On the imaging of rip currents in X-band radar

Rip currents pose a serious danger to visitors of bathing beaches; they are also an important process in surf zone circulation. The previous observations of Haller et al. [JWPCOE, 2014] demonstrated that shore-based X-band marine radar can be used to image rip current flows. However, little is known of the underlying radar imaging mechanics and the conditions under which rip currents are observable. Herein, we analyze the radar imaging mechanisms for currents and associated surf zone eddies. The radar imaging analysis utilizes the wave-action balance equation in order to quantify changes in the mean squared slope of the surface wind-generated waves due to the underlying surf zone flow fields. Both the rip current and the surf zone eddy are modeled using pre-existing analytic solutions for the mean flow. Results show that radar imaging of rip currents is dependent on two components of the deformation tensor: the surface current divergence and the surface strain in the wind direction. The level of dependency on these two components varies between the rip current and surf zone eddy. Comparisons are made between the numerical results and the rip current observations from the field.