Orographic flow representation in weather and climate models

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Orographic flows are atmospheric flows that are caused or influenced by nearby mountains. Examples include the wellknown foehn wind which brings warm dry air over and down mountains; barrier winds which transport air parallel to amountain range on the upwind side; and tip jets which accelerate winds around the ends of mountain ranges. Foehn winds are ubiquitous in mountainous areas such as the Alps, the Rockies, the Antarctic Peninsula and the Grampian mountains, where they have a profound influence on local weather conditions, for example increasing leeside temperatures by up 20oC. Barrier winds and tip jets are common adjacent to north-south oriented mountain ranges and especially in the Polar Regions, where they play a key role in coupling the atmosphere and ocean, so are a critical component of the climatesystem. The societal and environmental impacts of orographic flows are significant and diverse. Strong orographically-forced winds regularly cause damage to property and infrastructure and have implications for air-sea-ice interactions, sea ice distribution, ocean temperatures and ocean circulation. The warmth brought by foehn has implications for agriculture, ecosystems and climate systems. It can increase the risk of avalanches or floods, melt glaciers, and contribute to the disintegration of ice shelves. The combination of warm, dry air and high wind speeds during foehn conditions promotes the ignition and rapid spread of wildfires; indeed foehn conditions (Santa Ana's) are the most common weather situation during Californian wildfires. As such, accurate representation of orographic flows in meteorological models is a challenge not only for day-to-day weather forecasts in mountainous regions, but for hazard assessment and management, and one that is made significantly harder by uncertainty in their governing mechanisms and challenges in their representation in models. In this project the student will investigate orographic flows using numerical model simulations and observations gathered during aircraft-based field campaigns. They will explore the atmospheric response to different mountain flow regimes and assess their representation in weather and climate models. The CASE partner will be the Met Office, one of the world's leading weather forecasting agencies and one of the world's primary centres for climate research and services. The student will use the Met Office's Unified Model (the MetUM): a state-of-the art numerical weather and climate prediction model, which is used for operational weather forecasting, seasonal forecasting and climate modelling on all scales. They will receive comprehensive training and guidance in using the MetUM, providing skills that are highly sought after for a range of public and private sector organisations in the UK and abroad. The project will entail simulating a range of orographic flow case studies with the MetUM and examining in detail therepresentation of the orographic flows with different settings and configurations of the model. Observations of orographic flows will be available from two upcoming aircraft-based field campaigns, based out of Iceland and Greenland, and basedout of Alaska, where orographic flows are key campaign targets. In addition observations from a previous campaign in Antarctica will be available. The observations will be used to validate and tune model settings. The project aims to optimise orographic flow representation and establish generic findings on how best to parameterize (i.e.represent) certain key physical processes in weather and climate models. The impact of these findings could include changes in the MetUM settings for the operational weather forecasting or climate versions of the model or the development of new parameterizations for the impacts of orographic flows.