Model-based directed drifter launches in the Adriatic Sea: Results from the DART experiment

A high-resolution numerical model of the Adriatic Sea is used to predict Lagrangian coherent structure boundaries, quantified by finite-size Lyapunov exponents (FSLE), for flow features in the region of the Gargano Peninsula during the course of the Dynamics of the Adriatic in Real Time (DART) observational program. FSLE fields computed from two-day model forecasts of the surface velocity indicate distinct regions of high relative drifter dispersion. Model predictions of such regions located on available ship-tracks were used to direct the launching of pairs of surface drifters on three days during March 2006, with the goal of maximize coverage of the sampling area. For two of the three launches, the observed trajectories separated at locations and along directions closely approximated by those predicted from the model FSLE fields. The third case acted as an inadvertent control experiment. Model predictions at release-time showed minimal FSLE structure at the launch locations and the observed drifter pair advected in a coherent fashion for two days. While there are considerable differences between individual drifter observations and trajectory envelopes computed from ensembles of synthetic drifters, the experiment confirms the model's ability to approximate the location and shape of energetic flow features controlling the near-time fate of quasi-Lagrangian particles. Overall, the combined use of FSLEs with realistic coastal circulation models appears to be a promising avenue to aid real-time-directed drifter launches in observational programs.