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A series of surface‐based measurements of microwave brightness temperatures and accompanying ice characteristics have been made on first‐year and multiyear sea ice in the Canadian archipelago and in the southeastern Beaufort Sea during the late spring and early summer of 1982. Radiometer data taken at frequencies of 10, 18.7, and 37 GHz show the transition from conditions of snow‐covered ice from the onset of snow melt through to full summer melt conditions. Initially, values of TB for multiyear ice were larger than for first‐year ice, but by the middle of the experiment this distinction disappeared. Drained surfaces often had emissivities above 0.95, but values dropped considerably lower during heavy melting events. The skewness of the distribution of TB appears to distinguish ice types further into the melt season. The standard deviation is not useful for ice type discrimination but is sensitive to the area covered by melt ponds. Microwave attenuation in various surface layers was found to range from 2 m−1 early in the experiment to >400 m−1 during full summer melt, indicating penetration depths from 250 to 2.5 mm, respectively.

The microwave emission properties of first-year sea ice were investigated from the R/V Polarstern during the Antarctic Winter Weddell Gyre Project in 1989. Radiometer measurements were made at 611 MHz and 10 GHz and were accompanied by video and visual observations. Using the theory of radiometric emission from a layered medium, a method for deriving sea ice thickness from radiometer data is developed and tested. The model is based on an incoherent reflection process and predicts that the emissivity of saline ice increases monotonically with increasing ice thickness until saturation occurs. 22,569 22,570 MENASHI ET AL.' LoW-FREQUENCY PASSIVE-MICROWAVE OBSERVATIONS OF SEA ICE 0i Greenland, Nature, 345, 795-797, 1990. Wadhams, P., M. A. Lange, and S. F. Ackey, The thickness distribution across the Atlantic sector of the Antarctic Ocean in midwinter, J. Geophys. Res., A. W. Lohanick, Naval Oceanographic and Atmospheric Research Laboratory, Polar Oceanography Branch Office, Hanover, NH 03755.

During March 1983 extensive high‐quality airborne passive Ka band (33.6 GHz) microwave imagery and coincident high‐resolution aerial photography were obtained of ice along a 378‐km flight line in the Beaufort Sea. Analysis of these data suggests that four classes of winter surfaces can be distinguished solely on the basis of 33.6‐GHz brightness temperature: open water, frazil, old ice, and young/first‐year ice. New ice (excluding frazil) and nilas display brightness temperatures that overlap the range of temperatures characteristic of old ice and, to a lesser extent, young/first‐year ice. Scenes in which a new ice or nilas are present in appreciable amounts are subject to substantial errors in classification if static measures of Ka band radiometric brightness temperature alone are considered. Textural characteristics of nilas and new ice, however, differ significantly from textural features characteristic of other ice types and probably can be used with brightness temperature data to classify ice type in high‐resolution single‐band microwave images. In any case, open water is radiometrically the coldest surface observed in any scene. Lack of overlap between brightness temperatures characteristic of other surfaces indicates that estimates of the areal extent of open water based on only 33.6‐GHz brightness temperatures are accurate.

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