# Ka-Band Doppler Scatterometry: A Strong Wind Case Study
## 6 Conclusions  
This paper presents field measurements of the sea surface radar backscatter cross-section and Doppler centroid in the Ka-band collected at rather strong (up to 33 m s\({}^{-1}\)) wind speeds with distinct events of spray formation above the sea surface. Water droplet propagation within the radar beam causes the volume scattering and high-frequency signatures in the Doppler spectrum with Doppler velocities up to or even exceeding 10 m s\({}^{-1}\). However, the high-frequency tail spectral energy is 3 to 4 orders of magnitude weaker than the spectral energy of the signal backscattered from the sea surface. Hence, the spray contribution to the total NRCS observed in our experiment is negligible. Using the previously developed NRCS decomposition approach [55; 58] and this paper's co-polarized radar measurements, polarized Bragg-like and non-polarized (NP) radar backscatter components are assessed in a wide range of wind speeds and at similar wave conditions. Several empirical radar geophysical model functions (GMFs) developed for the L-, C-, and Ku-bands are used to estimate the short-scale wind wave spectrum. Previously, using the same experimental setup but for data collected at weaker wind speeds, \(U<15\) m s\({}^{-1}\), a decrease in NP relative contribution with growing wind speed has been reported [23]. This feature is confirmed with the present data, which also show that NP contribution starts increasing at \(U>15\) m s\({}^{-1}\), thus leading to a local minimum in the relative NP contribution. Empirical GMFs show that such local minimum in the relative NP contribution, thus in the polarization ratio, is also present in other bands. Although the relative contribution of breaking wave backscattering component increases in strong winds, the short-scale roughness spectrum (hence Bragg-like polarized backscatter) saturates at \(U>15\) m s\({}^{-1}\). The balance between polarized and non-polarized backscattering components governs the wind speed dependence of the NRCS, which is still not yet fully saturated for \(U<30\) m s\({}^{-1}\). The Doppler spectrum centroid is dominated by \"slow\" surface rather than \"fast\" volume scattering. With several long records in hand, it is demonstrated how the conventional radar modulation transfer function (MTF) approach (adopted for the Doppler centroid estimation in the KaDOP model [26]) can be applied to changing wind and wave conditions. On average, the KaDOP model adequately represents sea surface radar measurements. However, the effect of rapid changes in wind and wave conditions cannot be represented well by this model without the precise knowledge of concurrent wave spectra. This paper findings indicate that Doppler centroid prediction depends strongly on adequate knowledge of surface wave parameters rather than wind parameters. Although it is demonstrated for young offshore waves, similar wave conditions can be present in the open ocean under rapidly changing winds. This is especially important for small incidence angles, \(\approx\)20\({}^{\circ}\), exhibiting stronger long-wave NRCS modulation and wave-induced Doppler velocity contribution. Thus, in contrast to traditional NRCS geophysical model functions that solely rely on wind speed for sea state representation, the Doppler radar modeling is expected to be more robust if wave information is directly used. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/rs14061348/s1](https://www.mdpi.com/article/10.3390/rs14061348/s1)]([https://www.mdpi.com/article/10.3390/rs14061348/s1](https://www.mdpi.com/article/10.3390/rs14061348/s1)), Video S1: The sea surface with spray clouds at 20-30 m/s wind speed observed during the experiment. V.N.K. and Y.Y.Y. conceived and designed the experiments; V.N.K., B.C. and S.A.G. provided the sea surface modeling; Y.Y.Y. performed the experiments, analyzed the data, and wrote the paper. All authors have read and agreed to the published version of the manuscript.