Determination of sea ice parameters with the NIMBUS 7 SMMR

Abstract

A method of determining sea ice parameters using dual‐polarized multispectral radiance data obtained with the NIMBUS 7 Scanning Multichannel Microwave Radiometer (SMMR) is presented. Sea ice concentration is determined both at a 60‐km resolution from the polarization at the 1.7‐cm wavelength and at a 30‐km resolution using the polarization at the 0.81‐cm wavelength. Multiyear sea ice fraction is obtained from the spectral gradient ratio, which is the difference of the 0.81 cm and the 1.7 cm vertically polarized radiances divided by their sum. In addition, an ice temperature is calculated from the 4.6‐cm vertical channel radiances. The use of radiance ratios greatly reduces uncertainties in the derived parameters resulting from temporal and horizontal spatial variations of ice temperature. Observed SMMR radiances from selected areas in the Arctic region for the period February 3–7, 1979, are used in computing algorithm coefficients. Polar maps of sea ice concentration, multiyear fraction, and ice temperature are illustrated for this period. The variation of the mean and standard deviation of ice concentration and multiyear ice fraction for a region of perennial ice cover over the first 11 months of SMMR operation is also presented. The standard deviation about the mean for the computed concentration varies from 2 to 5% over the 11‐month period, while that of the multiyear fraction is about 8% for all but the summer months. Discrimination between first‐year and multiyear sea ice during the summer period is indeterminate largely because the general surface melt conditions mask the distinguishing properties of the two ice types. Based on the time variation of ice concentration and multiyear fraction for the central Arctic region and on an analysis of histograms of these parameters, the precision of the calculated ice concentration is estimated to be in the range of 5–9% and that of the multiyear fraction in the range of 13–25%. Comparisons are made between the calculated sea ice parameters and information obtained from previous studies using aircraft, submarine, and surface observations. From these comparisons it is concluded that the absolute accuracy of the SMMR parameters remains uncertain. The precision of the sea ice concentration is sufficient to provide useful data on the large‐scale polar ice cover, but further study is required to increase the confidence in the multiyear ice fraction which at present contains significant uncertainties.