On remote sensing of vertical profile of ocean surface currents by means of single-frequency VHF radars

The processes in the upper few meters of the ocean are the key ones in controlling the ocean-atmosphere exchange. One of the most informative characteristics of this uppermost layer of the ocean would be the vertical profile of the drift current, which implicitly gives the turbulence intensity distribution and through this the parameters of heat and momentum flux exchange at the air/water interface. However, no technique allowing one to measure or even estimate the current vertical profile has been found yet. In the present work the authors propose an idea of how to estimate the subsurface shear by means of VHF radars. The idea is based upon a well established technique of remote sensing of surface currents employing singlefrequency radars operating in high-frequency (25 MHz, HF) and very-high frequency (50 MHz, VHF) ranges. The frequency spectra of radar echo reflected from the sea surface exhibit two pronounced peaks corresponding to the so-called Bragg lines. The discrepancy between the observed frequency of Bragg lines and the linear dispersion relation for resonant surface gravity waves is attributed to the Doppler shift of the frequency of resonant waves due to the presence of shear current. Indeed, this shift of the surface wave frequency due to shear currents is well resolved by the radar. The authors propose a way to estimate the current vertical shear using the same single-frequency radars. They employ the fact that the backscattered radar echo has one more discernible pair of smaller peaks, the so-called second harmonics peaks. The latter are due to the contribution of the second harmonics of two times longer water wave. These peaks being less pronounced are often still discernible and by means of more sophisticated signal processing technique they apply can be reliably resolved. This enables them to obtain one more parameter of the current, and, thus, to specify the shear