Snow covered area retrieval using Envisat ASAR wideswath in mountainous areas

It has previously been shown that wet snow can be detected using ERS SAR repeat pass imagery where a reference image is captured during cold dry snow conditions and subtracted from the image one want to classify. We have extended and validated this technique for retrieving snow covered area (SCA) using Envisat ASAR wide swath data (500 by 500 km swath coverage, 100 meter resolution), covering the mountainous regions of Southern Norway. The algorithm has also been extended to postulate dry snow above areas with wet snow, thus giving a total snow covered area that is comparable to SCA from optical sensors. A sliding window technique has been applied to facilitate the dry snow classification. The method has been implemented in a near-real time environment and has been run pre-operationally in Norway in 2004. The relatively large coverage allows SAR to become an operational tool for snow monitoring, as opposed to standard modes used in previous works. In order to improve snow classification we have used air temperature data from the Norwegian meteorological station network to create high-resolution surface air temperature maps. These maps are used to filter wet snow from reference images and prevent incorrect classification of dry snow. Snow covered area maps for South Norway has been derived for the spring melt season with a one-week temporal resolution. The results are validated against optical sensor retrievals (MODIS, ASTER) and high accuracy field measurements.