Wind-speed inversion from HF radar first-order backscatter signal

Abstract

Land-based high-frequency (HF) radars have the unique capability of continuously monitoring ocean surface environments at ranges up to 200 km off the coast. They provide reliable data on ocean surface currents and under slightly stricter conditions can also give information on ocean waves. Although extraction of wind direction is possible, estimation of wind speed poses a challenge. Existing methods estimate wind speed indirectly from the radar derived ocean wave spectrum, which is estimated from the second-order sidebands of the radar Doppler spectrum. The latter is extracted at shorter ranges compared with the first-order signal, thus limiting the method to short distances. Given this limitation, we explore the possibility of deriving wind speed from radar first-order backscatter signal. Two new methods are developed and presented that explore the relationship between wind speed and wave generation at the Bragg frequency matching that of the radar. One of the methods utilizes the absolute energy level of the radar first-order peaks while the second method uses the directional spreading of the wind generated waves at the Bragg frequency. For both methods, artificial neural network analysis is performed to derive the interdependence of the relevant parameters with wind speed. The first method is suitable for application only at single locations where in situ data are available and the network has been trained for while the second method can also be used outside of the training location on any point within the radar coverage area. Both methods require two or more radar sites and information on the radio beam direction. The methods are verified with data collected in Fedje, Norway, and the Ligurian Sea, Italy using beam forming HF WEllen RAdar (WERA) systems operated at 27.68 and 12.5 MHz, respectively. The results show that application of either method requires wind speeds above a minimum value (lower limit). This limit is radar frequency dependent and is 2.5 and 4.0 m/s for 27.68 and 12.5 MHz, respectively. In addition, an upper limit is identified which is caused by wave energy saturation at the Bragg wave frequency. Estimation of this limit took place through an evaluation of a year long database of ocean spectra generated by a numerical model (third generation WAM). It was found to be at 9.0 and 11.0 m/s for 27.68 and 12.5 MHz, respectively. Above this saturation limit, conventional second-order methods have to be applied, which at this range of wind speed no longer suffer from low signal-to-noise ratios. For use in operational systems, a hybrid of first- and second-order methods is recommended

Remote Sensing of the Surface Wind Field over the Coastal Ocean via Direct Calibration of HF Radar Backscatter Power

“…For the WHOI systems in particular, the Bragg waves and their energy follow the wind speed and direction closely for wind speeds of 2-9 m s 21 for most fetch and/or wave age conditions (see Shen et al 2012). Thus, this work assumes a direct relationship between the speed and direction of the wind and the direction and energy level of the Bragg waves observed by the radar.…”

“…In contrast, the dependence of power on wind speed has traditionally been thought to be both spatially and temporally variable, as well as dependent on unknown radar-specific parameters. However, Shen et al (2012) and Kirincich (2013) recently illustrated that in situ wind data can be used to calibrate the backscatter power of individual radar sensors, enabling estimates of wind speed via empirical calibration. While these efforts documented the short spatial scale of the calibration's validity, they also demonstrated that, to the accuracy of the radar observations themselves, only small amounts of in situ observations were needed to produce a sufficient calibration.…”

HF Radar Activity in European Coastal Seas: Next Steps toward a Pan-European HF Radar Network

“…The spectra (frequency on the left and directional on the right) show strong swell (also evident in the peak wave directions on the map) with wind waves increasing in amplitude and peak frequency further offshore. Other products that can be obtained by means of advanced signal processing include: winds (Heron and Rose, 1986;Shen et al, 2012), and tsunami (Lipa et al, 2006;Gurgel et al, 2011;Grilli et al, 2015). In addition, several works have proven the potential use of HFRs for ship detection (Ponsford et al, 2001;Dzvonkovskaya et al, 2008) and tracking (Maresca et al, 2014).…”

“…In general an HFR sends out modulated radio waves and listens to the returned signal, which is mainly affected by the surface waves propagating along radar look direction that are of the order of the transmitted wave length (Bragg scattering). From the measured backscatter several oceanic parameters can be obtained, such as: ocean surface currents (e.g., Paduan and Rosenfeld, 1996;Gurgel et al, 1999;Shrira et al, 2001), waves (e.g., Wyatt et al, 2006), winds (e.g., Shen et al, 2012), tsunami (e.g., Lipa et al, 2006) and discrete targets (ships) (e.g., Maresca et al, 2014).…”

Some considerations about coastal ocean observing systems

“…The Bragg peaks used to estimate surface velocities are flanked by a weaker "second-order" continuum because of double scattering from two freely propagating waves, as well as scattering from nonlinearly bound waves. This continuum contains contributions from all ocean wave components longer than the Bragg waves, and thus the second-order part of the power spectrum can be inverted to estimate the frequency-direction spectrum of the longer waves (Hasselmann 1971;Weber and Barrick 1977;Wyatt, Ledgard, and Anderson 1997) and near-surface winds via a wind-wave model (Heron andRose 1986, Paduan et al 1999;Shen et al 2012;Kirincich 2016). Although operational wave products are available from PA systems, spatially dependent estimates of winds and waves from DF systems are still under development.…”