Total sea ice concentration retrieval from the SSM/I 85.5 GHz channels during the arctic summer

Abstract

During the 1994 Arctic Ocean Section, a joint voyage across the Arctic Ocean by the U.S. Coast Guard Cutter Polar Sea and the Canadian Coast Guard Ship Louis S. St.-Laurent, telemetry from the Defense Meteorological Satellite Program (DMSP) polar orbiters was tracked by a shipboard antenna. Special Sensor Microwave Imager (SSM/I) data was used to generate maps of total sea ice concentration, using the NASA Team algorithm with the 19 GHz and 37 GHz channels, and using a polarization-based algorithm with the 85.5 GHz channels. When compared with shipboard ice observations, the total sea ice concentration estimates from the 85.5 GHz algorithm are at least as accurate as those from the algorithm that uses only the lower SSM/I frequencies, despite the potential for greater difficulty in dealing with cloud liquid water contamination in the 85.5 GHz signal during the Arctic summer. Near the edge of the ice pack, the 85.5 GHz algorithm often provided more accurate estimates of total ice concentration when compared with surface observations, most likely because of the finer grid spacing at 85.5 GHz (12.5 km vs. 25 km for 37 GHz). However, when using the 85.5 GHz algorithm over regions of lower ice concentration, the reference polarizations in a given image must be chosen with care because over lower sea ice concentration the polarization-based algorithm is more sensitive to cloud opacity and can easily and substantially underestimate the ice concentration. The 85.5 GHz total sea ice retrievals are compared with in situ snow wetness measurements. This comparison suggests that, despite the higher atmospheric opacity at 85.5 GHz, information about sea ice surface properties that affect emissivity can be obtained from these SSM/I channels.