Passive microwave signature observations of Baltic Sea ice

Passive microwave signatures of different Baltic Sea ice types and open water leads were measured in the spring of 1995 and in March 1997 with airborne non-imaging microwave radiometers (MWR) operating in the frequency range from 6.8 to 36.5 GHz. The Baltic Sea is a semi-enclosed brackish sea water (salinity <6 ppt) basin in northern Europe. The ice cover in the Baltic Sea begins to form in November. It reaches its maximum extent usually in late February or early March, and usually all ice melts by the end of May. The maximum annual ice cover ranges from 12% to 100% of the whole Baltic Sea area. On the average, the maximum is around 50%. The ice in the Baltic Sea occurs as fast ice and drift ice. Fast ice is found in the coastal and archipelago areas. Sea ice in the open sea occurs as drift ice which can be level, rafted or ridged with a 0-100% coverage. The MWR data sets are used to study: 1) number of main dimensions of data sets, 2) discrimination of open water from sea ice and classification of various ice types using different MWR channel combinations, and 3) suitability of the SSM/I ice concentration algorithms for mapping the Baltic Sea ice. The effect of snow cover wetness is also studied.