Short-Wave Directional Distribution for First-Order Bragg Echoes of the HF Ocean Radars

Abstract

Directional distributions of 0.505-Hz ocean waves were investigated using 24.5-MHz HF ocean radars. Recent observations of surface waves reported that the wave directional distribution was bimodal rather than unimodal in fetch-limited conditions. Therefore, the feasibility of using HF ocean radars to identify the bimodal distribution was investigated based on a Monte Carlo simulation. The accuracy of wave parameter estimation and the selection of the model function to describe the wave directional distribution were examined. The number of beam directions is critical to the estimation of wave parameters in the case of the bimodal distribution. It was found that the wave parameter estimation was good if the secondary directional distribution peak amplitude was small. The accuracy of estimation was poor if the secondary distribution peak amplitude was close to the primary distribution peak amplitude. It is necessary to check the error of estimated parameters by simulating first-order Bragg ratios from the estimated wave parameters and degrees of freedom of integrated Doppler spectra. Akaike's information criterion (AIC) was useful in selecting the model function, although there were some failures. As many radars as possible should be used to identify the bimodal distribution. Wave parameters were estimated from observed Doppler spectra, when an atmospheric front passed over the observation area. The estimated wave distributions were almost unimodal. However, since the wave fields were spatially inhomogeneous, it cannot be concluded that real wave distributions were unimodal.