Quality Assessment and Practical Interpretation of the Wave Parameters Estimated by HF Radars in NW Spain

Abstract

High-frequency (HF) radars are efficient tools for measuring vast areas and gathering ocean parameters in real-time. However, the accuracy of their wave estimates is under analysis. This paper presents a new methodology for analyzing and validating the wave data estimated by two CODAR SeaSonde radars located on the Galician coast (NW Spain). Approximately one and a half years of wave data (January, 2014-April, 2015 were obtained for ten range cells employing two different sampling times used by the radar software. The resulting data were screened by an updated method, and their abundance and quality were described for each radar range cell and different wave regime; the latter were defined using the spectral significant wave height (Hm0) and mean wave direction (Dm) estimated by two buoys and three SIMAR points (SImulación MARina in Spanish, from the wave reanalysis model by Puertos del Estado (PdE)). The correlation between the results and the particularities of the different sea states (broadband or bimodal), the wind and the operation of the devices are discussed. Most HF radar wave parameters' errors occur for waves from the NNE and higher than 6 m. The best agreement between the Vilán radar and the Vilano-Sisargas buoy wave data was obtained for the dominant wave regime (from the northwest) and the southwest wave regime. However, relevant contradictions regarding wave direction were detected. The possibilities of reducing the wave parameters' processing time by one hour and increasing the numbers of range cells of the radars have been validated.HF radar technology is based on the Doppler shift of the radar's wave backscatter after colliding with the ocean gravity waves [6]. While current parameters are extracted from the first-order peaks of the Doppler-spectrum, which are produced by ocean waves of length one half of the radar's length, and with direction either away or towards the radar, the complete wave directional spectrum is based on the second-order peaks, produced by the interaction between any pair of ocean waves normalized by the previous first-order spectra [7,8]. Several designs of HF radars have been developed for obtaining ocean waves' information [9][10][11][12], and the one used in this paper is a broad-beam direction finding of CODAR, whose basis for estimating the directional wave spectrum has been extensively explained [8,13,14]. The present model of these radars, the SeaSonde, can use electromagnetic waves of frequencies between 4.4-50 MHz, but in oceanography, the most typically used are 5, 12 and 25 MHz. One of the benefits of these radars is to have only one or two monopole antennas (for emitting and receiving) which facilitate the location and installation [15,16].However, HF radars have some limitations [17,18]; their work frequency determines a threshold of the ratio between the wave backscatter and the noise floor of the signal that limits the minimum wave height that can be measured by the radar, and determines also a maximum wave height that, when exceeded, ensures the second-ord...

Wind Direction Data from a Coastal HF Radar System in the Gulf of Naples (Central Mediterranean Sea)

“…The first-order echoes are typically used to retrieve surface currents, while second order ones are analyzed to reconstruct the wave field [4]. While the analysis of surface currents is well established and the retrieval of wave parameters is an emerging topic [6][7][8][9], the use of first-order peaks to measure wind direction is still poorly explored in the SeaSonde system, while in the literature some investigation on acquisition and validation using phased-array systems are available. The first numerical model to extract the wind direction from HFr backscatter was suggested by Long and Trizna [10].…”

HF Radars for Wave Energy Resource Assessment Offshore NW Spain

“…Considering the previously discussed limitations of these radars [44] and the CODAR recommendations [64], the software was configured to process waves with periods between 5 and 17 s. When the received signal was too low to calculate the wave parameters, the radar software flags the corresponding samples [64], which we will refer to hereafter as nulls. Filtering based on the quality control defined by the Copernicus Marine in situ TAC (Thematic Assembly Centre) [39,68] was applied to the radar data series. In addition, considering Basañez et al [39], we flagged 2020 results as fails, corresponding to the samples that met the following conditions.…”

“…Likewise, the outcomes of SWAN depend on the wave patterns selected for its propagation [3,29]. As an alternative to models and buoys, new remote sensing technologies such as the oceanographic high-frequency radars (HF radar) are currently being applied to obtain current and wave data with high spatial resolution [34][35][36][37][38][39]. Radar technology is based on the Doppler effect that the wave emitted by the radar experiences as it backscatters from the ocean surface [40].…”

“…Despite many works having shown the efficiency of these radars [36,[43][44][45][46][47]] some limitations have also been described [39,48,49]. Firstly, the signal only returns to the radar when it backscatters from waves (called Bragg waves) with a wavelength one-half that of the radar and it propagates towards or away from the radar.…”

“…The two radars which are the focus of this work are the Seasonde models of CODAR. These have been proven to produce reliable wave data, especially regarding spectral significant wave height [36,39,59,60], but also some disadvantages, such as the lack of detection of most of the waves below 3 m [39], a significant disagreement with buoy data regarding wave period and some deviations of the wave direction [36,39,59,60]. However, some of these issues might be determined by the uncertainties that each location can bring to the validations, such as the heterogeneity of the sea state and the complexity of the coverage area [36,39].…”