Methods for digital analysis of AVHRR sea ice images

Algorithms for obtaining information on sea ice conditions from Advanced Very High Resolution Radiometer (AVHRR) digital imagery are presented. These algorithms comprise three steps: discrimination of cloud-covered areas from ice-covered and open water areas, estimation of ice concentration, and measurement of the sizes of ice floes and open water areas (polynyas) within the ice field. For ice-water-cloud discrimination, a dynamic threshold technique is applied to six combinations of the five spectral channels of the AVHRR. This results in an image that is useful for ice analyses. To derive ice concentration estimates, a simple linear relationship between pixel intensity values and amount of ice cover is applied to the visible channel reflectance data. The method for obtaining size frequency distributions is based on a size transform of binary realizations of the ice concentration result and is equally applicable to measuring polynyas and ice floes