Effects of Rain on the Ocean Radar Backscatter at Near-Nadir Incidence Angles: Insights From the Ku-Band SWIM/CFOSAT Under Low to Strong Wind Observations

The surface wave investigation and monitoring (SWIM) instrument, onboard China and France Oceanography Satellite (CFOSAT), is a Ku-band real-aperture radar, with six beams that illuminate the ocean surface at near-nadir incidences ranging from 0° to 10°. In this study, we investigate the effect of rain on the normalized radar cross section (NRCS) measured by SWIM under both tropical cyclone (TC) and non-TC conditions. Rain primarily attenuates the radar backscatter from the ocean surface. Under non-TC conditions, when wind speeds are below 21 m/s, the NRCS reduction is small (<1 dB) during light to moderate rain (<5 mm/h), but becomes very significant (>4–5 dB) under heavy rain (>15 mm/h) conditions. The impact of rain generally decreases as wind speed increases, especially at the smallest incidence angle. At low wind speeds, the NRCS reduction is also sensitive to the incidence angle. Based on a simplified model and the differing sensitivities of NRCS to surface roughness at two near-nadir incidence angles, we find that rain also influences the surface signal at incidence angles below 10°, in addition to the dominant effect of atmospheric attenuation, which leads to a consistent NRCS reduction across all wind speeds and incidence angles. Specifically, it contributes positively to NRCS at wind speeds above 7 m/s and negatively at lower wind speeds. This behavior is attributed to increased surface roughness from splashes and ring waves at low wind speeds, and to decreased roughness due to wave damping at moderate and high wind speeds. In TC environments, the NRCS tends to saturate under light to moderate rain rates (<5 mm/h) when wind speeds exceed approximately 35 m/s. In contrast, under heavy rain (>15 mm/h) conditions, the NRCS continues to decrease as wind speed increases. In both TC and non-TC conditions, beyond atmospheric attenuation, the impact of rain on surface roughness must be considered to fully explain the observed NRCS variations with wind and rain rate.