Identification of cloud‐free pixels in inhomogenous surfaces from AVHRR radiances

Abstract

A new method of distinguishing clear and cloud‐contaminated pixels from AVHRR data is described. Whereas most previous methods have difficulty in identifying cloud‐contaminated pixels when the background is inhomogenous, such as is the case for coastal regions or continental regions containing vegetated land and bare soil or land and water, the present method is particularly suited to these situations. Examination of the measured reflectivities in channels 1 and 2 of the AVHRR for these backgrounds and for clear skies suggests that the reflectivities are related linearly. The linear relationship is understandable in terms of pixels being comprised of linear combinations of two different surface types. Advantage is taken of this relationship as well as the radiance in channel 4 to identify cloud‐free pixels for complex surface conditions. The technique is readily automated and combined with other techniques that may be more suitable for homogenous backgrounds.