Relating Microwave Backscatter Azimuth Modulation to Surface Properties of the Greenland Ice Sheet

Azimuth modulation of the normalized radar crosssection in satellite data sets over Greenland is investigated. Data sets from the NASA Scatterometer (NSCAT) and from the European Remote Sensing Advanced Microwave Instrument (ERS) are employed. Azimuth dependence is clearly observed. The largest azimuth dependence occurs in the C-band ERS data with peak-to-peak azimuth modulations up to 3.0 dB. The Kuband NSCAT data exhibits slightly smaller modulations of up to 2.0 dB. Azimuth modulation is largest in the lower dry snow zone for ERS and in the dry to percolation transition zone for NSCAT. The incidence angle dependence of the azimuth modulation is parameterized over the ice sheet. In general, the azimuth modulation is found to either decrease with increasing incidence angles, or be relatively independent of incidence angle. Regions of large incidence angle dependence for the azimuth modulation include the western dry snow zone for ERS and the northeast dry snow to percolation transition zone for NSCAT. The second order azimuth modulation orientation is highly correlated with wind direction. A new simple surface model is introduced to relate azimuth modulation to surface properties. Using this model, the size and orientation of surface sastrugi are estimated.