# Ka-Band Doppler Scatterometry: A Strong Wind Case Study
## 1 Introduction  
The characteristics of Ka-band sea surface radar backscatter have drawn growing attention, with continuing efforts to propose new Doppler scatterometer space mission concepts [1, 2, 3, 4, 5, 6] for direct concurrent monitoring of global surface winds and currents. Indeed, switching to shorter but still atmospheric transmission radial wavelengths could allow achieving an improved measurement accuracy at a given antenna size [7].  
Following relatively rare past Ka-band radar studies [8, 9, 10, 11, 12], significant recent observation [13, 14, 15, 16, 17, 18] and modeling [19, 20, 21, 22] efforts have been undertaken. These include our group multi-year field experiments. From a research tower in the coastal Black Sea, in situ measurements have allowed us to parameterize the Ka-band normalized radar cross-section(NRCS) [23], examine its modulation by waves [24; 25], and propose a Doppler centroid model [26].  
However, high-wind Ka-band backscatter properties have not been addressed by any of these previous studies. For longer C-to-Ku-band wavelengths, a clear NRCS saturation has been reported for co-polarized signals at winds above 25-30 m s\({}^{-1}\)[27; 28; 29; 30]. For shorter Ka-band e.m. wavelengths, the sea surface radar backscatter properties are majorly unknown at such strong winds. Moreover, the Doppler characteristics, required if we are to understand the potential of Doppler scatterometry under extreme environmental conditions, have not ever been reported. Complete understanding of the physics of Ka-band radar backscatter at harsh winds is also challenged by the impact of sea spray commonly present in the air-sea boundary in such conditions. Interestingly, the early radar sea echo theories were based on the volume backscattering by sea spray only [31]. Indeed, the theory of sea spray radar contribution is well established [32; 33], but mainly for precipitation radar applications [34]. For radar backscattering studies, field data on sea spray impacts radar are scarce, especially for Ka-band measurements. Field measurements of surf breakers [35] generally correspond to enhanced radar backscatter events within the tail of Doppler spectra. Later, the dominant role of surface backscattering was revealed and the sea spray role downscaled [36]. Still, laboratory Ku-band measurements [37] suggested that sea surface radar backscatter can be significantly affected by relatively large droplets, developing very close to the surface.  
This paper aims to complement our previous studies by reporting a case study conducted at strong (up to 33 m s\({}^{-1}\)) winds. During wind gusts above 20-25 m s\({}^{-1}\), the generation of sea spray clouds was visually observed (see the Supplementary Materials, Video S1). Radar measurements made during these conditions can then help to more precisely assess the impact of sea spray on Ka-band backscatter cross-section and its Doppler characteristics.  
The paper is organized as follows. The experimental setup, environmental conditions, and Doppler radar processing brief are given in Section 2. Sections 3 and 4 focus on NRCS and Doppler characteristics at strong winds, respectively.