Modelling the small-scale impact of leads on the polar atmospheric boundary layer using a new nonlocal turbulence closure

Inhomogeneous forcing by wind and ocean currents always generates leads in the polar pack ice. Their width ranges from meters (cracks) to sometimes kilometers and is mostly much smaller than the grid size of regional climate and weather forecast models. Due to the small lead sizes the lead impact on atmospheric and oceanic processes is still unclear. A new modelling approach is presented for the simulation of the winter time convective flow over leads using a small-scale model with grid sizes of dx = 100-200 m. This allows to resolve the plumes developing over medium sized leads while the effect of the small eddies and thermals has to be included in the turbulence closure. Since traditional closures are restricted to horizontally homogeneous conditions and the flow over leads is characterized by a strong horizontal inhomogeneity a new nonlocal closure was developed to account for this inhomogeneity in the turbulent fluxes on scales smaller than dx. This new turbulence closure will be presented including a comparison between results for ten cases obtained by the microscale modelling and by large eddy simulation (LES). It will be shown that results agree well and that regions of counter-gradient and along-gradient transport of heat in the plume region is reproduced in the small-scale model similar as in the LES, while the small-scale model requires much less computational costs than the large eddy simulation model. Finally, the small-scale model is applied to a large domain with several leads. Results are discussed with a focus on the domain averaged profiles of wind and temperature as well as turbulent fluxes for several cases differing by the prescribed pattern of leads. It is shown that presently available surface flux parameterizations used in climate models fail to reproduce the impact of leads on both turbulent fluxes of heat and momentum with sufficient accuracy.