Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone

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Author
Spydell, Matthew S.
Feddersen, Falk
Guza, R.T.
MacMahan, Jamie
Date
2014Metadata
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Concurrent Lagrangian and Eulerian observations of rotational, low-frequency (1024 to 1022 Hz)
surfzone eddies are compared. Surface drifters were tracked for a few hours on each of 11 days at two
alongshore uniform beaches. A cross-shore array of near-bottom current meters extended from near the
shoreline to seaward of the surfzone (typically 100 m wide in these moderate wave conditions). Lagrangian
and Eulerian mean alongshore velocities V are similar, with a midsurfzone maximum. Cross-shore dependent
Lagrangian (rL) and Eulerian (rE) rotational eddy velocities, estimated from low-pass filtered drifter and current
meter velocities, respectively, also generally agree. Cross-shore rotational velocities have a midsurfzone maximum
whereas alongshore rotational velocities are distributed more broadly. Daily estimates of the Lagrangian
time scale, the time for drifter velocities to decorrelate, vary between 40 and 300 s, with alongshore time
scales greater than cross-shore time scales. The ratio of Lagrangian to apparent Eulerian current meter
decorrelation times TL/TA varies considerably, between about 0.5 and 3. Consistent with theory, some of the
TL/TA variation is ascribable to alongshore advection and TL/TA is proportional to V/r, which ranges between
about 0.6 and 2.5. Estimates of TL/TA vary between days with similar V/r suggesting that surfzone Lagrangian
particle dynamics vary between days, spanning the range from ‘‘fixed-float’’ to ‘‘frozen-field’’ [Lumpkin et al.,
2002], although conclusions are limited by the statistical sampling errors in both TL/TA and V/r.
Description
The article of record as published may be located at http://dx.doi.org/10.1002/2013JC009415
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Collections
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