Understanding snow in depth – ESA paves the way
An ESA campaign carried out in Austria has demonstrated how snow, a key component in the Earth's water cycle and important source of fresh water, can be measured from space using radar. The campaign supports one of the main objectives of the candidate Earth Explorer CoReH2O.
Cold Regions Hydrology High-Resolution Observatory(CoReH2O) is one of six candidate Earth Explorer missions that has just completed an assessment study and will be presented to the science community at a User Consultation Meeting in January 2009. Subsequently, up to three of the missions will be selected for the next stage of development (feasibility study), leading to the eventual launch of ESA's seventh Earth Explorer mission around 2016.
With climate change becoming more evident, there is a need to understand more about how a warming climate is altering the distribution of snow and ice and how this, in turn, is influencing the water cycle and associated feedbacks within the Earth system. The magnitude of feedbacks linking snow and ice to other components of the Earth system are very uncertain in current climate predictions.
In addition, snow and ice are important sources of fresh water with more than one-sixth of the Earth's population relying on seasonal snow packs and glaciers for their main supply. Decreasing snow and ice cover, due to a changing climate, will affect fresh water resources and consequently impact such things as human health, regional food security and biodiversity.
If selected, the CoReH2O mission would quantify how much water is held in snow and ice by employing dual-frequency (9.6 GHz X-band and 17.2 GHz Ku-band) Synthetic Aperture Radars (SAR) in two consecutive mission phases to deliver all-weather, year-round information on regional and continental snow-water equivalent.
To ensure that this concept would work from space, a number of experimental campaigns have been carried out to simulate the measuring technique. One such campaign – HeliSnow-2008 – was carried out in two separate one-week operations in the Austrian Alps earlier this year.
The campaign involved a helicopter being skilfully flown through alpine valleys and over glaciers to target different test sites chosen to cover a range of terrains. The different terrains included meadow, agricultural land, forest, high Alpine-pasture and glaciers. A scatterometer instrument, called Multi3Scat, carried aboard a helicopter took measurements at five different frequencies, including X- and Ku-band. It was the first time that this scatterometer, operated by the University of Hamburg (Germany), had been used for measuring snow.
The HeliSnow-2008 campaign was aimed at the validation and advancement of theoretical backscatter models and inversion algorithms for a wide range of different snow conditions. To achieve this, the Multi3Scat backscatter measurements were supported by detailed measurements of the physical properties of snow at several field stations. As well as measuring the depth of the snow layer, the snow was painstakingly characterised by grain size, density and shape.
"The HeliSnow-2008 campaign delivered a unique multi-frequency backscatter dataset that helped to advance and test algorithms for the retrieval of snow parameters. There has been a lack of backscatter data of snow extending into the Ku-band frequency range. This campaign helped to diminish this data gap significantly," commented Professor Helmut Rott from Environmental Earth Observation IT GmbH (ENVEO).
The HeliSnow-2008 campaign complements an earlier campaign carried out in 2007 called SARAlps, where Ku-band and X-band backscatter data of snow had been measured with a ground-based SAR over two sites in the Austrian Alps.
The large amount of data from both campaigns is expected to generate considerable interest since a complete remote-sensing dataset and simultaneously acquired ground data provide a valuable reference. Now that the dataset has been fully analysed it will soon be made available to the scientific community through ESA.