On the radiation characteristics of Antarctic Sea Ice

Abstract

Radiative measurements were carried out continuously during a cruise from Australia to Antarctica during austral summer 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 covered pack ice gave the highest albedo values (<70%), while flooded sea ice and thin ice reflected much less (<30%). • For each sea‐ice type, short term observations showed a good relationship between albedo and 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 cover. 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.