Visibility of River Plume Fronts with an X-Band Radar

Abstract

A land-based X-band radar was employed to observe river plume fronts at the mouth of the Tenryu River, Japan. Time-averaged radar images captured fronts extending offshore from the river’s mouth as bright streaks. Comparisons between satellite optical images and radar images confirm that streaky features in the radar image represent color river plume fronts. Further corroboration comes from field observations of water temperature, salinity, and turbidity conducted simultaneously with the radar measurements. When a survey ship crossed the front, the measured properties varied discontinuously, suggesting that water from the river and sea converged there and also that a downwards current was present. Variation of visibility of the fronts was assessed and compared with the rate of variation of water level and the wind speed and direction. The radar is able to image fronts when the water level is decreasing during ebb tide and the wind speed is over 3 m/s along shore. Surface ripple waves are generated by the local wind, and if they propagate across the front, wave heights increase, causing higher backscatter of the emitted radar beam. This observation gives further evidence on the imaging mechanism of river plume fronts with X-band radars in relation to wind direction.