An advanced version 4.3 of the Regional Atmospheric Modeling System (RAMS) has been made available by Colorado State University to the scientific community. This version includes an improved Land Ecosystem-Atmosphere Feedback model (LEAF-2) to represent surface features including vegetation, soil, lakes, and oceans, and snow cover and their influence on each other and on the weather. The parallel version of RAMS 4.3 has been tested in multiprocessor computers to infer the feasibility of running simulations that require large grids at very small grid spacings such the large eddy simulations (LES). The purpose of this paper is to report on sensitivity experiments carried out using RAMS 4.3 at different horizontal and vertical grid spacings to test the influence of land-surface heterogeneities (vegetation and soil type) on circulations in the boundary layer. A target of study has been a cloudfree day of 23 July 1995 over the Atmospheric Radiation Measurement (ARM) Cloud and Radiation Testbed (CART) domain in Oklahoma and Kansas. Simulations with initial and boundary data from NCEP reanalysis have been conducted over a small domain at grid spacings ranging from 6.5-km to 50-m (LES). The coarse grid simulations show that soil and vegetation features are associated with relatively deep circulations in the boundary layer accompanied by transport of heat, moisture and momentum to upper levels. The preliminary LES results show more intense vertical motions (5 m/s) in the upward branches of convective cells (Benard-like convection) than in coarser grids. It is hypothesized that in neutral and unstable conditions these circulations can give origin to severe weather (storm initiation). LES work will be extended to days with cumulus clouds to test this hypothesis.