A note on Doppler processing of coherent radar backscatter from the water surface: With application to ocean surface wave measurements

Abstract

[1] The technique for extracting wave period and wave direction from radar backscattering intensity is well developed, but the determination of spectral density or wave height is hindered by the complex nature of the modulation transfer function. In contrast to backscattering intensity, the Doppler signal of a coherent radar is originated from the radial velocity of the scattering objects. Its oscillatory component is contributed by ocean waves. The peak component of the Doppler velocity spectrum can be used to obtain the spectral peak wave period and the significant wave height can be calculated from the variance of the Doppler velocity. Analyses of coherent radar measurements collected from the ocean show that with radar range coverage on the order of 10 dominant wavelengths, a good estimate of peak wave period and significant wave height is achievable with radar data as short as a few seconds.Citation: Hwang, P. A., M. A. Sletten, and J. V. Toporkov (2010), A note on Doppler processing of coherent radar backscatter from the water surface: With application to ocean surface wave measurements,

A simple method for retrieving significant wave height from Dopplerized X-band radar

“…Assuming that wave breaking and the incidence angle dependence are small, linear wave theory can be applied to transform the orbital velocity spectrum to a wave amplitude spectrum. Hwang et al (2010) showed that peak frequencies and wavelengths can be estimated reasonably well, while the integral spectral energy differs considerably from the one retrieved by a buoy. The authors attributed this to various non-trivial uncertainties, including directional distribution, shadowing effect, radar look direction with respect to wave propagation, swell modification and difference between spatial and temporal measurements.…”

“…With this accuracy, the proposed method performs at least as well as the best results using traditional methods, however, without the need for any calibration and sophisticated filtering techniques (Nieto-Borge et al, 1999;Vincent Bueno et al, 2012). In the following, the results are compared to the method suggested by Hwang et al (2010) and discussed with respect to the physical explanation of this purely empirical relationship. For this, the geophysical interpretation of the Doppler signal from the ocean surface has to be discussed briefly.…”

“…This is not necessary for the 15 min long records used in this study. However, Hwang et al (2010) used the rms of the Doppler velocity which, after their detrending, is expected to be close to the standard deviation, which is used here (u DRMS ∼ = σ D ) because no trend is present in the time domain. Apparently the wave heights obtained using Hwang's method are significantly overestimated in high sea states, due to the fact that high significant wave heights during storm situations are also associated with large peak wave periods.…”

“…7b with a correlation coefficient of 0.96, a standard deviation of 0.31 m and a bias of −0.08 m. The calibration coefficient X is found to be 0.82 for the VV antenna used here. Note that Hwang et al (2010) had to detrend their datasets using a Butterworth filter technique because of the extremely short duration of their radar records. This is not necessary for the 15 min long records used in this study.…”

“…Only a few studies exist on the ability to retrieve wave field information from coherent radar measurements of the sea surface. Hwang et al (2010) discussed a method to retrieve significant wave heights from space-time Doppler records of the ocean surface with an upwind pointing radar antenna. They suggested an empirical relationship, H s = 4X u DRMS /ω p , where u DRMS is the root mean square value of the Doppler velocity, X an empirical coefficient and ω p the peak wave frequency.…”