Surface currents in Raritan Bay, New Jersey: Importance of HF radar first-order Doppler settings

Abstract

A surface current observation system based on high-frequency Coastal Ocean Dynamics Application Radar (CODAR) has been developed for Raritan Bay and the coastal waters of New York and New Jersey. This unique oceanographic tool is capable of measuring currents and waves at the ocean surface using resonant Bragg scatter return from transmitted radio signals. Surface current is measured by analysing the high-frequency (HF) radar sea-echo for the Doppler frequency shift where the frequencies of the first-order (FO) Doppler peaks that separate the surface current reflectance from the higher-order receiver signals are empirically determined.The presence of strong currents and horizontal shear in the near-surface flow results in spreading of the FO Doppler region, making it difficult to distinguish FO Doppler peaks from higher-order signals.Differentiating the FO Doppler region using empirical frequency cut-off parameters is one of the important steps in spectral processing of the HF radar receiver signals. The present work focuses on the surface current circulation in Raritan Bay and the New York Bight (NYB) Apex using HF radar observations, and to understand the importance of empirical determination of the FO Doppler region of the HF radar system in a strong, tidally dominated estuarine-ocean circulation.Comparison of HF radar observations with historic mooring observation and three-dimensional ocean model simulation shows that the HF radar system is highly sensitive to FO Doppler region settings. Strong tidal currents and wave-current interaction introduces spreading of the FO Doppler spectrum which results in underestimation of surface currents near the mouth of the New York/New Jersey (NY/NJ) harbour estuary.