High-Resolution Nearshore Wave Direction Retrievals from X-Band Radar Using an Adapted Slope-Array Method

Coastal observing platforms typically lack the spatial resolution needed to monitor the transformation of surface gravity waves, which impact nearshore circulation patterns, wave-driven runup, and sediment transport. Previous studies have demonstrated the effectiveness of deriving surface wave information from X-band radar by applying three-dimensional Fast Fourier Transforms (3D FFTs) to backscatter images in the open ocean. Near the coast, however, X-band retrievals are more challenging to interpret since wave fields are spatially inhomogeneous due to bathymetrically driven shoaling and refraction. Here, the classical four-point slope array method, traditionally applied to pressure sensors, is adapted to radar backscatter imagery for estimating wave direction at meter-scale resolution. Using data collected at a planar beach near Point Sal, CA, slope-array-derived wave statistics are compared with in situ Spotter buoys, a spectral refraction model, and the 3D FFT retrieval method. Additionally, a signal-to-noise-based quality control approach is proposed for coastal radar wave measurements to identify sampling conditions that degrade observations and the optimal wave and meteorological conditions for radar-based wave retrievals at this site are characterized. Quality controlled slope array and 3D FFT-based wave retrievals yield RMSE values of 13° and 11° relative to an in situ Spotter buoy time series. Spatial analyses indicate that slope-array-derived wave directions are in closer agreement with spectral refraction model predictions while providing directional estimates at 12 m spatial resolution, compared to 543 m using the 3D FFT approach. The results demonstrate the ability of X-band radar to capture small-scale coastal wave refraction, providing new opportunities to study nearshore wave dynamics.