Detecting new snow on ice sheets

Robert Bindschadler/970
Hyeungu Choi/SAIC

Al Chang was a leader in using the microwave emissions from snow to determine important properties of the seasonal snowpack. His pioneering insights also apply to snow that falls on ice sheets and is eventually incorporated into the body of the ice sheet. This snowpack is permanent with older snow buried by subsequent snowfalls. The relevant issues are determining when, where and how much snow falls in single accumulation events. The amounts of added snow tend to be small, typically a few centimeters and the snowpack depth can be treated as an infinite half-space on the scale of microwave wavelengths.

Our approach focuses on detecting a change in the surface emission characteristics that we relate to the addition of new snow. Fresh snow, regardless of the size or shape of the crystals, immediately begins to metamorphose into more rounded shapes to reduce the surface free energy of the individual crystals. This is expected to reduce the microwave emissivity, particularly at the higher frequencies.

We compare the temporal signature of 85GHz microwave brightness temperatures of ice sheets with field measurements of meteorological conditions and of surface height. We show that most rapid increases in 85GHz brightness temperature are associated with new snow. We also illustrate that the effect of cloud scattering on the 85GHz brightness temperatures is small compared to the new-snow effect. Other events that can change the surface emission, such as wind and sun crusts, also occur and cannot be separated from the new-snow events. Finally, we apply this approach to both Antarctica and Greenland to produce an animation of when and where snow accumulated over these ice sheets for the years 1999 to 2003. Work to attempt to quantify the snowfall amounts with ICESat data are continuing.

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