High resolution modelling of stable boundary layers over complex terrain

Near-surface temperatures, fog formation and pollutant trapping associated with stable night-time conditions over complex terrain are frequently poorly forecast and yet are often associated with adverse economic and safety impacts in the UK. Such conditions are often associated with cold air pooling and drainage flow episodes within the valleys. Improving forecasting of such conditions and also their representation in coarse resolution numerical weather prediction (NWP) and climate models are strategic priorities for the Met Office. This collaborative project addresses important scientific questions through work by academic and Met Office researchers and follows this through to operational implementation of model improvements. The science is challenging because the small scale of the terrain means that drainage flows, gravity waves and hence stable boundary layer (SBL) variability and fog formation all occur on small spatial scales, which are often smaller than the resolution of current forecast models. More fundamentally, the physical processes leading to the decoupling of the air in a valley from the flow aloft are not well understood. In this project high-resolution model simulations will be conducted and compared with recent field observations in order to improve the representation of processes leading to the formation of cold air pools and valley fog. The recent COLPEX field campaign provides a unique set of high density, medium term observations over moderate scale terrain typical of the UK. This approach will deliver validated improvements to forecasting capability. Integration at all stages between the academic and Met Office collaborators will ensure that the scientific advances are translated into improved forecast skill.