Atmospheric transmittance and surface shortwave cloud radiative forcing are determined for a springtime voyage of RSV Aurora Australis between Tasmania and East Antarctica, 47-69 deg S. Broadband solar irradiance was measured by two pyranometers on opposite sides of the ship. Visible irradiance is inferred from measurements by a quantum sensor. Comparison of the two pyranometers allows testing for errors due to shadowing and tilt. Surface albedo is obtained from hourly visual observations of fractional areas of ice types taken throughout the two-month voyage from September to November 1996, together with measured albedos of each type.
We define raw cloud transmittance (trc) as the ratio of the downward irradiance under cloud to that under clear sky at the same solar zenith angle. Clear sky is much more common over the sea ice, but when clouds do occur they appear to be of similar optical thickness to those over the open ocean. In the sea-ice zone trc under cloud is greater principally because of multiple reflections over the higher surface albedo. Transmittance through even the thinnest clouds was usually 20% less than that of clear sky, probably due to the threshold nature of the aerosol-to-cloud-droplet transition.
The shortwave cloud radiative forcing at the surface is smaller over sea ice due to both the higher surface albedo and the less-frequent occurrence of clouds. Cloud optical depth is calculated from a simple parameterization relating clear-sky and cloudy-sky downward fluxes to surface albedo, solar zenith angle, and cloud droplet number density derived by Wiscombe in 1973 for Arctic stratus clouds. Clouds over sea-ice and open water exhibit a similar range of optical depths. The values found are comparable to those found by Leontyeva and Stamnes over Barrow in the Arctic (0-100) and much larger than those found by Mahesh, Walden and Warren over the South Pole (0-6).
In future work the analyses will be repeated for other voyages in different seasons and prior years for which the same instrumentation is available, to obtain representative seasonal values of cloud optical thickness and transmittance in the Southern Ocean and Antarctic sea-ice zone.