Assessing the fidelity of surface currents from a

Authors: Liang Kuang Alan F Blumberg Nickitas Georgas

Publish Date: 2012/06/27

Volume: 62, Issue: 8, Pages: 1229-1243

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Abstract

The rapid expansion of urbanization along the world’s coastal areas requires a more comprehensive and accurate understanding of the coastal ocean Over the past several decades numerical ocean circulation models have tried to provide such insight based on our developing understanding of physical ocean processes The systematic establishment of coastal ocean observation systems adopting cuttingedge technology such as high frequency HF radar satellite sensing and gliders has put such ocean model predictions to the test by providing comprehensive observational datasets for the validation of numerical model forecasts The New York Harbor Observing and Prediction System NYHOPS is a comprehensive system for understanding coastal ocean processes on the continental shelf waters of New York and New Jersey To increase confidence in the system’s ocean circulation predictions in that area a detailed validation exercise was carried out using HF radar and Lagrangian drifterderived surface currents from three drifters obtained between March and October 2010 During that period the root mean square RMS differences of both the east–west and north–south currents between NYHOPS and HF radar were approximately 15 cm s−1 Harmonic analysis of NYHOPS and HF radar surface currents shows similar tidal ellipse parameters for the dominant M2 tide with a mean difference of 24 cm s−1 in the semimajor axis and 14 cm s−1 in the semiminor axis and 3° in orientation and 10° in phase Surface currents derived independently from drifters along their trajectories showed that NYHOPS and HF radar yielded similarly accurate results RMS errors when compared to currents derived along the trajectory of the three drifters were approximately 10 cm s−1 Overall the analysis suggests that NYHOPS and HF radar had similar skill in estimating the currents over the continental shelf waters of the Middle Atlantic Bight during this time period An ensemblebased set of particle tracking simulations using one drifter which was tracked for 11 days showed that the ensemble mean separation generally increases with time in a linear fashion The separation distance is not dominated by high frequency or short spatial scale wavelengths suggesting that both the NYHOPS and HF radar currents are representing tidal and inertial time scales correctly and resolving some of the smaller scale eddies The growing ensemble mean separation distance is dominated by errors in the mean flow causing the drifters to slowly diverge from their observed positions The separation distance for both HF radar and NYHOPS stays below 30 km after 5 days and the two technologies have similar tracking skill at the 95 level For comparison the ensemble mean distance of a drifter from its initial release location persistence assumption is estimated to be greater than 70 km in 5 daysThis paper is made possible with the help from Dr Josh Kohut of Rutgers University who kindly provided the hourly HF radar surface currents and answered so many questions we had Thanks also to Dr Arthur Allen of the US Coastal Guard and to Dr Eoin Howlett of Applied Science of Associates for providing the SLMDB drifter data We also thank Ms Caitlin O’ Connor of NOAA for providing technical advice with the GNOME software This work was supported by NOAA under Grant NA11NOS0120038 “Phased Deployment and Operation of MidAtlantic Regional Coastal Observing System MARCOOS” administered through Rutgers University

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