Investigating the typical occurrence of cold-air-pools during the COLd air Pool EXperiment (COLPEX)

Cold-air-pools in complex terrain can form within a dip, hollow, sink hole, mountain basin or valley and are a well known meteorological phenomena. The most telling sign for cold-air-pool existence is a well defined layer of fog or frost seen at the bottom of a valley. Cold-air-pools are characterised by a temperature inversion and a stable boundary-layer. Significant vertical temperature differences can occur over relatively small spacial scales (0.1 ^oC/m). They begin to form around sunset when ideal weather conditions prevail; conditions often synonymous with high atmospheric pressure. In the majority of cases cold-air-pools decay soon after sunrise as the convective boundary layer grows during the morning transition.

Specifically, cold-air-pool presence can be a cause of heightened concern at night-time, when their occurrence is mostly observed. This is especially true during winter months when the risk of localised frost and/or fog formation is higher, therefore leading to increased risk of dangerous driving conditions. Cold-air-pools, and the hazards associated with their occurrence, are particularly difficult to forecast using current operational weather forecast models, due to the fact that the models cannot represent the high variability of their occurrence over small spacial scales. This talk will present results from the COLd air Pool EXperiment (COLPEX). The COLPEX project is a collaboration project between the UK Met Office, the University of Leeds and the National Centre for Atmospheric Science (UK). COLPEX is a combination of both field experiments and model investigations of cold-air-pools. The COLPEX field experiment was conducted from Summer 2009 to Spring 2010, in the Clun valley region of Shropshire, UK. The aim of COLPEX is to improve scientific understanding of cold-air-pools and assess the predictive capability of the Met Office Unified Model for cold-air-pool representation in moderate valley systems; which typify many regions across the UK. This talk will focus on the observations gathered during the COLPEX field experiment. Results will have an emphasis on understanding the “typical” occurrence of cold-air-pools in the Clun Valley region; in part this will be achieved by investigations of the surface energy budget. Climatology investigations of cold-air-pool occurrence will be presented – how does cold-air-pool strength relate to the large scale pressure gradient and geostrophic wind?