A Numerical Study of the Thermally Driven Plain-to-Basin Wind over Idealized Basin Topographies

Abstract

Numerical experiments have been carried out with a two-dimensional nonhydrostatic mesoscale model to investigate the diurnal temperature range in a basin and the thermally driven plain-to-basin winds. Under clear-sky conditions, the diurnal temperature range in a basin is larger than over the surrounding plains due to a combination of larger turbulent sensible heat fluxes over the sidewalls and a volume effect in which energy fluxes are distributed through the smaller basin atmosphere. Around sunset, a thermally driven plain-to-basin flow develops, transporting air from the plains into the basin. Characteristics of this plain-to-basin wind are described for idealized basins bounded by sinusoidal mountains and the circumstances under which such winds might or might not occur are considered. In contrast with a previous numerical study, it is found that the height of the mixed layer over the plains relative to the mountain height is not a critical factor governing the occurrence or nonoccurrence of a plain-to-basin wind. The critical factor is the horizontal temperature gradient above mountain height created by a larger daytime heating rate over the basin topography than over the plains. Subsidence and turbulent heat flux divergence play important roles in this heating above mountain height.