Description: The interaction between polynyas and the atmospheric boundary layer is examined in the Laptev Sea using the regional, non-hydrostatic Consortium for Small-scale Modelling (COSMO) atmosphere model. A thermodynamic sea-ice model is used to consider the response of sea-ice surface temperature to idealized atmospheric forcing. The idealized regimes represent atmospheric conditions that are typical for the Laptev Sea region. Cold wintertime conditions are investigated with sea-iceocean temperature differences of up to 40 K. The Laptev Sea flaw polynyas strongly modify the atmospheric boundary layer. Convectively mixed layers reach heights of up to 1200 m above the polynyas with temperature anomalies of more than 5 K. Horizontal transport of heat expands to areas more than 500 km downstream of the polynyas. Strong wind regimes lead to a more shallow mixed layer with strong near-surface modifications, while weaker wind regimes show a deeper, well-mixed convective boundary layer. Shallow mesoscale circulations occur in the vicinity of ice-free and thin-ice covered polynyas. They are forced by large turbulent and radiative heat fluxes from the surface of up to 789 W m-2, strong low-level thermally induced convergence and cold air flow from the orographic structure of the Taimyr Peninsula in the western Laptev Sea region. Based on the surface energy balance we derive potential sea-ice production rates between 8 and 25 cm d-1. These production rates are mainly determined by whether the polynyas are ice-free or covered by thin ice and by the wind strength.

Description: Mesoscale model simulations were conducted for the Weddell Sea region for the autumn and winter periods of 2008 using a high-resolution, limited-area, non-hydrostatic atmospheric model. A sea ice–ocean model was run with enhanced horizontal resolution and high-resolution forcing data of the atmospheric model. Daily passive thermal and microwave satellite data was used to derive the polynya area in the Weddell Sea region. The focus of the study is on the formation of polynyas in the coastal region of Coats Land, which is strongly affected by katabatic flows. The polynya areas deduced from two independent remote sensing methods and data sources show good agreement, while the results of the sea ice simulation show some weaknesses. Linkages between the pressure gradient force composed of a katabatic and a synoptic component, offshore wind regimes and polynya area are identified. It is shown that the downslope surface offshore wind component of Coats Land is the main forcing factor for polynya dynamics, which is mainly steered by the offshore pressure gradient force, where the katabatic force is the dominant term. We find that the synoptic pressure gradient is opposed to the katabatic force during major katabatic wind events.

Description: Arctic flaw polynyas are considered to be highly productive areas for the formation of sea-ice throughout the winter season. Most estimates of sea-ice production are based on the surface energy balance equation and use global reanalyses as atmospheric forcing, which are too coarse to take into account the impact of polynyas on the atmosphere. Additional errors in the estimates of polynya ice production may result from the methods of calculating atmospheric energy fluxes and the assumption of a thin-ice distribution within polynyas. The present study uses simulations using the mesoscale weather prediction model of the Consortium for Small-scale Modelling (COSMO), where polynya area is prescribed from satellite data. The polynya area is either assumed to be ice-free or to be covered with thin ice of 10 cm. Simulations have been performed for two winter periods (2007/08 and 2008/09). When using a realistic thin-ice thickness of 10 cm, sea-ice production in Laptev polynyas amount to 30 km3 and 73 km3 for the winters 2007/08 and 2008/09, respectively. The higher turbulent energy fluxes of open-water polynyas result in a 5070% increase in sea-ice production (49 km3 in 2007/08 and 123 km3 in 2008/09). Our results suggest that previous studies have overestimated ice production in the Laptev Sea.

Description: The development of coastal polynyas, areas of enhanced heat flux and sea ice production strongly depend on atmospheric conditions. In Antarctica, measurements are scarce and models are essential for the investigation of polynyas. A robust quantification of polynya exchange processes in simulations relies on a realistic representation of atmospheric conditions in the forcing dataset. The sensitivity of simulated coastal polynyas in the south-western Weddell Sea to the atmospheric forcing is investigated with the Finite-Element Sea ice-Ocean Model (FESOM) using daily NCEP/NCAR reanalysis data (NCEP), 6 hourly Global Model Europe (GME) data and two different hourly datasets from the high-resolution Consortium for Small-Scale Modelling (COSMO) model. Results are compared for April to August in 2007–09. The two coarse-scale datasets often produce the extremes of the data range, while the finer-scale forcings yield results closer to the median. The GME experiment features the strongest winds and, therefore, the greatest polynya activity, especially over the eastern continental shelf. This results in higher volume and export of High Salinity Shelf Water than in the NCEP and COSMO runs. The largest discrepancies between simulations occur for 2008, probably due to differing representations of the ENSO pattern at high southern latitudes. The results suggest that the large-scale wind field is of primary importance for polynya development.

Description: Global Change and its predicted key impact on the Arctic bring the Laptev Sea to the centre of climate-related polar research. This Shelf Sea is known as being a highly productive area for the formation of new ice throughout the winter season. A main part of the ice production occurs in flaw polynyas which appear recurrently at the edge of the fast ice surrounding the coastal zones during wintertime. This work attempts to provide a method to reliably estimate the ice production in the Laptev Sea polynyas from model result s of the numerical weather prediction model COSMO. Our modeling approach contains the use of COSMO with 15 and 5 km horizontal resolution nested in global GME/ERA-Interim data to calculate ice production in polynyas. To account for realistic polynya representation polynya area is prescribed to the COSMO model by daily AMSR-E satellite data. The potential volume ice production is calculated from atmospheric net radiation fluxes. In contrast to preceding studies our new COSMO-based method takes into account the effect of polynyas on the atmosphere. Over open water, warmer 2m temperatures (COSMO in comparison to NCEP) lead to lower ice production. Over thin ice, surface temperature depends on air temperature and reduced air surface temperature gradients cause lower heat fluxes and less ice production than over open water. As warm-biased NCEP values are balancing the effects of our improvements the comparison of ice production retrieval based on NCEP data with our results show minor total differences only. Both methods are leading to results in same order of magnitude if the polynya is assumed to be covered with 10cm of thin-ice. This supports the thesis that either of them leads to feasible ice production values if thin ice within the polynya is accounted for in the calculation. In case of an open water polynya, however, our study underlines the impact of the atmospheric data on the ice production. Thus we conclude that it is of major importance to choose a validated ice thickness parameterization for the model.

Description: The Ronne Ice Shelf is known as one of the most active regions of polynya developments around the Antarctic continent. Low temperatures are prevailing throughout the whole year, particularly in winter. It is generally recognized that polynya formations are primarily forced by offshore winds and secondarily by ocean currents. Many authors have addressed this issue previously at the Ross Ice Shelf and Adélie Coast and connected polynya dynamics to strong katabatic surge events. Such investigations of atmospheric dynamics and simultaneous polynya occurrence are still severely underrepresented for the southwestern part of the Weddell Sea and especially for the Ronne Ice Shelf. Due to the very flat terrain gradients of the ice shelf katabatic winds are of minor importance in that area. Other atmospheric processes must therefore play a crucial role for polynya developments at the Ronne Ice Shelf. High-resolution simulations have been carried out for the Weddell Sea region using the non-hydrostatic NWP model COSMO from the German Meteorological Service (DWD). For the austral autumn and winter (March to August) 2008 daily forecast simulations were conducted with the consideration of daily sea-ice coverage deduced from the passive microwave system AMSR-E. These simulations are used to analyze the synoptic and mesoscale atmospheric dynamics of the Weddell Sea region and find linkages to polynya occurrence at the Ronne Ice Shelf. For that reason, the relation between the surface wind speed, the synoptic pressure gradient in the free atmosphere and polynya area is investigated. Seven significant polynya events are identified for the simulation period, three in the autumn and four in the winter season. It can be shown that in almost all cases synoptic cyclones are the primary polynya forcing systems. In most cases the timely interaction of several passing cyclones in the northern and central Weddell Sea leads to maintenance of a strong synoptic pressure gradient above the Ronne Ice Shelf. This strong synoptic forcing results in a moderate to strong offshore surface wind. It turned out that these synoptic depressions lead to strong barrier winds above the northwestern Ronne Ice Shelf and along the eastern flank of the Antarctic Peninsula. The fact, that these barrier winds often appear prior or during the initial break up of sea ice at the shelf ice edge, suggest that this mesoscale wind phenomenon plays a crucial role for polynya development. Furthermore, even mesoscale cyclogenesis above the Ronne Ice Shelf and the following northeastward passage of such a system can break up sea-ice cover under large-scale stationary weather conditions.

Description: Simulations for Greenland with focus on the wind regime are presented using the high-resolution non-hydrostatic model COSMO (Consortium for Small scale modeling). The simulations are performed at 15 km, 5.5 km and 1.3 km resolution for the time period of June 2010. The Nares Strait, including the North Water (NOW) polynya, in northwest Greenland was selected as focus of the simulations, since comprehensive measurements of the structure of the boundary layer are available from an aircraft study. The observations on four different days show a shallow stable boundary layer over the polynya and a pronounced low-level jet associated with the flow channeling in the Nares Strait, particularly at Smith Sound. The reproduction of the vertical patterns of wind and temperature by the simulations is realistic at all resolutions and best results are found for 5.5 km and 1.3 km resolution. A vertical displacement of the patterns and an overestimation of the temperature was found. The measured low-level inversion is not simulated well, but overall the vertical structures of the simulation and observation correlate highly. Thus, the model is well suited for simulations in particular for the situation of flow channeling in a topographically complex area. The analysis of the synoptic situations associated with channeled flow through the Nares Strait shows that the wind speed increases with higher pressure difference between the Lincoln Sea and Baffin Bay. Channeling effects lead to a prevailing flow direction towards Baffin Bay. A strong increase of the wind speed occurs at Smith Sound, where the flow also passes over mountains of the Greenland coast. The wind maximum is found downstream of Smith Sound, and typical low-level jets with wind speeds of around 20 m/s occur at a height of 100 m.

Description: Radical environmental changes are forecasted to occur in the Shelf areas of the Siberian Arctic during this century. The Laptev polynyas play a key role due to their impact on ice production and related feedback processes in the ocean and atmosphere. Observations and model studies have been performed within the BMBF founded project "Polynya systems face changes" (2007-2010) which is embedded in the IPY-project "Complex Investigations of Seasonal Cycle in the Arctic Seas". Four automatic weather stations which were installed along the fast ice edge in April 2008 reveal that the GME analyses (Global Model of Deutscher Wetterdienst) describe the synoptic conditions accurately (e.g. absolute error of wind speed between 0.2 and 0.5 ms-1 and correlation coefficients between 0.8 and 0.9). Thus, these analyses are an excellent data set for nesting meso-scale atmosphere models and forcing ocean models. Realistic and artificial case studies are presented with the non-hydrostatic atmospheric model COSMO (Consortium for Small-scale Modeling, Deutscher Wetterdienst) and the Finite Element Sea Ice Ocean Model (FESOM, Alfred Wegener Institute) on a grid with a horizontal resolution of 5km. These simulations show that the polynyas modify the atmosphere till a height of several kilometers. Furthermore, an accurate simulation of ice surface temperature is essential to quantify ice production realistically. The highest ice production rate was simulated for a cyclone case at the end of December 2007, whereas ice production is marginal for the April 2008 polynya cases.