Three‐dimensional model study of organized mesoscale circulations induced by vegetation

Abstract

Organized mesoscale circulations induced by vegetation discontinuities and their effects on the vertical transport of heat and moisture are investigated under realistic atmospheric conditions. A regional climate model (RegCM2) is used to simulate local circulations which result from contrasts in surface thermal forcing over land. The limited area domain, with a grid resolution of 10 km, is forced at the boundaries by analyses of observations for the 30‐day period of June 1990. Surface vegetation and soil moisture are idealized to provide strong, geometric contrasts in surface heating. Control (homogeneous surface) and perturbation (surface thermal contrast) simulations are required to isolate the circulation signal. Differences in the wind, temperature, and humidity fields show the diurnal evolution of coherent circulations during the simulated month. On three days, circulations having strength similar to a land‐sea breeze occur. Vertical fluxes of heat and moisture, which result from the circulations, show a different vertical structure and diurnal development than turbulent fluxes. The vertical transport of moisture is found to be comparable to or larger than transport by turbulent processes. Vertical transport of heat by the circulations is small. An increase is seen in the monthly total large‐scale precipitation downwind of the ascending region of the circulations. Of the large‐scale forcings studied, the circulation is most closely correlated to the mean horizontal temperature gradient.