Influence of external meteorology on nocturnal valley drainage winds

Abstract

We are able to describe changes in the depth of drainage and volume in flux in terms of the influence of external wind and radiative effects on the collection of cool air in a valley airshed, and on erosion of an established drainage by turbulent entrainment. We describe evidence for internal buoyancy waves and rotors that can have a major effect on transport and dispersion in the nocturnal cool-air drainage regime. The transition layer that bounds the top of the drainage depends on the thermal stability and wind speed and direction at ridge level. Standing internal waves, predicted a Froude number criterion, may govern the encroachment of ambient air into the local valley regime. Brush Creek Canyon is a narrow-steep-walled valley that may frequently exhibit a shear-induced helix imposed upon the down-valley drainage. This circulation is supported by meteorological and tracer data and can significantly influence pollution distributions. -from Authors