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Principal Investigator: J. Carter Ohlmann (UCSB)


Project Objectives


The primary goals of this research were to collect surface current data over the inner-shelf of the Santa Barbara Channel with Pacific Gyre’s “Microstar” Lagrangian drifters, and use the data to:  identify characteristic features of the flow field such as convergences, divergences and cross shelf transports, determine the surface velocity and velocity variance distributions, examine flow patterns on scales that are too small to be resolved in CODAR current measurements, and investigate how well particle paths determined from Eulerian CODAR fields represent measured Lagrangian flows. 


Summary of Research


The final set of drifter deployments directly related to the project was carried out on 12, December 2003.  This gives a total of nearly 200 high-resolution drifter tracks collected during the project.  The data processing codes were rewritten to properly handle parsing of multiple deployments of a single drifter in a day, to eliminate data when drifters are clearly caught in kelp beds, and to eliminate data records recorded during times when drifters were aboard boat.  The final processed data are now available via the Web:    (http://www.icess.ucsb.edu/~kirk/drifter/analysis.htm).


The short drifter tracks collected do not resolve a complete tidal cycle.  Therefore, as a first analysis step, high frequency (h.f.) radar data and a tidal model were incorporated to resolve tidal signals.  A considerable effort went into modeling tides to demonstrate that the drifters are not just measuring tidal currents. 


Drifter data have been used for an improved validation of h.f. radar derived velocities.  As the number of drifters sampling within an h.f. radar grid cell increases, so does the rms error in currents from the two sampling platforms.  This suggests that up to 50% of the error previously reported in h.f. radar data is due to a discrepancy in sampling scales, and not due to error in h.f. radar or its sampling technique.  A comparison between actual drifter trajectories and trajectories derived from h.f. radar fields shows a bias in position of more than 1 km after 3 hours, with the greatest discrepancy in the along-shore component.  Such differences must be reconciled if h.f. radar fields are to be used to understand and forecast how passive tracers such as pollutants and objects lost-at-sea move through the coastal ocean. 



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