Valley Winds and the Diurnal Circulation over Plateaus

Abstract

The role of valley winds in the diurnal circulation of large-scale mountain massifs like the Tibetan Plateau is investigated. We consider a circular plateau with radial valleys leading to the surrounding lowlands. A low-resolution grid point model is used to compute the diurnal circulation above the plateau and in the valleys. The circulation is driven by the diurnal variation of the surface temperature. The model produces valley winds and a diurnal circulation above the plateau in qualitative agreement with observations. The inflow towards the plateau in the valleys is strongest in the afternoon. There is nocturnal outflow. During the day, a low pressure system with a corresponding cyclonic circulation around the massif resides at the plateau and we find high pressure well above it. At night it is the reverse. It is demonstrated that the valley winds contribute significantly to the fluxes of mass and moisture in the plateau's diurnal circulation. Linear calculations for a neutrally stratified atmosphere and narrow valleys show that it is mainly the elevation of the sources and sinks of heat at the plateau which is responsible for the circulation.