Microwave Radar Detection of Rip Currents: Observations and Characterization from a Coastal Monitoring Station in Southwestern Taiwan

Offshore rip currents are among the primary causes of drowning incidents for beachgoers. Maritime radar has demonstrated potential for monitoring rip currents. To investigate the characteristics of rip currents as captured in radar imagery, a radar-based monitoring station was established along the southwestern coast of Taiwan. This station acquires nearshore radar echo images every 20 minutes, and the observational experiments have been ongoing for over six months. Rip current features detected in radar images can be categorized into two types.

The first type is the offshore flow channel (channel rip) occurring within the surf zone. The highly irregular surface structures in the surf zone increase radar wave scattering intensity, resulting in strong electromagnetic echoes in radar imagery. Conversely, wave breaking within the offshore flow channel is often reduced compared to the surrounding areas, leading to weakened radar wave scattering.

The second type is the offshore rip head extending beyond the surf zone. Floating debris on the sea surface, influenced by the rip current, is transported offshore, forming a streak-like region. Compared to clean seawater, these floating materials generate stronger sea surface echoes. Additionally, interactions between the offshore-directed rip current and onshore-directed waves increase sea surface roughness, further enhancing radar backscatter intensity.

To better elucidate the rip current features observed in radar images, we conducted supplementary experiments during the radar monitoring period, including field surveys of bathymetry, aerial photography, and drifter experiments. Cross-validation of these diverse datasets aims to clarify the feasibility of microwave radar for detecting rip currents comprehensively.