The effect of advection and accumulation of downslope cold air on nocturnal cooling in basins

Abstract

The effect of advection and accumulation of cold air from surrounding mountain slopes on the nocturnal cooling at the bottom of basins was studied with the use of one-dimensional numerical model. In the model, horizontal advective cooling was parameterized by using the results of heat budget analyses and power law profiles of air temperature. Calculated results show that the nocturnal cooling in the basin, the depth of which is 500m for example, is about 15% (in winter) and about 25% (in summer) larger than that at flat terrains while it is about 40% (in winter) and about 90% (in summer) larger than that at the top of mountains or hills. These excesses of the nocturnal cooling in basins are due to the difference in downward long wave radiation which is caused by the advection and accumulation of cold air inside basins. This effect of cold air accumulation on the nocturnal cooling is remarkable in deep basins. On the other hand, the nocturnal cooling at the foot of a mountain or in valleys is small compared with that in basins. This can be explained by the difference in the amount of cooling due to the advection and accumulation of downslope cold air among those topographies. Calculated results agreed well with observed values.