Analysis of Sea Ice Processes in Coastal Polynias based on SAR Satellite Imagery, Model Simulation and Data Fusion
Hollands, Thomas; Dierking, Wolfgang; Haid, Verena; Timmermann, Ralph
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, GERMANY
Polynias are open water areas in sea ice. Hence they represent gaps in the border between ocean and atmosphere and thereby allow an increased heat- and moisture exchange between both media. While sensible heat polynias occur due to warm water currents melting the ice cover, latent heat polynias form when parts of the ice cover are forced away by e.g. winds. A prominent example for this kind of polynias is the group of costal polynias, which occur when strong offshore winds move ice away from the coast. Dependent on the meteorological conditions new ice may be formed instantly. Latent heat polynias are therefore sometimes also referred to as sea ice factories. We will present a study on sea ice motion at the Ronne Polynia, Antarctica, where we used the combination of a high-resolution drift algorithm and a coupled sea ice ocean model, focusing on two events of polynia evolution. For our investigations we employed two time series of Envisat ASAR WS images, which were acquired in February and June 2008. We analysed the time series with a recently developed pattern matching approach to identify the displacement of corresponding sea ice structures and floes as a function of time. Afterwards we compared theses observations with simulations from a coupled sea ice – ocean model. Since the atmospheric forcing is the dominating factor for ice motion and the generation of latent heat polynias, the wind data have a strong influence on the simulation results. In the model runs we therefore employed different atmospheric forcings, which vary in temporal and spatial resolution. The simulation results suggest that the general kinematics within a polynia can be realistically reproduced by modern sea ice models. However, they show a strong dependence on the employed atmospheric forcing in the case of fast changing wind conditions. Besides the kinematics of sea ice in a polynia, we focus specifically on the changing extent during polynia formation. Therefore we compared the water/ice border visually derived from the SAR images with model results based on thresholds for ice concentration.
In the second part we present the extension of the polynia project started recently. The objective at this stage is to combine data of multiple sensors operating in the optical, infrared and microwave region. Based on such data combinations, we expect to improve the retrieval of different geophysical parameters characterizing polynia evolution and regression, such as ice thickness and ice type distributions within and close to the polynia. One outcome of this analysis is an assessment of the potential of future missions like Sentinel-1 and 3 for polynia research.