Trapped lee waves as a source of low-level drag on the atmosphere

被困的背风波是大气层低层阻力的来源

• 批准号: 2740527
• 金额: --
• 依托单位: University of Reading
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2740527
• 关键词: Trapped; lee; waves; source; low

Atmospheric models used for numerical weather prediction and climate projections have significant uncertainties in their representation of small-scale processes that contribute to errors in the modelled atmospheric circulation. Indeed, the latest report from the Intergovernmental Panel on Climate Change (IPCC) places low confidence on projected tropospheric circulation changes under global warming scenarios, deduced from atmospheric models. One particular process which both contributes significantly to the atmospheric circulation and is not fully represented in atmospheric models is the drag (i.e. frictional) force produced by small-scale mountains. In this project you will investigate the importance of this missing process using theory, high-resolution numerical model simulations and observations.When the atmosphere flows over mountains, internal atmospheric waves (known as orographic gravity waves) are generated that exert a drag force on the atmosphere, acting to decelerate the large-scale circulation both locally and remotely. A large proportion of this drag is caused by mountains at horizontal scales that are either partially or totally unrepresented in models typically used for weather forecasting or climate and seasonal projection, so their influence on the circulation is accounted for through approximations called parameterizations. Existing parametrizations focus on the drag produced by vertically propagating orographic gravity waves, which typically acts within wave breaking regions at high altitudes, sometimes reaching as high as the stratosphere (the atmospheric layer above about 10 km altitude) or even above. However, other orographic gravity waves, known as trapped lee waves, are also known to exert a drag on the atmosphere. These trapped lee waves have even smaller horizontal scales and propagate horizontally at lower levels (being made visible by cloud alignments - see figure below), but are not parametrized in most weather and climate models.In strongly stably stratified atmospheric boundary layers (common at high latitudes over land), turbulence co-exists with gravity waves. In an effort to minimize errors in the predicted surface temperature and surface wind angle (which affects, for example, the decay rate of extratropical cyclones), turbulent mixing is often unrealistically increased. This increase has the negative side-effect of overestimating the boundary layer depth and the height of the low-level jet (a feature of nocturnal boundary layers) by factors as large as two, and can result in biased predictions of minimum and maximum temperatures. A likely reason for these problems is the omission of trapped lee wave drag and its misrepresentation as turbulent form drag, which necessarily degrades forecasts, as the dependence of each of these processes on the flow parameters is different.This project aims to clarify the contribution of trapped lee waves to low-level drag exerted on the atmosphere using theory, numerical simulations and observations. Theory assuming atmospheres with a representative layered structure will be used to establish the rough dependence of the drag on basic flow parameters. Very high-resolution (1km grid spacing) numerical simulations using the Met Office's operational weather forecast model (the Unified Model, MetUM) will be used to test the theory in idealized and realistic cases, and improve the representations of trapped lee waves suggested by theory for weather and climate prediction. Lower-resolution simulations will be used to test these ideas in an operational context. A ground-truth for verification of results from theory and numerical simulations will be provided by available observations of trapped lee waves, including aircraft observations made in the UK by the FAAM research aircraft.

用于数值天气预报和气候预测的大气模型在表示造成模拟大气环流误差的小尺度过程方面有很大的不确定性。事实上，政府间气候变化专门委员会(IPCC)的最新报告对从大气模型推导出的全球变暖情景下预测的对流层环流变化缺乏信心。小规模山脉产生的阻力(即摩擦力)是一个对大气环流有重大贡献但在大气模式中没有得到充分反映的特殊过程。在本项目中，您将使用理论、高分辨率数值模式模拟和观测来研究这种缺失过程的重要性。当大气在山脉上方流动时，会产生大气内部波(称为地形重力波)，对大气施加阻力，从而在本地和远程减缓大范围环流的速度。这种阻力的很大一部分是由水平尺度的山脉造成的，而在通常用于天气预报或气候和季节性预测的模型中，这些山脉要么部分或完全没有代表，所以它们对环流的影响通过被称为参数化的近似来解释。现有的参数化集中在垂直传播的地形重力波所产生的阻力上，这种重力波通常作用在高海拔的波浪破碎区域内，有时高达平流层(大约10公里高度以上的大气层)甚至更高。然而，众所周知，其他地形重力波，即被捕获的背风波，也会对大气施加阻力。这些被捕获的背风波具有更小的水平尺度，并在较低的水平传播(通过云线可见-见下图)，但在大多数天气和气候模式中没有被参数化。在高度稳定的层结大气边界层(通常在陆地上的高纬度)，湍流与重力波共存。为了尽量减少预测地表温度和地表风角的误差(例如，这会影响温带气旋的衰减率)，经常不切实际地增加湍流混合。这种增加的负面影响是，高估了边界层深度和低空急流高度(夜间边界层的一个特征)，高估了多达两个因素，并可能导致对最低和最高温度的有偏见的预测。造成这些问题的一个可能原因是忽略了捕获的背风波阻力，并将其错误地表示为湍流形式的阻力，这必然会降低预报，因为这些过程对流动参数的依赖是不同的。本项目旨在通过理论、数值模拟和观测来阐明捕获的背风波对大气施加的低层阻力的贡献。假定大气具有典型的分层结构的理论将被用来建立阻力与基本流动参数的粗略关系。使用气象局业务天气预报模式(统一模式，MetUM)的极高分辨率(1公里网格间距)数值模拟将用于在理想和现实情况下检验该理论，并改进用于天气和气候预报的理论所建议的捕获背风波的表示。较低分辨率的模拟将用于在操作环境中测试这些想法。通过对捕获的背风波的现有观测，包括FAAM研究飞机在英国进行的飞机观测，将提供用于验证理论和数值模拟结果的地面真相。