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

Abstract

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-resolution, five-channel Advanced Very High Resolution Radiometer (AVHRR) satellite imagery of the Antarctic continent and the adjacent ocean areas. The scheme is based on multiple independent tests to determine whether a pixel is cloudy or cloud-free. Tests used include the use of various channel thresholds, inter-channel differences and assessment of the spatial coherence of infrared data. During that part of the year when there is solar radiation, the channel 3 (3.7 m) data are the most valuable as clouds composed of supercooled water droplets can be detected via their high reflectivity against the dark ice surface. The scheme is successful at detecting most types of cloud, but some problems still remain. During the daytime, low-level, optically thin cloud can be difficult to detect, while during the winter, thick, featureless cloud with a temperature that is similar to the surface is difficult to identify. Assessment of the early climatological fields suggests that the scheme gives a realistic distribution of cloud over the southern part 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 coastal polynya, and the lower cloud fraction down the spine of the Peninsula.