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Wave-induced Doppler shift of Ka-band radar signal backscattered from the sea surface

The Wave-Induced Doppler velocity (WIDV) arising from correlated modulations of sea surface velocity and its normalized radar cross-section (NRCS) is evaluated from Ka-band radar measurements taken from the Black Sea research platform. The WIDV measurements are compared to the WIDV simulations using an empirical Ka-band Modulation Transfer Function (MTF) and wind wave spectrum parameterization zeroed at frequencies below the observed peak frequency to account for fetch-limited waves typical of the Black Sea. Given a good agreement between measured and simulated WIDV, we extrapolate the WIDV onto the open ocean conditions by using the fully developed wave spectrum. Simulated open ocean WIDV depends on incidence angle, azimuth, radar polarization, and wind speed. The WIDV magnitude has a peak at small incidence angles (θ = 20°), with downwind magnitude larger than upwind magnitude. At larger θ, the WIDV decreases. The wind speed dependence of WIDV is explained by two competing factors, MTF decrease with the wind and peak wave orbital velocity amplification with the wind. The typical magnitude of WIDV is 3 to 8 times larger than either Bragg wave phase velocity or wind drift velocity for winds between 5m/s and 15m/s. Thus, the WIDV is important for the sea surface currents retrieval from satellite Doppler radar measurements.