Remote sensing of snow with passive microwave radiometers

This report considers passive microwave radiometry and its application for satellite remote sensing of snow at a global scale. The microwaves observed from a dry snow pack are emitted from the ground beneath the snow. The bulk effect of the snow cover on the micro‐ waves is that the measured brightness temperature will decrease. This depends on the mass of the snow (snow water equivalent – SWE), but also on the snow grain size and internal layering. Other important factors include the forest type. The microwave sensitivi‐ ty to the snow is strongly dependent on the microwave frequency. The microwave feature to be related to snow is therefore the difference in brightness temperature between two frequencies. Simple empirical algorithms are valid only for limited areas, and global algo‐ rithms should therefore consider most of the factors that influence the satellite observa‐ tions. Radiative transfer models, which aim at predicting the microwave signal from the snow variables, can be valuable for validating algorithms or they may be part of them. As a basis for global algorithm the HUT model and its corresponding algorithms are suggested.