Passive microwave in situ measurements over snow covered Siberian sea ice compared with SSM/I

Snow on sea ice affects the energy and mass balance of sea ice on seasonal and synoptic scales and is, with its high albedo, responsible for a net cooling effect on global climate. To quantify the distribution of snow on a large scale satellite based measurements are necessary. To determine the influence of snow on the passive microwave signal snow thickness measurements and brightness temperatures at 11, 21 and 35 GHz with different incidence angles and polarizations were performed over sea ice in the Kara and Laptev Sea during the Arctic expedition ARK-12 of R.V. Polarstern in September 1996. Comparison of in situ data with two emissivity models including one based on the Combined Radiative Transfer-Strong Fluctuation Theory show that the most important influence on the spaceborne passive microwave signal in the Siberian Arctic is scattering in the snow while the outliers can be explained by absorption in the snow due to high free water content at temperatures near 0/spl deg/C. The brightness temperature gradient ratio of the vertically polarized 19 and 37 GHz channels is used to derive a retrieval algorithm for the snow thickness from SSM/I data. Its sample application to the whole Arctic shows an increase of snow thickness with latitude in accordance with measurements from Russian drifting stations.