The Thermally Driven Cross-Basin Circulation in Idealized Basins under Varying Wind Conditions

The Weather Research and Forecasting model is used to perform large-eddy simulations of thermally driven cross-basin winds in idealized, closed basins. A spatially and temporally varying heat flux is prescribed at the surface as a function of slope inclination and orientation to produce a horizontal temperature gradient across the basin. The thermal asymmetry leads to the formation of a closed circulation cell flowing toward the more strongly heated sidewall, with a return flow in the upper part of the basin. In the presence of background winds above the basin, a second circulation cell forms in the upper part of the basin, resulting in one basin-sized cell, two counterrotating cells, or two cells with perpendicular rotation axes, depending on the background-wind direction with respect to the temperature gradient. The thermal cell near the basin floor and the backgroundwind-induced cell interact with each other either to enhance or to reduce the thermal cross-basin flow and return flow. It is shown that in 5–10-km-wide basins cross-basin temperature differences that are representative of eastand west-facing slopes are insufficient to maintain perceptible cross-basin winds because of reduced horizontal temperature and pressure gradients, particularly in a neutrally stratified atmosphere.