Abstract
Abstract. The Antarctic Ice Sheet exhibits a strong anisotropy in microwave backscatter, both as a function of azimuth angle and incidence angle. This anisotropy arises as a result of i) the alignment of roughness elements and other wind-related surface and sub-surface features, as well as ii) internal layers and snow grain size gradient within the snowpack. As a result of its antenna configuration, the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Advanced SCATterometer (ASCAT) satellite instrument is able to observe much of the continent with a large azimuth and incidence angle diversity. A lack of azimuth and incidence diversity has restricted previous backscatter parameterisations to relatively simple bi-sinusoidal (azimuth angle) and linear (incidence angle) parameterisations. Using ASCAT, we show that a better fit can be obtained using a cubic incidence angle function and a Fourier series of up to four terms for parameterisation of the azimuth angle anisotropy. Scatterometer instruments have previously been used in Greenland to retrieve accumulation rate by observing the change in backscatter as a function of incidence angle. Here we present preliminary results of an empirical study linking the isotropic component and incidence angle dependence to snow accumulation rate in Antarctica, using snow stake measurements as ground truthing.
Highlights
Surface mass balance (SMB) is the net input term for ice mass balance calculations and is a critical parameter for accurate estimation of the Antarctic Ice Sheet (AIS) contribution to sea level rise (Allison et al, 2009)
“Gridded-swath” ASCAT Level 1B data were obtained from the EUMETSAT data centre, and gridded to a polar stereographic projection, with a ground resolution of approximately 12.5 km
We find a lower RMS residual in the empirical fit between SMB and A (0.20 m accumulation/annum), than with SMB and B (0.24 m/annum) or B1 (0.27 m/annum)
Summary
Surface mass balance (SMB) is the net input term for ice mass balance calculations and is a critical parameter for accurate estimation of the Antarctic Ice Sheet (AIS) contribution to sea level rise (Allison et al, 2009). Scatterometers are typically spaceborne, active instruments originally designed to obtain microwave backscatter measurements from small scale roughness on the surface of the ocean for multiple azimuth angles. From this information, surface wind strength and direction can be inferred. Since many scatterometers observe the surface at a range of incidence angles, the dependence of backscatter on incidence angle can be exploited to retrieve physical parameters of the snow/ firn, e.g., snow grain size (Ulaby et al, 1996) and SMB (Drinkwater et al, 2001). A more complete comparison of parameterisations will be presented in a forthcoming publication (Fraser et al, in prep.)
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