Melting of small Arctic ice caps observed from ERS scatterometer time series

[1] Time series of active microwave backscatter data from the ERS Wind Scatterometers are used to demonstrate (1) the timing of melt onset can be observed over small ice caps, as well as the major ice sheets and multi-year sea ice; and (2) temporally integrated backscatter reduction (R) is strongly correlated with total annual positive-degree-days (PDD), as measured at four automated weather stations on the surface of the Greenland Ice Sheet. The latter is particularly interesting because unlike melt onset, R is a measure of melt processes integrated over an entire ablation season. Both methods work owing to the well-known sharp decrease in radar backscatter triggered by the appearance of liquid water in a glacier snowpack. Melt onset and R are determined for 14 small Arctic ice caps from 1992–2000. Interannual and regional variability in the timing of melt onset are comparable, averaging 33 ± 9 and 38 ± 13 days, respectively. Regional variability in R exceeds interannual variability, averaging 906 ± 164 and 309 ± 130 dB d, respectively. The brevity of the derived nine-year records limits their statistical testing for trend. However, their qualitative interpretation suggests that the timing of melt onset is more variable for Arctic ice caps than for sea ice, and the intensity of summer surface melting increased by about 20% in the 1990s.