The dynamics of cold pools in narrow valleys

During calm, clear nights the air temperatures in hollows and valleys can fall several degrees lower than across the surrounding hill tops. These cold pools are associated with hazardous conditions such as localised fog, icy road surfaces and poor air quality. Accurate forecasting of such phenomena is important. However, the small-scale nature of cold pools means they are often poorly represented in even the highest resolution operational numerical weather prediction (NWP) models.

The COLd-air Pooling EXperiment (COLPEX) is a research programme concerned with understanding and improving predictions of valley cold pools during stable nighttime conditions. The field phase of COLPEX has involved long-term detailed measurements of atmospheric flows in and around a small-scale valley; the Clun Valley in Shropshire, UK.

Very high resolution simulations of the flows observed during COLPEX have been conducted using the Met Office Unified Model. Nested simulations, for which the inner-most grid has a horizontal grid spacing of 100 m, have been performed for a number of cold-pool and fog episodes within the Clun Valley. Comparisons between the model predictions and the COLPEX observations suggest that in general the simulated cold pools are very realistic.

The model results have been used to study the key physical processes involved in cold pool formation within the Clun Valley and to address some fundamental questions regarding cold pool dynamics. For example, to what extent is the cooling controlled by local processes at the valley bottom, such as radiative loss of heat from the ground and turbulent mixing of heat in the air above? Or is the drainage of cold dense air down from the hill tops and side valleys important in establishing the cold pool in the valley?

Results will be presented which shed light on the above questions. The implications for numerical weather prediction will be discussed.