence of horizontal gradients in soil moisture, soil type, vegetation, or snow cover. The expense of extensive observational networks and the relatively small-scale circulations involved has made examining these circulations very difficult. Recent numerical studies have indicated that sharp gradients in soil or vegetation properties may induce mesoscale circulations in the absence of synoptic forcing. The current study employed a three-dimensional, hydrostatic mesoscale model to evaluate the effects of horizontally heterogeneous soil moisture and soil type on the passage of a summer cold front in the central United States. Grid-scale condensation, precipitation, latent heat release, and cumulus conviction are not accounted for in this model; moisture was affected only by advection, diffusion, and evaporation. Numerical simulations demonstrated that evaporation of soil moisture significantly affected the boundary layer structure embedded in the baroclinic circulation. Although the position of the front was not altered, the thermal and momentum fields were effected enough to weaken the front near the surface. Evaporated soil moisture was advected ahead of the cold front, far from its source region. Moisture convergence was significantly enhanced in several locations, indicating that soil moisture may play an important role in modifying the spatial distribution and intensity of precipitation. The impact of surface inhomogeneities in soil moisture and soil type on the atmosphere is expected to be highly dependent on the particular synoptic conditions.