On the radiation characteristics of Antarctic sea ice

Radiative measurements were carried out continuously during a cruise from Australia to Antarctica during austral slimmer 1995/96. Both shortwave and longwave radiative fluxes were measured. Some of the results are: The incoming solar radiation had a mean value of 217 W m(-2); this was a relatively weak value due to the large amount of fractional cloud cover observed. The sun was, for a large part of the trip, above the horizon for 24 hours a day. The reflectivity varied widely not only as a function of sea-ice concentration, but also as a function of ice type. Snow coveted pack ice gave the highest albedo values (approximate to 70%), while flooded sea ice and thin ice reflected much less (approximate to 30%). For each sea-ice type, short term observations showed a good relationship between albedo mid ice concentration The albedo increased with decreasing solar elevation. The net longwave radiation was negative (mean -27 W m(-2)); this small absolute value is due to a high amount of fractional cloud covet: There was a weak diurnal variation with a maximum loss (-33 W m(-2)) in the early afternoon. On the average, the net radiation was positive for 17 hours, and negative for 7 hours a day. However, the duration of a positive balance depended strongly on the surface albedo. For the observed albedo values, modelling results showed that the net radiation was always positive when averaged over a day The magnitude, however; depended strongly on the surface albedo, varying by more than the factor of three.