Relative dispersion observations in the coastal ocean and Lagrangian modeling with HF radar surface current data
Carter Ohlmann, Libe Washburn
University of California, Santa Barbara
(Abstract received 08/14/2009 for session A)
ABSTRACT
Eddy diffusivity values and related parameters, not well observed in coastal regions, are obtained from in situ surface drifter observations and presented along with a discussion of their utility in Lagrangian stochastic models. Lagrangian trajectory information in the coastal ocean is important for problems in larval transport, contaminant movement, search-and-rescue, and in oceanographic research where “advective” terms in the Navier-Stokes equations are non-negligible. Eulerian high frequency (HF) radar data can provide the basis for easily determining large numbers of observationally based trajectories. However, Monte Carlo simulations of trajectories from HF radar require knowledge of unresolved eddy, or sub grid scale, information corresponding to the gridded, time-averaged mean currents. Triplets of GPS tracked surface drifters, drogued at 1 meter depth, were repetitively deployed in the Santa Barbara Channel with initial horizontal spacing of ~ 10 meters. Drifter sets were deployed on 24 occurrences during the July 2004 through June 2005 period and left to sample their position every 10 minutes for 1 to 2 days. The drifter triplets demonstrate that eddy diffusivity values for spatial scales from 10’s to 100’s of meters range from 10^-2 to 10^1 m^2 sec^-1. These values follow closely with Richardson’s four-thirds power law scaling and Okubo’s historical empirical fits. The drifter data are used to evaluate Lagrangian stochastic model results using mean currents from HF radar. Both the mean (center of mass) and relative dispersion of modeled trajectories are evaluated.
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