The Development of a C-Band Advanced Scatterometer (ASCAT) Geophysical Model Function at NOAA/NESDIS

The Advanced Scatterometer (ASCAT) is a radar instrument on board the MetOp-A satellite that designed specifically to measure ocean surface wind speed and direction. The ASCAT transmits 5.255 GHz (C-band) microwave energy using vertically polarized fan-beam antennas to the ocean surface and measures the returned radar backscatter signal from small-scale wind-driven sea surface roughness. ASCAT incidence angle ranges 25°-65° from antenna beams directed at +/-45°, +/-90° and +/-135° relative to the sub satellite track, from which 550-km dual swaths are formed.  As the satellite travels forward, the normalized radar cross section (sigma0) are measured from the fore-, mid- and aft-beams respectively; resulting in a triplet of sigma0 measurements for each wind vector cell. Given knowledge of the ASCAT measurement geometry parameters and the relationship between the sigma0 and wind vector know as the Geophysical Model Function (GMF), the wind speed and direction can be retrieved unambiguously. Validation of the ASCAT wind vectors show 2 m/s RMS errors for wind speeds > 15 m/s, however they exhibit and increasing low bias beyond 15 m/s, and less than 20° RMS directional errors for wind speed > 5 m/s.

An examination of ASCAT sigma0 revealed some additional sensitivity at the higher wind speeds that was not adequately represented by the current GMF. A revised GMF is empirically derived to find a relationship between sigma0 and wind speed using a near real-time QuikSCAT as a surface truth, where the GMF is usually modeled as two harmonic cosine Fourier series. A new DC term in the GMF is derived and replaced in the operational CMOD5.5 GMF. Validation of the revised GMF shows that the wind speed retrievals beyond 15 m/s are closer to QuikSCAT than the operational retrievals, while wind direction retrievals remain relatively the same for all wind speeds as expected.