Assessing the fidelity of surface currents from a coastal ocean model and HF radar using drifting buoys in the Middle Atlantic Bight

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 cutting-edge 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 drifter-derived surface currents from three drifters obtained between March and October 2010. During that period, the root mean square (RMS) differences of both the eastwest 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 M 2 tide, with a mean difference of 2.4 cm s −1 in the semi-major axis and 1.4 cm s −1 in the semi-minor 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 ensemble-based 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 days.

Inshore/offshore water exchange in the Gulf of Naples

“…They provide current fields on a spatial regular grid that are also well suited for validating coastal circulation models (e.g. Kuang et al, 2012) and for being assimilated into them (e.g. Barth et al, 2008;Breivik and Saetra, 2001).…”

Estimating Lagrangian transport blending drifters with HF radar data and models: Results from the TOSCA experiment in the Ligurian Current (North Western Mediterranean Sea)

“…In particular, open ocean models assimilating altimetric data appear able to capture several transport features induced by large and mesoscale structures (Olascoaga and Haller, 2012). Capturing smaller scale coastal currents and their effects on transport can be more challenging for models, unless in regions with long standing observatories (Haza et al, 2007;Kuang et al, 2012) where models have been thoroughly validated and often local in situ data are assimilated.…”

Impact of assimilating high frequency radar surface currents on the fidelity of a Middle Atlantic Bight circulation model

“…4. Compared with the mean total surface currents without DA, the mean total surface currents did not change much for the majority of the region given the fact that HF radar and NYHOPS have similar performance [10]. However, the mean total surface currents had more changes in the regions where relatively bigger RMSD was shown.…”

“…Coverage within the study domain was best away from the coast. It was also relatively low in the northern and southern parts of the MAB region, and in the region next to the continental shelf-break [10]. The specific type of drifter used here is called the Self-Locating Marker Datum Buoy (SLMDB).…”