Observations and Simulations of Cold Air Pooling in Valleys

Abstract

A series of surveys was carried out with an instrumented vehicle along the A361 in south-west England between 20 January and 4 April 2007, primarily during clear calm nights in order to observe the formation of pools of cold air in valleys. High resolution simulations were performed for three of these cases using the Met Office Unified Model (MetUM®), which was shown to successfully reproduce many of the features of the observed along-route variations in air temperature. Analysis of the observations and the results of the simulations suggest that the cooling of air within these moderate valleys was the result of sheltering, which reduces the turbulent flux of heat down from aloft. Valleys that experience a large degree of cooling are those that become fully decoupled, allowing the near-surface air within the valley to cool rapidly in response to the surface longwave radiative cooling. The behaviour of the valley cooling intensity is shown to be similar to that observed in idealised simulations of the sheltering mechanism (Vosper and Brown, 2008, Boundary-Layer Meteorol 127:429–448).