Influence of External Meteorology on Nocturnal Valley Drainage Winds

Abstract

Thermally driven local circulation in valleys has been studied for many years with the result that the underlying physics are reasonably well understood. ASCOT experiments of the early 1980s were formulated to help quantify predictive models and to apply the resulting methods to transport and dispersion of airborne materials. During the performance and analysis of experiments in two quite different valleys, it became clear that important aspects of the structure of the valley circulation depend on subtle differences in the ambient atmospheric conditions. In this paper we interpret nocturnal drainage structure in terms of ambient characteristics. We are able to describe changes in the depth of drainage and volume 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. Under ideal conditions of radiative cooling and light ambient winds the drainage depth fills the valley to the ridge level. The radiative driving factor is strongly suppressed by low cloud ceilings and this is reflected in drainage depths shallower than 25% of the valley depth. Further erosion of cold air drainage by turbulent entrainment of ambient air under conditions of moderate to strong ridge top winds results in a linear regression of the form:where v is the ridge top wind in m s?1. With these dependencies on clouds and wind it is not surprising that the ?Climatology? of valley drainage winds favors the seasons of weak synoptic activity and low thunderstorm frequency. The transition layer that bounds the top of the drainage is variable in time and space and 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 Crek Canyon is a narrow, steep-waled 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.