Surface drifters in the German Bight: model validation considering windage and Stokes drift

Callies, Ulrich; Groll, Nikolaus; Horstmann, Jochen; Kapitza, Hartmut; Klein, Holger; Maßmann, Silvia; Schwichtenberg, Fabian

doi:10.5194/os-13-799-2017

Cited by 39 publications

(54 citation statements)

References 63 publications

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“…This agrees well with the observations of this study, which also show the highest drifter velocities for both components in tidal inlets generated by tidal jets (Figure 9, and Supplementary Video 2) which seem to overshadow the wind effects in the coastal area. These findings are also in good agreement with recent studies by Callies et al (2017) that compared trajectories of six surface drifters with results of offline particle simulations in the inner German Bight.…”

Section: Discussionsupporting

confidence: 92%

A State-of-the-Art Compact Surface Drifter Reveals Pathways of Floating Marine Litter in the German Bight

et al. 2019

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Lagrangian observations are important for the understanding of complex transport patterns of floating macroscopic litter items at the ocean surface. Satellite-tracked drifters and numerical models are an important source of information relevant to transport processes as well as distribution patterns of floating marine litter (FML) on a regional to global scale. Sub-mesoscale processes in coastal and estuarine systems have an enormous impact on pathways and accumulation zones of FML and are yet to be fully understood. Here we present a state-of-the-art, low-cost and robust design of a satellite-tracked drifter applicable in studying complex pathways and sub-mesoscale dynamics of floating litter in tidally influenced coastal and estuarine systems. It is compact, lightweight <5 kg, capable of refloating, easily recovered and modified. The drifter motion resolves currents of the ocean surface layer (top 0.5 m layer) taking into account wind induced motions. We further showcase findings from seven of our custom-made drifters deployed from RV Heincke and RV Senckenberg in the German Bight during spring and autumn 2017. Drifter velocities were computed from high resolved drifter position data and compared to local wind field observations. It was noted that the net transport of the drifters in areas far away from the coast was dominated by wind-driven surface currents, 1% of the wind speed, whereas the transport pattern in coastal areas was mainly overshadowed by local small-scale processes like tidal jet currents, interactions with a complex shoreline and fronts generated by riverine freshwater plumes.

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Section: Discussionsupporting

confidence: 92%

A State-of-the-Art Compact Surface Drifter Reveals Pathways of Floating Marine Litter in the German Bight

et al. 2019

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“…However, when wave breaking is not occurring, the surface Stokes drift appears to account for much of the discrepancy between the drogued drifter velocity and the thin surface drifters. This assumes that the impact of Stokes drift on the drogued drifters is negligible, an assumption supported by the analysis of Kenyon [10], but differing from some previous studies [25,26]. A possible explanation is that the thin drifters have very small diameters, allowing them to closely follow orbital trajectories of very short surface gravity waves.…”

Section: Discussionmentioning

confidence: 91%

Measurement Characteristics of Near-Surface Currents from Ultra-Thin Drifters, Drogued Drifters, and HF Radar

et al. 2018

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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.

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“…10a refers to drifter A 5 that is not subject of our study on drifter pairs. Drifter A 5 travelled, however, for nearly 49 days (see Callies et al, 2017) so that the length of data recorded compares to the time span underlying the Eulerian spectrum in Fig. 9.…”

Section: Kinetic Energy Spectramentioning

confidence: 99%

“…In the German Bight its horizontal resolution is 900 m. Although the vertical coordinate in BSHcmod is dynamical (Dick et al, 2008), re-gridded archived output represents surface currents in terms of the mean in an upper 5 m water column. Callies et al (2017) found that an additional wind drag in terms of 0.6 % of the 10 m wind velocity u 10m is appropriate to compensate for the lack of vertical grid resolution in archived model output. Therefore, for simulating drifter location x as function of time t, the following equation is used:…”

Section: Drifter Simulationsmentioning

confidence: 99%

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Submesoscale dispersion of surface drifters in a coastal sea near offshore wind farms

Callies¹,

Carrasco²,

Floeter³

et al. 2018

Preprint

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Abstract. We analyse relative dispersion of surface drifters released as pairs (6 instances) or triplets (2 instances) during three field experiments in the German Bight in close proximity to wind farms. Drifter pairs can be classified in a remarkably clear way into those with spatial separation growing either exponentially or non-monotonously. There is some tentative evidence that exponential relative dispersion growth rates preferably occur for drifter pairs that are most exposed to the possible influence of a wind farm. Kinetic energy spectra and velocity structure functions suggest that turbulent energy could be injected by tides, possibly also via an interaction between tidal currents and wind turbine towers. Applicability of inertial range turbulence theory, however, can be doubted given distinct peaks of overtides observed in velocity power spectra. More comprehensive studies would be needed to better separate submesoscale effects of wind farms, tides and possibly baroclinic instabilities on observed drifter behaviour in a complex coastal environment.

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Surface drifters in the German Bight: model validation considering windage and Stokes drift

Cited by 39 publications

References 63 publications

A State-of-the-Art Compact Surface Drifter Reveals Pathways of Floating Marine Litter in the German Bight

A State-of-the-Art Compact Surface Drifter Reveals Pathways of Floating Marine Litter in the German Bight

Measurement Characteristics of Near-Surface Currents from Ultra-Thin Drifters, Drogued Drifters, and HF Radar

Submesoscale dispersion of surface drifters in a coastal sea near offshore wind farms

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