The effect of tidal currents on radar backscatter from the sea around Portland Bill

Low grazing angle radar data of the sea surface were collected using three different frequencies (3, 10 and 16 GHz) from a cliff-top site on the south coast of England. A number of features were observed in the radar imagery that could be related to the tidal current flow around Portland Bill. The strongest, most obvious features occurred near the time of low water, and these features had significantly reduced backscatter levels in horizontal polarization, with reductions up to 20 dB, or more, below the clutter levels around them. In vertical polarization, the reduction in the clutter level was typically somewhat smaller (IOdB or so) than the horizontally polarized backscatter. No convincing explanation of this effect has been found. The strain rate component in the radar line of sight was estimated from measurements of the current component calculated from the radar data. A comparison of range-time intensity images of the radar backscatter and the strain rate showed a number of strongly correlated features that repeated with the semi-diurnal tidal period. The maximum strain rate was around 0.0005 s(-1), which produced modulations in the radar backscatter of around 2-3 dB. On occasions a number of bright streaks with a separation of around 100 in were also observed, moving away from the radar at a few cm s(-1). A satellite image from European Remote Sensing Satellite (ERS)-2 of the Portland area suggests that the slow-moving streaks may be internal waves generated by tidal flow over raised bathymetry.