Cryosat-2 thickness retrievals of freshwater lake ice

Abstract:

The European Space Agency’s (ESA) Cryosat mission was launched to improve our knowledge of the trends in the thickness of sea ice and glaciers. The new Synthetic Aperture processing method allows for significantly enhanced along-track resolution compared to traditional pulse-limited radar altimeters. Satellite observations have revealed rapid changes in the duration of the seasonal snow and ice cover of subarctic lakes. The often smooth, homogeneous ice cover of lakes is an excellent target for detailed studies of radar altimeter and imaging radar backscatter behavior. Furthermore, there is only limited information available regarding the changes in ice thickness of these lakes. Here we present and validate a method to retrieve the thickness of lake ice using CryoSat L1B data. In contrast to sea ice measurements where ice thickness is derived from isostatic freeboard retrievals, we obtain ice thickness from radar returns from both the ice surface and bottom, assuming that CryoSat’s Ku-band radar pulses can penetrate through freshwater ice. The seasonal evolution of ice thickness of Great Bear Lake and Great Slave Lake, two large lakes in northern Canada, thus observed is compared to in-situ measurements, SAR imagery, scatterometer data, the results of a freezing-degree-day model, and previous studies. These confirm that the Ku-band signal often penetrates through the low-loss freshwater ice and is scattered from both the snow/ice and the ice/water interfaces. We examine the data for scattering from within the snow pack and the ice as this introduces uncertainty in the retrieval of ice thickness by masking the signal from snow/ice or ice/water interfaces. Although not designed for freshwater lake ice studies, CryoSat-2 and future SAR/SARIN mode satellite altimeter missions offer new possibilities to monitor Arctic and sub-Arctic lakes.