Observations of the Antarctic ice sheet with the Seasat scatterometer: relation to katabatic-wind intensity and direction

Abstract

The Seasat A satellite scatterometer radar, initially designed to measure ocean-wind intensity and direction, also provided observations on the Antarctic ice sheet. The signal of the back-scatter coefficient decreases strongly from 10 to −20 dB when the incidence angle of the observations increases from 0° to 65°. An additional 5 dB signal is found, which is correlated with the direction and intensity of katabatic winds, independent of the incidence angle and polarization of the signal. By using simplified models of the volume-scattering within the snowpack (which is mostly sensitive to snow grain-size) and surface-scattering from the air-snow interface (which depends on roughness), it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle, and of micro-roughness and volume back-scatter at incidence angles greater than 25°. The instrument therefore provides a means of measuring the direction and intensity of katabatic winds.

Observation and characterization of radar backscatter over Greenland

“…Using a simple physical model, we [18] find that azimuth dependence of can be directly related to aeolian-formed surface features such as sastrugi, i.e., the physical model indicates that azimuth modulation of over Greenland is related to katabatic wind flow, which is similar to findings over Antarctica [16], [17], [19], [20]. Using a physical model provides valuable insights into the relationship between surface features and backscatter; however, inversion of the model requires secondorder iterative nonlinear regression.…”

“…The models previously proposed for the azimuth signature over Antarctica each include one or more terms of a Fourier Series. Ledroit et al [19] use a bisinusoid based on a theoretical model of ocean backscatter to model azimuth dependence of Seasat-A scatterometer measurements over Antarctica, finding azimuth dependencies as large as 5 dB at Ku-band. Similarly, Young et al [20] use a bisinusoid to model ERS variations over Antarctica, finding azimuth modulations of similar magnitude (5.5 dB peak-to-peak maximum) for C-band.…”

“…Ledroit et al [19] state that large-scale roughness corresponds with azimuth modulation at low incidence angles, and small-scale roughness corresponds to azimuth modulation at high incidence angles. Thus, the variation of azimuth modulation with incidence angle is an indication of the relative roughness of the surface features driving the modulation.…”

WindSat Passive Microwave Polarimetric Signatures of the Greenland Ice Sheet

“…For example, Ledroit et al [21] attributed the azimuthal modulation of normalized radar cross-sectional measurements to sastrugi and correlated such a modulation with the katabatic wind direction. Ashcraft and Long [8] assume that snow surface roughness at the 3-300-m scales are the primary mechanism driving the modulation, and developed successfully an ERS scatterometer model to extract the azimuthal modulation and surface snow properties.…”

Polar Applications of Spaceborne Scatterometers

“…At Ku-band, microwaves are more effectively scattered by the snow grains and thin ice layers, thereby resulting in larger Ku-band values in the dry snow zone, in the center of the Greenland. Multiple layers from annual accumulation complicate this when large-scale coherent structures, such as snow dunes or sastrugi are involved [94]. These features can introduce a wind-dependent azimuth variation to the backscatter not seen in sea ice.…”