Land surface processes play an important role in regulating diurnal variations of temperature and moisture in the planetary boundary layer. The surface processes play an essential role in the realistic simulation of near surface moisture convergence. Motivated by this requirement, an advanced and robust land-surface scheme was coupled to a cloud resolving model. The coupled-land-cloud resolving model was tested and evaluated in single column mode against observations from the Cabauw CEOP site in the Netherlands. Both the land and atmospheric conditions were allowed to evolve for two different time periods: (a) August 20-24, 2001 when no precipitation was observed, and (b) August 3-12, 2001 when precipitation events were distributed throughout. The first case is relatively simple as the complex interactions of radiation calculations and clouds were absent. The surface energy fluxes and the diurnal variations of land surface and air temperatures were found to be in good agreement with the measured data. The second period of simulation elucidates the sensitivity of results with respect to radiative processes, and especially the important role of the cloud fraction parameterization.

The model is next applied to the mesoscale simulation of the North American Monsoon over Central Mexico. The emphasis of this study is to understand the mechanisms responsible for the enhancement of moisture convergence at the ridge-valley scale in the Grande de Santiago river basin. Numerical simulations were performed for a 48 hour period using nested grids with the finest grid resolution being 1 km in horizontal directions and 10 m in the vertical. The results help assess the influence of land-atmosphere interactions (specifically latent and sensible heat fluxes), monsoonal moisture flux and land-sea contrast on moisture convergence patterns. The results suggest that the space-time organization of the precipitation features are regulated by the topographically induced gravity waves at large-scale, whereas at small-scales, the diurnal thermodynamic gradients and topography-flow geometry relationships were found to be more important. These features were compared and found to be close in agreement with the TRMM (Tropical Rainfall Measurement Mission) satellite data.