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

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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.