Greenland Flow Distortion.

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Greenland has a major influence on the atmospheric circulation of the North Atlantic-Western Europe region; dictating the location and strength of mesoscale (100-km scale) weather systems around the coastal seas of Greenland and directly influencing synoptic-scale (1000-km scale) weather systems both locally and downstream over Europe. One can think of this sizeable 3000-m high barrier deflecting flow both over and around itself, i.e. distorting the atmospheric flow, with both local and remote consequences. Hurricane-stength winds associated with the local weather systems can induce large air-sea fluxes of heat (over 600 Watts per square metre, more than the daily average from the sun), moisture and momentum in a region that is critical to the overturning of the thermohaline circulation (the primary circulation of the world's oceans). Hence the flow distortion plays a key role in controlling the coupled atmosphere-ocean climate system. This project will investigate the role of Greenland in defining the structure and the predictability of both local and downstream weather systems, through a programme of aircraft-based observation and numerical modelling. The Greenland Flow Distortion Experiment (GFDex) will provide some of the first detailed in situ observations of the intense atmospheric forcing events that are thought to be important in modifying the ocean in this area (but are presently poorly understood): namely tip jets, barrier winds and mesoscale cyclones. Tip jets form at the southern tip of Greenland, at Cape Farewell, through the forcing of flow over and around the topography. Barrier winds occur when the large-scale flow is piled up against the southeast coast of Greenland, forcing winds parallel to the coast. While located off this southeast coast is an area of frequent mesoscale cyclogenesis. GFDex will also investigate Greenland's role in atmospheric flow predictability by carrying out upstream observations that are 'targeted' at investigating the sensitivity of the downstream flow to the details of the upstream flow and at improving subsequent forecasts over Europe. Greenland's flow distortion can trigger large-scale atmospheric 'Rossby' waves which influence weather systems thousands of kilometres away and several days later. These waves are by nature predictable, so by adapting our observing strategy to target specific areas, improvements in subsequent forecasts over the United Kingdom are possible. Numerical modelling experiments after the field campaign will be used to assess any improvements from the additional 'targeted' observations. While further numerical modelling studies of the high impact local weather systems will be evaluated and refined using the aircraft-based observations. This will increase our understanding of these systems and, through comparisons with other observations and data sets, provide accurate fields of air-sea heat and moisture fluxes for driving ocean and climate models.