# Ka-Band Doppler Scatterometry: A Strong Wind Case Study
## 3 Sea Surface Cross-Section  
### Sea Spray Signatures  
Signatures of sea spray in instantaneous Doppler spectra, \(S(f_{\rm d},t)\) can be well identified. As a typical example, a part of Record #7 collected at \(\theta=45^{\circ}\) is shown Figure 3. The spectrum can be speculatively partitioned into a low Doppler frequency, \(f_{\rm d}\), \"core\", associated with orbital motions of surface waves (\(|V_{\rm d}|<2\) m s\({}^{-1}\)), and high \(|f_{\rm d}|\) transient tails, largely associated with radar signal spikes (mostly positive for upwind look geometry). Most of these spikes can also be associated with peaks in the \"core\" and/or wave breaking events (see more examples in the dedicated study [42]). However, some events occur independent of wave characteristics within the radar footprint. These latter events are likely produced by spray clouds generated upstream of the radar footprint. For further insights, we select data with stable northerly wind direction and varying wind speed (Records #5 from 01:00 UTC, #6, and #7). Instantaneous Doppler spectra are then averaged over different wind speed intervals (Figure 4), to illustrate some wind-dependent high-frequency Doppler spectrum features. The upper limit of the observed line-of-sight Doppler velocities is about 10 m s\({}^{-1}\) (based on the radar signal-to-noise characteristics) or about 14 m s\({}^{-1}\) (if converted to ground range speed). In our opinion, the only explanation for such high Doppler velocities is sea spray clouds embedded in the airflow. Regardless of polarization and incidence angle, the \"tail\" spectrum level is 3-4 orders of magnitude lower than the \"core\" spectrum level. Thus, though the spray contribution can be distinguished in Doppler spectra due to faster propagation speed, its net NRCS contribution is apparently negligible, at least at observed winds, \(U<33\) m s\({}^{-1}\). Figure 3: Sample record fragment showing HH-polarization Doppler spectrum at \(U\approx 20\)–\(25\) m s\({}^{-1}\), \(\theta=45^{\circ}\). The mean NRCS wind dependencies (Figure 5) do not explicitly exhibit any specific features, suggesting a backscattering regime change at strong winds. Mean NRCS dependencies are qualitatively similar to the background Ku-band geophysical model functions (GMF) derived from the Tropical Rainfall Measurement Mission (TRMM, the closest \(\theta=18^{\circ}\) is used, see the bottom curve in their Figure 7 [30]) and NSCAT4 [43, 44] satellite data. The TRMM GMF possibly suggests a noticeable NRCS decrease for \(U>35\) m s\({}^{-1}\), i.e., wind speeds exceeding our observation range. In general, this behavior is in line with the often-reported co-polarized backscatter saturation, possibly due to a wind stress attenuation at strong winds [45, 46, 47, 48, 49]. Figure 4: Doppler spectra averaged at different wind speeds (color-coded) for **(a,b)**\(\theta=20^{\circ}\) and **(c,d)**\(\theta=45^{\circ}\). **(Left column)**—VV-polarization, **(right column)**—HH-polarization. Figure 5: Measured NRCS at \(\theta=20^{\circ}\) and \(\theta=45^{\circ}\) for different radar-to-wind azimuths in comparison with KaDPMod, NSCAT-4, and TRMM GMFs. **(a–c)**—VV-polarization, **(d–f)**—HH-polarization. Our data are thus consistent with the background Ku-band GMF at low incidence angles, \(\theta=20^{\circ}\), for which strong wind speed dependence is not expected (Figure 5a,b,d,e). Our former NRCS parameterization (Ka-band Dual Polarization Model, KaDPMod, [23]) built on the same instrumental setup, but using more common wind conditions, \(U<18\) m s\({}^{-1}\), agrees well with the present dataset at strong winds, \(U>10\) m s\({}^{-1}\). The change of radar-to-wind azimuth (compare left and middle columns in Figure 5) does not significantly alter the NRCS wind dependence. At larger incidence angles, \(\theta=45^{\circ}\) (Figure 5c,f), the observed NRCS starts to be more wind-dependent, but stronger discrepancies are found compared to the Ku-band NSCAT4 GMF.