A new technique for the retrieval of near‐surface vertical current shear from marine X‐band radar images

Abstract

AbstractA new method to determine near‐surface vertical current shear from noncoherent marine X‐band radar (MR) data is introduced. A three‐dimensional fast Fourier transform is employed to obtain the wave number‐frequency spectrum of a MR image sequence. Near‐surface currents are estimated from the Doppler‐shifted surface gravity wave signal within the spectrum. They represent a weighted mean of the upper ocean flow. The longer the ocean waves on which the current estimates are based, the greater their effective depth. The novelty lies in the wave number‐dependent retrieval method, yielding ∼100 independent current estimates at effective depths from ∼2 to 8 m per ∼12 min measurement period. First, MR near‐surface vertical current shear measurements are presented using data collected from R/V Roger Revelle during the 2010 Impact of Typhoons on the Ocean in the Pacific experiment in the Philippine Sea. Shipboard acoustic Doppler current profiler (ADCP) and anemometer measurements as well as WAVEWATCH III (WW3) model results are used to demonstrate that results are in accord with physical expectations. The wind and wave‐driven Ekman flow is obtained by subtracting ADCP‐based background currents from the radar measurements. At ∼2 m, it is on average ∼1.6% of the wind speed and ∼38.9° to the right of the wind. With increasing effective depth, the speed factor decreases and the deflection angle increases. Based on WW3 results, the MR‐sensed Stokes drift speed is ∼50% of the Ekman flow at ∼2 m and ∼25% at ∼8 m. These findings are consistent with previous observations and Ekman theory.