Growing-Season Synoptic and Phenological Controls on Heat Fluxes over Forest and Cropland Sites in the Midwest U.S. Corn Belt

Spatial variations in land use/land cover (LULC) in the Midwest U.S. Corn Belt—specifically, deciduous forest and croplands—have been suggested as influencing convective rainfall through mesoscale circulations generated in the atmosphere’s boundary layer. However, the contributing role of latent and sensible heat fluxes for these two LULC types, and their modulation by synoptic weather systems, have not been determined. This study compares afternoon averages of convective fluxes at two AmeriFlux towers in relation to manually determined synoptic pressure patterns covering the nine growing seasons (1 May–30 September) of 1999–2007. AmeriFlux tower U.S.-Bo1 in eastern Illinois represents agricultural land use—alternating between maize and soybean crops—and AmeriFlux tower U.S.-MMS in south-central Indiana represents deciduous forest cover. Phenologically, the latent and sensible heat fluxes vary inversely across the growing season, and the greatest flux differences between cropland and deciduous forest occur early in the season. Differences in the surface heat fluxes between crop and forest LULC types vary in magnitude according to synoptic type. Moreover, statistically significant differences in latent and sensible heat between the forest and cropland sites occur for the most frequently occurring synoptic pattern of a low pressure system to the west and high pressure to the east of the Corn Belt. The present study lays the groundwork for determining the physical mechanisms of enhanced convection in the Corn Belt, including how LULC-induced mesoscale circulations might interact with synoptic weather patterns to enhance convective rainfall.