# Ka-Band Doppler Scatterometry: A Strong Wind Case Study
## 3 Sea Surface Cross-Section
### NRCS Decomposition  
The last Record #7 (see Figure 2a) offers a unique opportunity to separate different backscattering components in changing wind speed conditions. Providing that the fetch is limited at the northerly wind direction, the wind waves should rapidly adjust (in minutes) to wind changes. Thus, this 2.5-h-long Record #7 can be treated as the one composed from quasi-stationary samples of fetch-limited wind waves at different wind speeds. First, we focus on the observed polarization ratio (PR), \(P=\sigma_{0}^{\rm hh}/\sigma_{0}^{\rm vv}\), a parameter directly reflecting the partition between polarized and non-polarized backscattering components. A small value of PR, \(P<1\) (or \(P<0\) dB), indicates a dominance of the resonant Bragg backscattering mechanism, while PR values approaching 1 (or 0 dB) suggest a dominance of the non-polarized (NP) backscattering mechanism. The latter mechanism generally results from quasi-specular reflection, either from a regular (non-breaking) surface at low incidence angles or a steep wave breaking surface (at larger incidence angles). At \(\theta=45^{\circ}\), the probability of regular specular reflections is quite small, thus the NP backscattering component can be fully attributed to wave breaking events. Record #7 radar data are shown in Figure 6, including VV and HH NRCS in Figure 6a (also shown in Figure 5c,f) and PR in Figure 6b. These observation data are compared with the following GMFs and models:  
* Ka-DPMOD, the Ka-band dual-co-polarized GMF based on data obtained with the same experiment setup as in the present study;
* Ka-TSM, the two-scale (TSM) resonant Bragg model based on the radar imaging model (RIM) [50]. The above Ka-band estimates are augmented by empirical GMFs in other microwave bands to evaluate differences due to radar wavelength:  
* L-AQUARIUS, L-band (1.4 GHz) GMF inferred from Aquarius scatterometer data [51];
* C-SARMOD, C-band (5.255 GHz) GMF based on joint VV and HH data from Envisat Advanced SAR (ASAR), Radarsat-2, and Sentinel-1 SARs [52] for \(\theta\leq 42^{\circ}\);
* C-MOD/(H), a combination of C-band CMOD/7 (for VV polarization, [53]) and CMODH (for HH polarization, [54]) GMFs based on Envisat ASAR, Radarsat-2, and Sentinel-1 data for \(\theta\leq 42^{\circ}\);
* Ku-NSCAT-IFR, Ku-band GMF developed by the French Research Institute for Exploitation of the Sea (IFREMER) for NSCAT scatterometer data [55];
* Ku-NSCAT4, Ku-band GMF (version 4) developed by the Royal Netherlands Meteorological Institute (KNMI) for NSCAT scatterometer data [44]. In line with previous studies [37; 50; 56; 57], the observed PR \(\approx-2\) dB apparently exceeds TSM values, PR \(\approx-7\) dB (Figure 6b). Noticeably, empirical GMFs in Figure 6b suggest that PR decreases for longer radar wavelengths. However, its wind speed behavior is less obvious. While most of the GMFs in Figure 6b suggest that HH/VV ratio increases for \(U>10\) m s\({}^{-1}\) (in line with expected stronger NP component contribution due to wave breaking intensification), the CMOD7(H) and KaDPMod suggest a decrease. This different behavior may be related to the way they were derived. In particular, the CMOD7(H) is a combination of independent VV and HH GMFs, which may have affected their ratio, i.e., the PR. In comparison to other GMFs, the KaDPMod is applicable in a relatively narrow wind speed range, \(U<18\) m s\({}^{-1}\), which may affect its wind speed dependencies. For \(U\geq 12\) m s\({}^{-1}\), the observed Ka-band PR fit closely follows the Ku-NSCAT4 PR. An interesting feature of the observed PR wind dependency is the presence of a local minimum at about \(U=10\) m s\({}^{-1}\) (Figure 6b). A similar PR minimum is also present in both Ku-band GMFs but at lower wind speed, \(U=7\)-8 m s\({}^{-1}\). For longer radar wavelengths (C- and L-bands), such minimum in PR wind dependence cannot be firmly identified.