Ground Validation of Proposed SWE Retrieval Algorithms

The Northern high latitudes are now experiencing some of the most rapid and severe climate change on Earth. Arctic warming, almost twice the rate as that of the rest of the world over the last 30 years. It is expected to accelerate, contributing to major bio-geophysical changes. Snow cover appears as a key indicator in such processes, and its dynamics need to be fully understood. Seasonally, it covers up to 50 million square kilometres affecting atmospheric circulation and climate from regional to global scales. Snow cover and glaciers are vital resources of fresh water in high latitude regions and many densely populated areas at mid and low latitudes. Accelerating shrinkage of seasonal snow pack and mountain glaciers due to global warming threatens water supply in particular in regions, where snow and glaciers are dominant sources of runoff. This is not only the case in high and mid latitudes, but snow and ice also provide headwaters to many major rivers of the world that supply water to populated areas in semi-arid and arid regions. Himalayan snow and glaciers, for example, feed many of Asia's great rivers, and snow and ice of the Andes/Rockies chain provide crucial water resources to many areas along the mountains. It is estimatede that over a billion people depend on for their water resources from snow melt. Fresh water is already a limited resource in many of these regions. Within the next decades the population growth will further impair the water availability. Even in Central Europe, glacier and snow melt has an important regulatory function for water supply. Climate change seriously threatens the abundance of these resources, calling for immediate action to improve the understanding of climatic effects on the water cycle. The European Space Agency has recognized the importance of reliable estimates of snow water equivalent (SWE, the amount water stored in snow) and snow depth, and is supporting a satellite mission that will use microwave radar imagery to measure these two important parameters. It will also measure the spatial extent, mass and type of snow (such as how 'dry' or 'wet' it is). The mission will provide a new type of high quality observations, which will be used to initialize, run and validate water, weather and climate models for prediction and environmental monitoring applications.It will use an innovative combination of images measured at frequencies 10GHz and 17GHz. The two-frequency combination is needed to sample a sufficient range of snow depth. One-way penetration depths are typically 4m at the higher frequency, which is sufficient to penetrate most natural dry snow cover, but the lower frequency with a typical penetration depth of 10m is needed for deeper snow in mountainous areas. To remove contamination to the signal by any from any returns coming from the ground underneath, so that we only measure the snow signal, we use the knowledge that the certain properties ('polarisation') of the returned signal will be different depending upon whether it reflected from the snow or ground. A programme of work is proposed here utilizing the UK's Ground-Based SAR (GB-SAR) System in a field measurement campaign to validate and develop the proposed two frequency combination for snow measurement prior to satellite mission. GB-SAR is a portable radar imaging system, which provides well-calibrated, high quality imagery. It will deployed at a test site in the Alps during the winter and measure the radar behaviour of snow for different depths, state, and environmental conditions. Complementary detailed measurements of snow properties and weather conditions will be made. Together, they provide the unique opportunity to carry out precise measurements in controlled conditions in which all system and environmental parameters are known, in order to obtain a better understanding of the relationships between them.