ABSTRACT
The physical basis of snow retrievals by means of microwave radiometry is reviewed. Effects of atmospheric attenuation and emission on the microwave radiation measured at satellite altitude are discussed and examples for the modification of microwave radiation of the earth surface due to atmospheric effects are shown. Algorithms are presented which enable the inversion of satellite measurements in respect to areal extent, water equivalent, and depth of the snow cover. Intercomparisons of different algorithms for snow mapping show considerable differences of the retrieved snow cover products. Reasons for these differences include (i) regional variations in snow morphology and land cover type, which affect the relation between the emitted radiation and the depth and water equivalent of the snow cover; (ii) the use of different radiometer channels for the retrievals; (iii) differences in the multitemporal treatment of the data. Existing algorithms have been developed and verified at regional scales, but an accurately verified global snow retrieval algorithm is still missing. Such an algorithm needs to include factors accounting for regional diversities of snow morphology and land cover type, in particular forest cover. Enhanced efforts in this respect are needed to further utilize the great potential of microwave radiometry for monitoring the extent and mass of the global snow cover, a crucial element for climate research and for the global water and energy cycle.
