Description: A new turbulence parametrization is developed for a non-eddy-resolving microscale model to study the effects of leads (elongated open-water channels in sea ice) of different width on the polar atmospheric boundary layer (ABL). Lead-dominated sea ice regions are characterized by large horizontal inhomogeneities of the surface temperature causing strong convection. Therefore, the new parametrization is based on a previous formulation where inhomogeneous conditions of dry convection over leads and nonlocal effects on heat fluxes had already been taken into account for a fixed lead width. A nonlocal lead width dependent approach is applied now for both heat fluxes and momentum fluxes in the convective region. Microscale model results obtained with the new, the previous nonlocal, and a local parametrization are shown, where 10 idealized cases of a lead-perpendicular, near-neutral ABL-flow below a strong capping inversion are considered. Furthermore, time-averaged large eddy simulation (LES) results of those cases are considered for analyzing the integrated effects of the dry convection on ABL characteristics. Microscale model results obtained with the new nonlocal parametrization agree well with the LES for variable lead widths and different atmospheric forcing although there is a room for further improvement. Furthermore, several features obtained with a local closure clearly disagree with LES. Thus, the microscale study also points to difficulties that might occur in mesoscale studies over regions where leads dominate the flow regime when local closures are applied.

Description: In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research primarily focused on the Atlantic sector of the Southern Ocean. In parallel, the German National Science Foundation (Deutsche Forschungsgemeinschaft DFG) started a Priority Program ‘Antarctic Research’ (since 2003 called SPP-1158) to foster and intensify the cooperation between scientists from different German universities and the AWI as well as other institutes involved in polar research. Here, we review the main findings in meteorology and oceanography of the last decade, funded by the priority program. The paper presents field observations and modelling efforts, extending from the stratosphere to the deep ocean. The research spans a large range of temporal and spatial scales, including the interaction of both climate components. In particular, radiative processes, the interaction of the changing ozone layer with large-scale atmospheric circulations, and changes in the sea ice cover are discussed. Climate and weather forecast models provide an insight into the water cycle and the climate change signals associated with synoptic cyclones. Investigations of the atmospheric boundary layer focus on the interaction between atmosphere, sea ice, and ocean in the vicinity of polynyas and leads. The chapters dedicated to polar oceanography review the interaction between the ocean and ice shelves with regard to the freshwater input and discuss the changes in water mass characteristics, ventilation and formation rates, crucial for the deepest limb of the global, climate relevant meridional overturning circulation. They also highlight the associated storage of anthropogenic carbon as well as the cycling of carbon, nutrients, and trace metals in the ocean with special emphasis on the Weddell Sea.

Description: Leads (open-water channels in sea ice) play an important role for surface-atmosphere interactions in the polar regions. Due to large temperature differences between the surface of leads and the near-surface atmosphere, strong turbulent convective plumes are generated with a large impact on the atmospheric boundary layer (ABL). Here, we focus on the effect of lead width on those processes, by means of numerical modeling and turbulence parametrization. We use a microscale atmosphere model in a 2D version resolving the entire convective plume with grid sizes in the range of L/5 where L is the lead width. For the sub-grid scale turbulence, we developed a modified version of an already existing nonlocal parametrization of the lead-generated sensible heat flux including L as parameter. All our simulations represent measured springtime conditions with a neutrally stratified ABL capped by a strong temperature inversion at 300 m height, where the initial temperature difference between the lead surface and the near-surface atmosphere amounts to 20 K. We found that our simulation results obtained with the new approach agree very well with time-averaged results of a large eddy simulation (LES) model for variable lead widths with L ≥ 1 km and different upstream wind speeds. This is a considerable improvement since results obtained with the previous nonlocal approach clearly disagree with the LES results for leads wider than 2 km. In conclusion, considering L as parameter in a nonlocal turbulence parametrization seems to be necessary to study the effect of leads on the polar ABL in non-eddy-resolving small-scale atmosphere models.

Description: Even in the central polar ocean regions, the sea ice cover cannot be regarded as a homogeneous and permanently closed surface. Due to divergent sea ice drift, even in winter elongated open-water channels, so-called leads, develop. Leads play an important role for surface-atmosphere interactions since they enable a direct contact between ocean and atmosphere. Especially in winter, when temperature differences amount up to 40 K, strong convective plumes are generated over leads with upward heat fluxes in the order of 10^2 Wm^(-2) . This has considerable effects on the structure of the atmosphere. Those effects mainly depend on the meteorological conditions close to the lead, but also on the lead geometry as, for example, the lead width. To understand the determining processes which lead to the formation and the decay of the plumes, we use numerical model simulations of a lead-perpendicular flow. We use a small-scale atmosphere model, which resolves the entire plume, but not the transport due to sub-grid scale turbulence. Hence, that transport has to be parametrized to close the system of the model equations. Thus, we developed an improved parametrization of the convection over leads, mainly for the turbulent heat flux. A previous parametrization, where one particular lead width (1 km) was considered, acts as a starting point and we derived a more general approach by including the lead width as a parameter. To validate our results, we use time-averaged results of a large eddy simulation (LES) model. We show that also for different lead widths small-scale model results obtained with our modified parametrization agree well with LES. In addition, our approach is still robust against variations of wind speed and surface temperature differences. Thus, our parametrization represents a clear improvement since now variable lead widths can be considered for a detailed investigation of the effects of lead-generated convection on the polar atmosphere.