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IOVWST Meeting
Portland, Maine, USA, 2019

ASCAT Normalized Radar Cross Section Validation

Presenting Author: Dr. Seubson Soisuvarn

The Advanced Scatterometer (ASCAT) is a radar instrument, flying on board the MetOp-A satellite, designed specifically to retrieve 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 (σ0) from small-scale wind-driven sea surface roughness. ASCAT wind retrievals have been validated against NWS Global Data Assimilation System (GDAS) global wind fields, QuikSCAT satellite scatterometer wind retrievals, and oceanic buoy wind measurements. The result shows that ASCAT wind retrieval performance is within 2 m/s RMS error for wind speeds from 0-15 m/s under all weather conditions, and 20° RMS directional error for wind speeds > ~5 m/s. However, ASCAT wind speed retrievals are significantly underestimated compared to GDAS and QuikSCAT winds for wind speeds > ~15 m/s. To further investigate the ASCAT high wind speed retrieval performance, ASCAT σ0 measurements from fore-, mid- and aft-beams are compared with predicted σ0 measurements to determine measurement residual errors and possible adjustments in the model function to improve ASCAT retrievals at higher wind speeds. A wind vector from a collocation dataset of ASCAT with the GDAS and QuikSCAT are being used with the CMOD5 to calculate the model σ0. These σ0 are compared with both of ASCAT measured σ0 at 25- and 50-km resolutions. The results show that at wind speeds > 15 m/s there are σ0 measurement residual errors across the swath and that these errors increase as a function of wind speed. An empirical relationship between the measured σ0 and “true” wind vectors are established and new model function coefficients are calculated. The harmonic response functions of wind speed for the CMOD5 and the empirical model function are compared.

2019 International OVWST Meeting
May 29-31 in Portland, Maine, USA
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