High resolution modelling of stable boundary layers over complex terrain

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

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.

Publications

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Hughes J (2015) Assessment of valley cold pools and clouds in a very high resolution NWP model in Geoscientific Model Development Discussions

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JemmettSmith B (2019) A casestudy of coldair pool evolution in hilly terrain using field measurements from COLPEX in Quarterly Journal of the Royal Meteorological Society

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Price J (2011) COLPEX: Field and Numerical Studies over a Region of Small Hills in Bulletin of the American Meteorological Society

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Vosper S (2014) Cold-pool formation in a narrow valley Cold-pool Formation in a Narrow Valley in Quarterly Journal of the Royal Meteorological Society

 
Description Understanding of the processes leading to cold air pooling in small (UK) scale valleys.
Identification of physical issues with the current scheme to parametrise these flows in weather and climate models.
Identified issues with the way sub-grid humidity and cloud is handled in new high resolution forecasts / simulations.
Exploitation Route Work is being followed up by Met Office to improve parametrisation of drainage flows in NWP / climate models.
Also potential future work to look at the impact of sub-grid relative humidity in high resolution forecasts / simulations.
Process understanding is also feeding in to downscaling tools to improve route-based forecasing of road icing.
Follow on project focussing on fog formation in valleys linked to cold air pooling.
Sectors Environment,Transport,Other

 
Description Project was in close collaboration with the Met Office. The process based understanding from this research is being used to improve route based forecasing of road icing at the Met Office. The project has also identified several issues with current parametrisations of cold pools and drainage flows in weather and climate models. Further work by the Met Office will build on this research to rectify these issues. This project has also led to a follow on field experiment led by the Met Office focussing on fog formation and we have a NERC open CASE PhD studentship working on this.
First Year Of Impact 2013
Sector Environment,Transport
Impact Types Societal