Cloud cover analysis with Arctic AVHRR data: 1. Cloud detection

Abstract

Automated analyses of satellite radiance data have concentrated heavily on low and middle latitude situations. Some of the design objectives for the International Satellite Cloud Climatology Project (ISCCP) cloud detection procedure such as space and time contrasts are used in a basic algorithm from which a polar cloud detection algorithm is developed. This algorithm is applied to Arctic data for January and July conditions. Both advanced very high resolution radiometer (AVHRR) and scanning multichannel microwave radiometer (SMMR) data are utilized. Synthetic AVHRR and SMMR data for a 7‐day analysis period are also generated to provide a data set with known characteristics on which to test and validate algorithms. Modifications to the basic algorithm for polar conditions include the use of SMMR and SMMR‐derived data sets for the estimation of surface parameters, elimination of the spatial test for the warmest pixel, the use of AVHRR channels 1 (0.7 μm), 3 (3.7 μm), and 4 (11 μm) in the temporal tests and the final multispectral thresholding, and the use of surface class characteristic values when clear‐sky values cannot be obtained. Additionally, the difference between channels 3 and 4 is included in the temporal test for the detection of optically thin cloud. Greatest improvement in computed cloud fraction is realized over snow and ice surfaces; over open water or snow‐free land, all versions perform similarly. Since the inclusion of SMMR for surface analysis and additional spectral channels increases the computational burden, its use may be justified only over snow and ice‐covered regions.