HF radar Lagrangian trajectory calculations accounting for Stokes’ drift and the nonlinear Bragg wave phase speed correction term

Douglas Cahl, University of South Carolina Columbia, Department of Earth and Ocean Sciences, Columbia, SC, United States, George Voulgaris, University of South Carolina, Columbia, Earth and Ocean Sciences, Columbia, SC, United States and Nirnimesh Kumar, Scripps Institution of Oceanography, Integrative Oceanography Division, La Jolla, CA, United States
Surface trajectory pathway calculations using HF radar surface current observations in Long Bay, SC show drifting larvae originating in the Charleston Gyre do not possess sufficient shoreward progress to reach estuaries on the coast. To correct known deficiencies in trajectory calculations made from HF radar surface current observations, Lagrangian trajectory calculations are performed by taking into account the nonlinear correction term to the Bragg wave phase speed (Weber and Barrick, 1977) for each radial current measurement. Additionally, full surface Stokes’ drift is added to the corrected surface current measurement before surface trajectories are recalculated. The difference between trajectory calculations is a predominantly onshore component from Stokes’ drift, with differences exceeding 10km over a 24 hour period.

Calculations of the nonlinear correction term (filtered Stokes’ drift) of the Bragg wave phase speed and the full surface Stokes’ drift require a full directional-energy ocean wave spectrum. Wavewatch III (WWIII) hindcast reanalysis provides full spectra only at buoy locations and a partitioned data set of wave parameters covering most areas of HF radar coverage. Reconstruction of the full spectrum using the partitioned data set is used to calculate Stokes’ drift and filtered Stokes’ drift, which is found to compare favorably to calculations using the full spectrum at buoy locations from WWIII both in the South Atlantic Bight and the Southern California Bight. This reconstruction method is used for Stokes’ drift and filtered Stokes’ drift calculations over the HF radar coverage area in Long Bay for Lagrangian trajectory calculations.