A short climatological study of cold air pools and drainage flows in small valleys

This paper uses detailed observations from the COLd air Pooling EXperiment (COLPEX) to study the frequency and characteristics of cold air pooling in a small-scale valley typical of much of the southern UK and other lowland regions of the world. The field experiment took place in and around the Clun Valley, Shropshire, England during July 2009-April 2010, which was a particularly cold winter with a record low in the NAO index. Cold pools, defined here as where the minimum valley temperature overnight is at least 1°C colder than the surrounding hill tops, occur on 45% of nights over the observational period, with strong cold pools (>4°C temperature difference) occurring on 12% of nights. As might be expected, cold pool formation is closely linked to conditions with clear skies and light winds, often associated with high pressure situations. Cold pools are also closely linked with weak down valley drainage flows. This contrasts with non-cold pool nights or daytime conditions where several other mechanisms also contribute to the observed winds in the valley. The data set highlights the importance of cold air pools and drainage flows, even in quite moderate terrain, and the impact this can have on local microclimates.