Representation of the convective atmospheric boundary layer during cold-air outbreaks in regional models: a joined study based on observations, Large Eddy Simulation and mesoscale modelling

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The interaction between atmospheric processes in polar and mid-latitude regions is strongly affected by cold-air outbreaks (CAOs). During such events cold air originating from (sea)ice covered regions is transported over large distances towards lower latitudes, while over open water a shallow but strongly heated convective boundary layer develops. Although the present generation of weather and regional climate models captures CAOs qualitatively well, there are deficiencies caused by uncertainties in the representation of the multi scale physical CAO processes in models. Open questions concern e.g., the generation of rolls under conditions of strong thermal forcing over the open and partially ice covered ocean and under nonstationary synoptic conditions. Other questions are related to the parameterization of the strong turbulent fluxes, especially in coarse grid resolution. Hence this project aims to study the physical processes in polar CAOs with roll vortices using large eddy simulation (LES) and mesoscale modelling. Furthermore, parameterizations of turbulence in CAOs will be improved with a focus on the most modern low order closures and their applicability to regional scale models. To this end current and new closures are used in a mesoscale model with different resolutions and its results are compared with observations and with the results of LES.