MONITORING OF SPRING SNOWMELT WITH ENVISAT ASAR WS IN THE EASTERN ALPS BY COMBINATION OF ASCENDING AND DESCENDING ORBITS

ENVISAT ASAR Wide Swath data from ascending and descending orbit have been normalized and combined in the eastern Alps in order to increase the covered area from 60-70% to up to 78%. Band rationing was applied for the identification of wet snow surface. Although that probability maps of thawing snow can be derived, combinations of morning and evening orbits are difficult to interpret in late spring. Wet snow could be identified for forested as well as un-forested regions. Areas, which have either thawed completely, only at the surface or such which have not yet undergone thawing, can be distinguished with the use of optical data such as SPOT acquired shortly after the ASAR images. ENVISAT Global mode data can also be used to detect snowmelt patters over large areas. Good temporal resolution allows the determination of snowmelt at altitudes above 2000 m in the study area.