Diagnosis of sub-grid valley cold air pools from numerical weather prediction (NWP) forecasts

On clear, calm nights, cold air pools form in valley bottoms, resulting in exaggerated temperature minima. This can cause problematic conditions such as frost and fog, crop damage, and has implications for the behaviour of future microclimates. Valleys in which cold pools form are frequently smaller than the grid size of operational forecast models. However, post-processing techniques may by applied to the gridded NWP data in order to downscale temperature forecasts and obtain more accurate overnight temperature minima in valleys and on hilltops. Met Office forecasts employ a downscaling algorithm which corrects for model height errors and simulates parametrically the formation of cold pools. Recent improvements to the initially simple scheme will be discussed.