# Automated Rain Detection by Dual-Polarization Sentinel-1 Data
## 1 Introduction  
Space-borne C-band Synthetic Aperture Radar (SAR) has now become an operational component of the existing Earth Observation (EO) network and is commonly used for investigating air-sea interactions from space. In particular, Copernicus, the European program for EO, includes a constellation of two C-band SAR, Sentinel-1A (2014) and Sentinel-1B (2016), and the Canadian Space Agency also recently launched the Radarsat Constellation Mission (2019), a new constellation of 3 satellites with a C-band SAR payload. These two operational programs have also defined dedicated acquisition strategies for SAR-based maritime applications. Copernicus level-2 ocean products encompasses waves (OSW), wind (OWI) and radial surface velocities (RVL) measurements. However, many other ocean and atmospheric geophysical phenomena such as oceanic front, convective cells, modulation of the stress in the marine atmospheric boundary layer or rain [1, 2] also affect the signal. Although routinely available since the Radarsat-2 mission, the use of dual-polarization measurements for ocean processes analysis and level-2 ocean product definition is still emerging. Up to now, most of the work has been devoted to the analysis of oceanic fronts [3, 4], surface currents [5], hurricanes wind forcing [6, 7, 8], slicks [9], or sea ice [10, 11, 12, 13]. Studies on rain signatures in SAR imagery is still limited although such signatures can lead to significant errors during wind field or ocean waves spectrum inversion asthey impact both the averaged signal and its wave-induced modulation when performing a spectral analysis. [PERSON] and [PERSON] [1] first documented the rain signatures for L-band and studies for X-band and C-band are then devoted [14; 15; 16; 17]. [PERSON] et al. [14] compared the rain signatures simultaneously acquired at L-, X-, C-bands based on SIRC/X-SAR data. For L-band, negative impacts of rain on the NRCS are observed usually attributed to the surface contribution through wave damping since the atmospheric effects are negligible with no attenuation for the large radar wavelengths. For X-band, surface scattering from ring wave and splash products together with the volume scattering in the atmosphere are expected to increase NRCS. The reduction behind the enhancement is interpreted by the attenuation in the atmosphere along the incident path. C-band lies between X- and L-band, and thus the impact of surface scattering on the radar signal of rain can give rise to both positive and negative contributions [18]. Additionally, the volume scattering and attenuation in the atmosphere cannot always be ignored especially under heavy precipitation [17; 19; 20]. Thus, the rain signatures at C-band are a combination of different contributions from the atmosphere (volume scattering and attenuation of hydrometeors) and sea surface roughness enhancement/decrease (surface scattering). The multiple contributions lead to the concurrent irregular bright and/or dark areas observed at C-band. In addition, the rain signature is also sensitive to incidence angle and polarization. [PERSON] and [PERSON] [18] investigated the signatures with multi-frequency and multi-polarization scatterers, including Doppler, has shown that the contribution due to scattering at stalks increases at HH polarization when the incidence angle increases, whereas the ring-wave impact decreases. Apart from the relative contribution from the surface scattering mechanisms, the attenuation and the volume scattering along the incident path in the atmosphere varies with incidence angle and thus the radar cross-section are incidence angle dependent. In the case of cross-polarization, [PERSON] et al. [14] showed that the signal at cross-polarization is more pronounced with respect to co-polarization. [PERSON] et al. [17] show several cases revealing that the signal-to-noise ratio is often too low for investigating the impact of rain effects such as the downdraft or the waves damping on the SAR images.