ACCURACY OF SEA-ICE TEMPERATURE DERIVED FROM THE ADVANCED VERY HIGH-RESOLUTION RADIOMETER

The accuracy of Arctic sea ice surface temperatures T-s derived from a dvanced very high resolution radiometer (AVHRR) thermal channels is ev aluated in the cold seasons by comparing them with surface air tempera tures T-air from drifting buoys and ice stations. We use three differe nt estimates of satellite surface temperatures, a direct estimate from AVHRR channel 4 with only correction for the snow surface emissivity but not for the atmosphere, a single-channel regression of T-s with T- air, and Key and Haefliger's (1992) polar multichannel algorithm. We f ind no measurable bias in any of these estimates and few differences i n their statistics: The similar performance of all three methods indic ates that an atmospheric water vapor correction is not important for t he dry winter atmosphere in the central Arctic, given the other source s of error that remain in both the satellite and the comparison data. The errors are not reduced by regression with both thermal channels an d the satellite scan angle. A record of drifting station data shows wi nter air temperature to be 1.4 degrees C warmer than the snow surface temperature. ''Correcting'' air temperatures to skin temperature by su btracting this amount implies that satellite T-s estimates are biased warm with respect to skin temperature by about this amount. A case stu dy with low-flying aircraft data suggests that ice crystal precipitati on can cause satellite estimates of T-s to be several degrees warmer t han radiometric measurements taken close to the surface, presumably be low the ice crystal precipitation layer. An analysis in which errors a re assumed to exist in all measurements, not just the satellite measur ements, gives a standard deviation in the satellite estimates of 0.9 d egrees C, about half the standard deviation of 1.7 degrees C estimated by assigning all the variation between T-s and T-air to errors in T-s .