High-Frequency Radar observation of strong and contrasted currents: the Alderney race paradigm

TL;DR

This paper presents an improved high-resolution radar technique to measure and analyze strong, contrasted currents at the Alderney Race, addressing challenges posed by extreme shear flows and providing valuable data for tidal energy development.

Contribution

It introduces a novel phased-array radar processing method tailored for extreme shear currents, enhancing azimuthal resolution and accuracy in complex hydrodynamic conditions.

Findings

Enhanced azimuthal resolution with phased-array radar

Range resolutions of 750 and 1500 m are insufficient for detailed current variations

Radar data agrees with numerical models and in situ measurements

Abstract

The Alderney Race has been identified as a future site for the development of tidal energy, due to its bidirectional strong current reaching 5 m/s during spring tides. This hydrodynamics is very difficult to measure by in situ or remote sensing means. High-frequency coastal radars can provide a synoptic and near-real-time view of such a complex circulation, but the classical processing algorithms are not adapted to the extreme situation of strongly sheared currents. We propose an improved high-resolution direction-finding technique for the azimuthal processing of such radar data. It uses phased-array systems and combines the advantages of the usual beam-forming technique to eliminate many problems related to the distortion of Doppler spectra by extreme currents. The method is evaluated with a unique data set of radar measurements at two radar frequencies (13 and 24.5 MHz) and three spatial resolutions (200, 750, and 1500 m). The radar-based surface currents are analyzed in the light of a high-resolution numerical model and also compared with in situ measurements. While high azimuthal resolution can be achieved in this way, it is shown that the typical range resolutions of 750 and 1500 m are insufficient to account for the strong spatial variations of the surface current at some specific times and locations.