Cloud detection and classification over oceans at night with NOAA-7

Abstract

Abstract Clouds above the ocean are detected with three thermal-infrared channels of the AVHRR on board NOAA-7. A classification into low, middle and high clouds is possible. As only spectral and not spatial information is used, it is possible to analyse each pixel individually. The classification has been developed with a radiative transfer model for clouds adapted to the spectral domain of the NOAA-7 infrared channels. The ruling parameter is the ratio between scattering and extinction. The spectral optical depth has slightly less influence, while different anisotrophy of scattering for individual types of clouds is not important. Two parameters are best suited to classifying clouds, namely, brightness temperature in channel 5(12·0 μm) and the difference of brightness temperature between channel 3 (3·7μm) and channel 4 (10·8 μm). The algorithm to classify clouds with thresholds referring to the standard atmosphere ‘autumn 45° north’ is applied to actual satellite data.