A study of the effects of sub-grid-scale land use differences on atmospheric stability in prestorm environments

The effects of sub-grid-scale land use differences in modifying the stability of prestorm environments has been studied using data from the Department of Energy Cloud and Radiation Testbed (CART) in Kansas and Oklahoma and a mesoscale model. To quantify the atmospheric instability, three indices were used: the lifted index, the modified K index, and convective available potential energy. The spatial variations of these indices were calculated from simulations using spatially varying and spatially uniform surface fluxes as lower boundary conditions. The CART is approximately 10(5) km(2) in area and is characterized by large areas of contrasting vegetation cover and surface sensible and latent heat fluxes. The spatially varying fluxes were calculated with the SiB2 model using data from the CART. Six days, during which isolated thunderstorms developed, were chosen for the study. The results suggest that sub-grid-scale variations in land use differences do not modify the spatial distribution of the stability indices in the southern Great Plains to any significant degree. Predictions of areas of preferred development of deep convection, based on changes in the indices, are not improved by accounting for sub-grid-scale variations in land use. The indices were found to be potentially useful but imperfect indicators of the occurrence of deep convective precipitation at scales smaller than that of the CART. At the model resolution used (grid spacing of 2.08 km) a close correlation was not found between regions of precipitation and regions with enhanced simulated vertical updrafts related to land-use differences, but a detailed study of local triggering has not been carried out.