Abstract
Co-located over extreme events, C-band co-polarized and cross-polarized normalized radar cross sections (NRCS) and L-band ocean surface roughness brightness temperature ( $T_{B,\text {rough}}$ ) are directly compared to analyze the similarities and differences between these two parameters at medium resolution (about 25 km). NRCS in VH-polarization and VV-polarization ( $\sigma _{0,\mathrm {VH}}$ , $\sigma _{0,\mathrm {VV}}$ ) were acquired by Sentinel-1 C-band synthetic aperture radar. $T_{B,\mathrm {rough}}$ is estimated from brightness temperatures ( $T_{B}$ ) measured by the L-band radiometer on-board the Soil Moisture Active Passive mission. When the rain rate is less than 20 mm/h, a striking linear relationship is found between active C-Band cross-polarized NRCS and passive L-Band $T_{B\mathrm {,rough}}$ : $\sigma _{0,\mathrm {VH}}(\theta _{\mathrm {SAR}}) \propto \tan (\theta _{\mathrm {SAR}}) \times T_{B,\mathrm {rough}}(\theta _{\mathrm {SMAP}}= 40^{\circ }$ ), without any apparent saturation for $T_{B,\mathrm {rough}}$ ranging from 3.5 to 17 K. Compared to both high $T_{B,\mathrm {rough}}$ and $\sigma _{0,\mathrm {VH}}$ , co-polarized $\sigma _{0,\mathrm {VV}}$ measurements saturate. As interpreted, this can correspond to a regime change of the air–sea interactions during extreme events. In heavy rain conditions, C-band co-polarized NRCS decreases for extreme situations. In these cases, the covariation between C-band cross-polarized NRCS and L-band $T_{B,\mathrm {rough}}$ is less evident. An accurate and unambiguous assessment of the impact of rain will deserve further investigations.
Published Version
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