DIabatic Processes during the North Atlantic Waveguide and Downstream impact EXperiment – DIP-NAWDEX

DIP-NAWDEX aims at better understanding the dynamics and predictability of mid-latitude atmospheric disturbances and their role in the formation of high-impact weather (HIW) events. New questions on the role played by diabatic processes (cloud microphysics, radiation, turbulence) in mid-latitude atmospheric disturbances and their implication in the formation of weather forecast errors and climate model biases will be addressed by using the collected NAWDEX dataset. NAWDEX (North Atlantic Waveguide and Downstream Impact Experiment) is a recently accomplished international field campaign (September-October 2016) that provides a unique observational dataset to assess the skills of current NWP and climate models. The underlying hypothesis of NAWDEX is that diabatic processes over North America and the North Atlantic have a major influence on jet stream structure, the downstream development of Rossby waves on the tropopause, and HIW events over Europe. Thanks to a close international collaboration among North American and European countries, four aircraft from three distinct countries have been operated in the Northeast Atlantic. Fine-scale observations of atmospheric disturbances extending over a large domain have been obtained with state-of-the-art airborne and ground-based instrumentation like the most recently developed lidars and cloud radars.
The focus will be made on warm conveyor belts (WCBs) that have been intensively observed by the French aircraft through released dropsondes and the remote sensing platform RALI composed of a doppler cloud radar, a high-resolution backscatter lidar and an infrared radiometer. WCBs correspond to air masses gaining humidity inside the warm sector of extratropical cyclones and ascending from the boundary layer to the tropopause level. A better knowledge of the processes at play within WCBs should help better characterizing the impact of diabatic processes on upper-tropospheric circulation like the jet stream, synoptic Rossby waves and larger-scale phenomena like blocking.
The project is decomposed into three main tasks. The first task dedicated to observational data analysis and tools development will help in carrying out process studies of the two other tasks. It includes a verification stage of the quality of the observational datasets by comparing various co-localized datasets (in-situ, airborne and spaceborne remote sensing measurements). Model-observation comparison tools will be developed like reflectivity simulators and cloud microphysics retrievals from RALI. A subtask dedicated to data assimilation of wind observations using the Météo-France operational global model is also planned. The second main task is dedicated to mesoscale process studies and local effects of WCBs. Convection-permitting simulations using different limited-area mesoscale models centred over Iceland and its southern region where the French Falcon was operated will be confronted to NAWDEX observations. Sensitivity experiments to diabatic processes and various parametrization schemes will be performed. Lagrangian diagnostics will be also used to separate the diabatic heating into distinct parts (radiation, turbulence, cloud microphysics). Finally, the third task will investigate large-scale interaction processes. We will in particular investigate the effect of WCBs on the downstream circulation by focusing on two case studies: a blocking formation in early October and a cut-off formation in mid-October that triggered a HIW event over southern France. Climate model biases will be identified by performing short-term simulations of two climate models starting from operational analyses. This should help identifying misrepresented fast physical processes whose errors grow within a few days.
To conclude, addressing the above research questions requires a combined methodological approach, which becomes possible due to the complementary expertise of the involved scientists.