Passive microwave in situ observations of winter Weddell Sea ice

Abstract

The microwave radiative characteristics of Antarctic sea ice during a winter period were investigated continuously from R/V Polarstern during the 1986 Winter Weddell Sea Project while the ship went through about 3000 km of ice from the marginal ice zone to the coastal region and back. Radiometer measurements at 6, 10, 18, 37, and 90 GHz in vertical and horizontal polarizations were complemented by visual and video observations and measurements at 60 stations of ice thickness, salinity, temperature, snow cover, density, and other physical characteristics. Two distinct types of ice cover were observed in the marginal ice zone: small pancakes evenly distributed during the southbound leg, and ice bands with wet pancakes during the northbound leg. Other ice types observed were first‐year ice covered by varying thicknesses and states of snow cover, and new and young ice found mainly in leads and polynyas. Analysis of the data shows a large variability in the multispectral microwave emissivities of these ice types, especially at 90 GHz. Over newly refrozen lead or polynya regions, several forms of new ice appear radiometrically distinct, while over thick consolidated ice with snow cover, the brightness temperatures observed at 90 GHz varied by as much as 100 K. Overall, however, at 18 GHz and lower frequencies, the emissivities of thick and cold first‐year ice are relatively stable with standard deviations of about ±0.02. At the marginal ice zone, the emissivity of the ice cover is a lot less predictable and could cause large uncertainties in ice concentration estimates. The use of the 90‐GHz channel in combination with a lower‐frequency channel shows strong potential for more detailed characterization of the ice cover including the identification of various forms of new ice and the quantification of varying snow cover and roughness.