Microwave Backscattering Measurements of Alpine Snowcover

Snow is the dominant environmental factor in the alpine zone for more than half of the year. The properties of the snow cover influence the processes and species composition of alpine ecosystems. The snow cover alters the land-atmosphere interactions and radiation while making snow water a significant storage term in the hydrological cycle. Snow pack data are used operationally in numerical weather prediction, and snow pack water storage estimates are routinely accomplished and contribute to flood and reservoir recharge forecasts. On a local scale, the snow structure and liquid water content have a relevant influence on stability of the snow cover and, hence, on the formation of avalanches. The remote detection of critical snow conditions could help in avalanche prognosis. Active and passive microwave remote sensing sensors have proved to be powerful tools to monitor the spatial and temporal behavior of the snow covered earth surface and extensive studies have been conducted during the last 30 years. The running passive satellite based microwave systems cover well the frequency range between 10 and 100 GHz although at low spatial resolution. While the low spatial resolution is convenient for large scale climatic applications its a severe constraint for small scale applications in the Alps with strong topography. High resolution satellite based imaging radar systems were so far limited to frequencies up to 5 GHz, a severe constraint for snow applications. At these large wavelengths and the given observation geometry dry snow is mainly transparent and thus the backscattering from the ground below the dry snow dominates. Recently launched missions such as TerraSAR-X and Cosmo/Skymed and envisaged missions such as CoReH2O will enhance the spectral coverage up to 20 GHz and have the potential to improve microwave remote sensing of snow on a local scale and in alpine areas. However, a lack of simultaneous X- and Ku-band backscattering measurements and standardized and reproducible snow characterisation to compare with was identified to validate the available models and for application development. In oder to close this important gap a new X- to Ku-band scatterometer is under development. The design of the new X- to Ku-band scatterometer and related snow campaign in the Alps is partly driven by the University of Bern backscattering snow signature catalogue at 5.3 GHz and 35 GHz covering a wide range of alpine snow situations with target information. In this presentation selected snow situations and backscattering signatures at 5.3 GHz and 35 GHz will be presented and discussed, and an overview on the development of the new stepped frequency scatterometer and the planned campaign in the frame of the ESA-ESTEC SnowScat project will be given.