An operational, real-time cloud detection scheme for use in the Antarctic based on AVHRR data

A description is given of an automatic cloud detection scheme that has been developed for year-round, routine use on full (1.1 km) horizontal-resoluti on, five-channel Advanced Very High Resolution Radiometer (AVHRR) satellite imagery of the Antarctic continent and the adjacent ocean areas. The schem e is based on multiple independent tests to determine whether a pixel is cl oudy or cloud-free. Tests used include the use of various channel threshold s, inter-channel differences and assessment of the spatial coherence of inf rared data. During that part of the year when there is solar radiation, the channel 3 (3.7 mum) data are the most valuable as clouds composed of super cooled water droplets can be detected via their high reflectivity against t he dark ice surface. The scheme is successful at detecting most types of cl oud, but some problems still remain. During the daytime, low-level, optical ly thin cloud can be difficult to detect, while during the winter, thick, f eatureless cloud with a temperature that is similar to the surface is diffi cult to identify. Assessment of the early climatological fields suggests th at the scheme gives a realistic distribution of cloud over the southern par t of the Antarctic Peninsula and resolves the greater amount of cloud that is present in King George VI Sound and over the Southern Weddell Sea coasta l polynya, and the lower cloud fraction down the spine of the Peninsula.