Orographic flow representation in weather and climate models

地形流是由附近山脉引起或影响的大气流。例子包括著名的焚风，它把温暖干燥的空气带到山上和山下;障碍风，它把空气平行地输送到逆风一侧的山脉;以及尖端喷气机，它加速了山脉末端的风。焚风在阿尔卑斯山、落基山脉、南极半岛和格兰扁山脉等山区无处不在，对当地的天气状况有着深远的影响，例如使背风面温度升高20摄氏度。障碍风和尖端急流在南北走向的山脉附近很常见，特别是在极地地区，它们在耦合大气和海洋方面发挥着关键作用，因此是气候系统的关键组成部分。地形流的社会和环境影响是显著和多样的。强烈的地形强迫风经常对财产和基础设施造成损害，并对空气-海冰相互作用、海冰分布、海洋温度和海洋环流产生影响。焚风带来的温暖对农业、生态系统和气候系统都有影响。它可以增加雪崩或洪水的风险，融化冰川，并有助于冰架的解体。焚风条件下温暖干燥的空气和高风速的结合促进了野火的点燃和快速蔓延;事实上，焚风条件（圣安娜）是加州野火期间最常见的天气状况。因此，在气象模型中准确表示地形流不仅对山区的日常天气预报，而且对灾害评估和管理都是一个挑战，而且由于其管理机制的不确定性和模型中表示的挑战，这一挑战变得更加困难。在这个项目中，学生将使用数值模型模拟和在基于飞机的实地活动中收集的观察来研究地形流。他们将探索大气对不同山流状态的反应，并评估它们在天气和气候模型中的代表性。CASE的合作伙伴将是气象局，世界领先的天气预报机构之一，也是世界上主要的气候研究和服务中心之一。学生将使用气象局的统一模型（MetUM）：最先进的数值天气和气候预测模型，用于所有尺度的业务天气预报，季节预报和气候建模。他们将接受全面的培训和指导，使用MetUM，提供的技能是非常抢手的一系列公共和私营部门组织在英国和国外。该项目将需要用MetUM模拟一系列的地形流案例研究，并详细研究在不同设置和模型配置下地形流的再现。即将进行的两次基于飞机的实地活动将对地形流进行观测，这两次活动分别以冰岛和格陵兰岛为基地，以阿拉斯加为基地，其中地形流是关键的活动目标。此外，还将提供以前在南极洲进行的一次活动的观测结果。观测值将用于验证和调整模型设置。该项目旨在优化地形流表示，并就如何最好地在天气和气候模型中参数化（即表示）某些关键物理过程建立通用结果。这些研究结果的影响可能包括MetUM设置的业务天气预报或气候版本的模型或新的参数化的地形流的影响的发展的变化。

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.