Abstract
Concurrent measurements by satellite tracked drifters of different hull and drogue configurations and coastal high-frequency radar reveal substantial differences in estimates of the near-surface velocity. These measurements are important for understanding and predicting material transport on the ocean surface as well as the vertical structure of the near-surface currents. These near-surface current observations were obtained during a field experiment in the northern Gulf of Mexico intended to test a new ultra-thin drifter design. During the experiment, thirty small cylindrical drifters with 5 cm height, twenty-eight similar drifters with 10 cm hull height, and fourteen drifters with 91 cm tall drogues centered at 100 cm depth were deployed within the footprint of coastal High-Frequency (HF) radar. Comparison of collocated velocity measurements reveals systematic differences in surface velocity estimates obtained from the different measurement techniques, as well as provides information on properties of the drifter behavior and near-surface shear. Results show that the HF radar velocity estimates had magnitudes significantly lower than the 5 cm and 10 cm drifter velocity of approximately 45% and 35%, respectively. The HF radar velocity magnitudes were similar to the drogued drifter velocity. Analysis of wave directional spectra measurements reveals that surface Stokes drift accounts for much of the velocity difference between the drogued drifters and the thin surface drifters except during times of wave breaking.
Highlights
The velocity of seawater very near the ocean surface is a critical variable for a large array of scientific and practical applications
Upper ocean velocity data from numerical models or measurements from drifters, Acoustic Doppler Current Profilers (ADCPs), High Frequency (HF) radar or other remote sensing technologies are used as proxies for the surface velocity (e.g., [1,2])
Surface drifters typically have drogues extending over some depth and measure currents acting over their drogue cross-sectional area
Summary
The velocity of seawater very near (within a few centimeters of) the ocean surface is a critical variable for a large array of scientific and practical applications. Various methods have been used to approximate surface currents from upper layer velocity from models or observations at some depth below the surface for applications such as surface drift of oil. The primary goal of this work is to compare measurements of velocity near the sea surface using different approaches, including a new design for an ultra-thin satellite tracked drifter for measuring currents within a few centimeters of the ocean surface, and ideally to explain these differences. Analyses of the observations provide insight into the behavior of the near-surface drifters relative to other techniques for measuring near-surface currents Understanding these differences is critical when using surface current measurements from different methods for applications such as surface material drift, air–sea flux calculations, and testing of new remote sensing technologies. TDheisstrinad, aFrLfr(eFqiguuernecy1)b.aTnhdismreaadsaurrefsrecquurreennctys abtaanpdpmroexaimsuarteeslycu2–r3remntsdeapt tahp[p1r5o].xHimoautrellyya2v–e3ramgedde6p-mth b[1in5]n.eHdovuercltyoravcuerraregnetds 6o-vmerbtihnendedrifvteercteoxrpceurrimreenntst roevgeironthwe edrreifotebrtaeixnpeedrifmroemntthreegCioonaswtaelrOe bosbetarvininegd Rfreosmeatrhche CanodasDtaelvOelbospemrveinngt CReensteearr.ch and Development Center
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