L- and C-Band SAR Backscattering Characteristics of Green Tide in the Yellow Sea

Green tides, caused by the excessive proliferation of green macroalgae (GMA), have become a recurring environmental disaster along the coasts of the Yellow Sea in China, inflicting substantial ecological and economic impacts each summer. Synthetic Aperture Radar (SAR) observations play an irreplaceable role in green tide monitoring due to their all-weather, all-day observation capabilities. Previous studies have shown that the floating GMA mats appear consistently brighter than the surrounding seawater at C- and X-bands, whereas either darker or brighter at L-band. However, the L-band findings were based on limited cases and lacked systematic verification, leaving the physical mechanism underlying this frequency-dependent imaging behavior unclear. This study examines five cases with co-located L-band, C-band, and optical observations. We propose a physical method to explain the frequency-dependent imaging characteristics of GMA-covered ocean surfaces systematically. First, the effective surface roughness is estimated by fitting SAR observations with the advanced integral equation model (AIEM) simulations, empirically constraining the effective correlation length and optimizing the root-mean-square height. Then, a relative roughness descriptor, namely RRD, is introduced to analyze the GMA-induced modulation of sea surface roughness. The satellite observations confirm that floating GMA mats are consistently brighter at C-band, generally darker at L-band, and only occasionally brighter when the ocean surface is very smooth. Simulation results further demonstrate that brighter GMA mats exhibit effective roughness enhancement, whereas darker patches exhibit effective roughness suppression. Moreover, control experiments reveal that the frequency-dependent imaging characteristics of GMA-covered surfaces are primarily governed by GMA-induced variations on effective surface roughness, with negligible contribution from the permittivity.