Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone

TitleRelating Lagrangian and Eulerian horizontal eddy statistics in the surfzone
Publication TypeJournal Article
Year of Publication2014
AuthorsSpydell M.S, Feddersen F, Guza RT, MacMahan J.
JournalJournal of Geophysical Research-Oceans
Volume119
Pagination1022-1037
Date Published2014/02
Type of ArticleArticle
ISBN Number2169-9275
Accession NumberWOS:000336261200024
Keywordsbeach; currents; diffusivity; dispersion; drifter observations; nearshore; north-atlantic; southern-ocean; transport; waves
Abstract

Concurrent Lagrangian and Eulerian observations of rotational, low-frequency (10(-4) to 10(-2) 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 (sigma(L)) and Eulerian (sigma(E)) 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 T-L/T-A varies considerably, between about 0.5 and 3. Consistent with theory, some of the T-L/T-A variation is ascribable to alongshore advection and T-L/T-A is proportional to V/sigma, which ranges between about 0.6 and 2.5. Estimates of T-L/T-A vary between days with similar V/sigma 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 T-L/T-A and V/sigma.

DOI10.1002/2013jc009415
Short TitleJ Geophys Res-Oceans
Integrated Research Themes: 
Student Publication: 
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