Summer Melt over Arctic Ice Caps

The Greenland ice sheet and larger Arctic ice caps have the potential to contribute significantly to global sea level rise if the rate at which they are losing mass changes in response to climate warming in the Arctic. Measurements of surface mass balance of these ice masses provide one way of quantifying the rate of mass loss and allow assessment of how this is related to prevailing climate conditions. However, long-term measurements of mass balance are sparse in the Arctic region and there is a need to upscale them to the whole ice cover. In the Arctic, inter-annual variability in the annual net mass balance is strongly correlated with the variability in summer balance – that is to say it arises largely from variability in summer melt. Fortunately active microwave satellite remote sensing offers opportunities to monitor summer melt across the whole of the Arctic ice cover.

Libo Wang initiated work on this topic in our group and the work is now being continued by Gabe Wolken. Libo used the Ku band backscatter measured by the Seawinds scatterometer on the QuikScat satellite to map the timing of melt onset and freeze-up, and the duration of the summer melt season across the Canadian Arctic ice caps and Greenland ice sheet for the period 2000-2004. Similar work is in progress for the Eurasian Arctic ice caps (Svalbard, Novaya Zemlya and Severnaya Zemlya).  The method relies upon identification of the sharp reduction in microwave backscatter that occurs once liquid water is present in a snow pack. The results of these studies provide a number of indices of summer melt conditions that may serve as proxies for the ice cap mass balance. They can also be used to investigate the relationships between synoptic climatic conditions and melt over ice caps, and to validate predictions of summer melt from regional climate and mass balance models. There is evidence from Prince of Wales Icefield on Ellesmere Island that the annual summer melt duration at a site may correlate with the annual melting degree day total, a parameter that can be used to estimate the summer melt intensity. Work is in progress to assess the potential for using the integrated reduction in backscatter that occurs over a summer melt season as a direct indicator of melt intensity. In future we plan to explore the potential of the 1km resolution EnviSat Global Mode ASAR data for monitoring summer melt at a much higher resolution than is possible using the enhanced resolution QuikScat data (2.25km resolution).

Active microwave remote sensing data can also provide valuable information about the distribution of snow and ice facies over large spatial areas. Snow and ice facies on glaciers, ice caps, and ice sheets include (from high to low elevation) the dry snow, percolation, saturation, superimposed ice, and glacier ice zones. The distribution of snow and ice facies is directly related to the annual processes of accumulation and surface melt and can thus provide valuable information about spatial and temporal variations in the surface mass balance of land ice in the Arctic. Gabe Wolken and others used enhanced resolution data from QuikScat to map the distribution of snow and ice facies and its inter-annual variability (1999-2005) in the Canadian High Arctic. Gabe Wolken and Martin Sharp applied similar methodologies to all ice covered areas in the Arctic during the period 1999-2008. In these studies, results indicated that increases in air temperature in the lower to mid-troposphere and melt duration were associated with a decrease in the areas of the percolation and saturation zones, an increase in the area of the glacier ice zone, and an increase in the elevation of inter-facies boundaries, including the equilibrium-line altitude.