Relating Lagrangian and Eulerian horizontal eddy statistics in the surfzone

Abstract

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.