Calibration of Backscatter Measurement From OSCOM Airborne Campaign

Ocean currents and winds are crucial parameters to understand ocean-atmosphere interactions, while the simultaneous retrievals using Doppler scatterometry can provide direct observational support. To obtain high-quality airborne Doppler scatterometer wind and current products, accurate estimation of the observed backscatter coefficient and reliable wind field inversion are essential. Based on the Ocean Surface Current Observation Mission (OSCOM) airborne experiment data collected using a Ka-band rotating pencil-beam Doppler scatterometer, we propose two different calibration methods for the backscatter coefficient to account for the larger-than-expected azimuthal modulation of the backscatter signal, as predicted by consolidated Geophysical Model Functions (GMFs) used in Ka-band scatterometry. Both methods are based on the numerical ocean calibration (NOC) approach, which is in turn based on the estimation of the mean backscatter differences between real measurements and simulated ones with the use of the GMF and reference winds. The first method employs an azimuth-dependent calibration, which can be implemented using either an overall ratio or a ratio per flight leg. The second method involves modifying the GMF to match the observed azimuthal modulation, with options for one or two GMF coefficient adjustments. The retrieved wind speeds range from 4 to 7 m/s, with wind directions around 155°. In comparison with collocated ECMWF winds, the wind speed biases of different methods are all lower than 1.2 m/s, and the wind direction standard deviations (SD) are lower than 9.3 °. The azimuth-dependent calibration method yields smaller wind speed biases but larger wind direction SDs compared to the modified GMF method. The azimuth-dependent calibration using leg-dependent ratios leads to the closest retrieved wind speeds and directions to ECMWF. The calibration methods proposed in this study provide data support for future simultaneous retrieval studies of ocean winds and currents. Additionally, these methods can be applied to other airborne Doppler scatterometer experiments.