Near-surface current shear measured by marine X-band radar

Abstract

This paper presents a new method to measure the near-surface currents' vertical shear by marine X-band radar (MR). The data used here were acquired from R/V Roger Revelle during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign in 2010. Existing techniques use MR surface wave signatures to retrieve a single near-surface current vector per analysis period. In essence, this is done by measuring the phase velocity of a wave on a current, which is then compared with the known still-water linear dispersion relationship. The resulting current corresponds to a depth-weighted average over the near-surface ocean layer. We introduce a new method that yields multiple estimates as a function of ocean wavenumber. The “effective” depth of the radar current measurement increases with the length of the ocean wave it was derived from. Our wavenumber-dependent near-surface currents thus provide information on vertical current shear. This method is analogous to an approach that has already proven successful for multi-frequency high-frequency (HF) radars. Here, we present first results which are put into the context of shipboard wind, surface wave, and background current measurements.