Methods of estimating ridging of Antarctic sea ice

Estimating the sea ice thickness surrounding Antarctica is important for determining the ocean-atmosphere heat fluxes, the salt flux to the ocean and for verifying global climate models. A widespread method of estimating ice thickness in the Antarctic is the use of ice observation data. These data are collected by ship-based observers who estimate the sea ice and snow properties hourly as the ship transects the pack. This is a relatively low-cost method of collecting data on ice properties, and data can be collected in all weather conditions, both day and night. However, it is limited to the region of the pack immediately around the ship, and consequently to ice regimes that the ship transects. The largest uncertainty in estimating the total ice volume is from the estimates of the ridging properties (average ridge height and percent ridging). To help quantify this error, we compared the ice observation data to ridge distribution data collected using aerial photography. The number of ridges, and the volume of ice contained in these ridges are estimated using 505 aerial photographs. Using a ridge cut-off height of 0.4 m, 0.8 m and 1.0 m, the ridge frequency is 60 km/sup -1/, 12 km/sup -1/ and 6 km/sup -1/, respectively. Using the same cut-off heights, these ridges are estimated to add 0.7 m, 0.4 m, and 0.3 m to the area-averaged, undeformed ice thickness. Different ridging regimes are also compared to backscatter values obtained from synthetic aperture radar (SAR) data collected from RADARSAT-1.