# Automated Rain Detection by Dual-Polarization Sentinel-1 Data
## 3 Co-Analysis of Rain Rate and C-Band SAR Backscattering
### Statistical Analysis
The legend in each sub figure stands for the NRCS in rain-free area, suggesting different wind regimes (0-4 m/s, 4-8 m/s, 8-16 m/s, and \(>\)16 m/s) as approximately derived with CMOD-5 model. Above histogram denotes the data amount for lines at each base reflectivity bin. However, the ASCAT scatterometer (as well as microwave radiometers) have resolution of tens of kilometers, whereas the statistics in this study is at a resolution of 1 km. Neglecting the atmosphere attenuation effect (reasonable for light rain at C-band), we anticipate the negative NRCSratio values reveal the localized effect of small-scales surface waves damping under light-to-medium rain rate. Because the data are in high resolution, this effect is not counterbalanced by wind sea from ambient wind or downdraft winds as is most likely the case for lower-resolution data. For wind speeds larger than 16 m/s (see red lines), the NRCSratio values are negative and almost constant in comparison with lower wind speeds. NRCSratio even exhibits a slight decrease for incidence angles lower than 37\({}^{\circ}\). Additionally, it is noted that the NRCSratio fluctuates around zero for wind speed of 8-16 m/s as incidence angle lower than 33\({}^{\circ}\). First, the decrease in sensitivity to rainfall in the C-band radar backscattering results from the competing effect of rain to change (mostly increasing) the NRCS and the background sea-state contribution. For stronger winds and severe sea state, the wind-waves already saturate the centimeter scales ocean surface waves and breaking waves become significant. In these conditions, rain-induced phenomena such as ring waves and splash products can hardly contribute to increasing the surface roughness and even probably contribute by smoothing the surface. This smoothing effect is pronounced at low incidence angle even though the wind speed is not that strong at a low incidence angle (See green line in Figure 7(a1)). Secondly, possible attenuation from the atmosphere can also add to the negative NRCSratio values. Please note that except for the lower incidence angle range, the ratio is for intermediate wind speeds (green lines) close to zero or negative for rain rates lower than 30 dBZ before it increases for larger rain rates. For given wind speed and base reflectivity, the NRCSratio increases with incidence angle. Rain signatures are thus expected to be larger at cross-polarization for large incidence angles than at co-polarization. Compared to co-polarization, few studies exist on rain signatures at cross-polarization. Case studies analyzed by [PERSON] et al. (2014), [PERSON] and [PERSON] (2006) and [PERSON] and [PERSON] (2017), show that the rain signatures at cross-polarization might be more pronounced than at co-polarization. As for VH, the NRCSratio statistics of VH-polarized images are presented in Figure 7(b1-b4). For light rain and low NRCS values (corresponding to light wind speeds), the NRCSratio values at cross-polarization are found to be significantly higher than in co-polarization. However, the signal-to-noise ratio is very low in cross-polarization and noise contamination impact cannot be excluded for this particular regime. Apart from this, the NRCSratio values are found to increase with base reflectivity for all wind speed regimes considered in this study. This sensitivity decreases with increasing wind speed. For a given base reflectivity, our analysis does not reveal a clear dependency of NRCSratio on incidence angle. This is explained by the fact that the cross-polarization backscattered signal from the ocean surface is much less sensitive to incidence angle than at co-polarization ([PERSON] et al., 2016). This suggests that rain signature should not be incidence angle dependent in cross-polarization. Overall, above statistics indicate that VH is more sensitive to rain than VV for the whole range of incidence angle and wind speed considered in this study.