The use of HF radar surface currents for computing Lagrangian trajectories: Benefits and issues

Surface coastal currents mapped by a pair of high frequency ground-wave radars (HFR) have been used to predict Lagrangian trajectories in the proximity of Heron Island (Capricorn Bunker Group, Great Barrier Reef, Australia), and to compare with the current data measured by an Acoustic Doppler Current Profiler (ADCP) at three mooring stations. Overall the HRF and ADCP absolute current speeds showed a difference less than ±0.15 m s^-1 for 68% of the observations. A good agreement between HFR (at a depth of 1.5 m) and ADCP (at a depth of 5.5 m) data were observed for the u-component (cross-shelf) which presented a stronger tidal signal, while a poor comparison was found for the v-component (north-south) more influenced by the south-easterly and northerly winds. The HFR allowed inclusion of not only the temporal, but also the spatial current variability in the tracking computation. This proved to be crucial because the Lagrangian trajectories were very sensitive to the starting position and time in the studied area, where the currents exhibit a large spatial variation imposed by tides, winds, large scale circulation and topography. One challenge in applying HFR data for Lagrangian tracking consists of estimating the missing values and including the effects of small scale fluctuations.