Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

Abstract

Flash rips and surf eddies are transient horizontal structures of the order of 10 to 100 m, which can be generated in the surfzone in the absence of bathymetric irregularities. They are traditionally evaluated in a depth averaged setting which involves intrinsic horizontal shear instabilities and the direct generation of vorticity by short-crested waves. In this article, we revisit the processes of surf eddy generation with a new three-dimensional wave resolution model (CROCO) and provide a plausible demonstration of new 3D non-hydrostatic instability and turbulent cascade. We first present a quick overview of a compressible free surface approach suitable for nearshore dynamics. Its ability to simulate the propagation of surface gravity waves and nearshore wave-driven circulation is validated by two laboratory experiments. Next, we present a real world application from Grand Popo Beach, Benin, forced by waves with frequency and directional spreading. The generation of surf eddies by the 3D model differs from depth-averaged models, due to the vertical shear associated with shallow breaking waves. In this case, the generation of eddies from both horizontal shear instability and the breaking of short-crested waves is hampered, the former by stretching the alongshore current and the latter by inhibiting the inverse energy cascade. Instead, the vertical shear flow is subjected to forced wave group variability and Kelvin-Helmholtz type instability at an inflection point. Primary and secondary instabilities generate spanwise and streamwise vorticity connecting small-scale eddies to larger horizontal surfzone structures. Streamwise 1 filaments, appearing as 5 m wide ribs or mini-rips, can extend beyond the surfzone but with moderate energy. These results appear consistent with the velocity spectra and observed patterns of tracers and suspended sediments at Grand Popo Beach. The timescale associated with the mean shear-induced turbulence is several times the wave period and suggests an intermediate range between breaker-induced turbulence and large-scale surf eddies.

Comparison of the Coastal and Regional Ocean COmmunity model (CROCO) and NCAR-LES in non-hydrostatic simulations

“…In this section, we compare the turbulence statistics simulated by the NCAR boundary-layer LES model (Moeng, 1984;Sullivan et al, 1994;Sullivan and Patton, 2011) and the Coastal and Regional Ocean COmmunity (CROCO) non-Boussinesq (NBQ) model (Auclair et al, 2018;Marchesiello et al, 2021). In addition, we test the sensitivity of the turbulence statistics to certain constants specific to the CROCO NBQ model.…”

“…The former avoids adding any explicit SGS terms and implicitly relies only on numerical diffusion. Here, the WENO5-Z improved version of the fifthorder-weighted essentially non-oscillatory scheme (Borges et al, 2008) is used for all advection terms (see Auclair et al, 2018, andMarchesiello et al, 2021, for more information). Unless explicitly mentioned otherwise, the CROCO runs shown here use the numerical-only option for the SGS parameterization because we are interested in understanding the performance of (unavoidable) numerical diffusion before adding explicit SGS terms (and associated parameters) which make the model behavior more complex.…”

African coastal camera network efforts at monitoring ocean, climate, and human impacts

“…Flash rips and surf eddies are transient horizontal structures of the order of 10 to 100 m, which can be generated in the surf zone in the absence of bathymetric irregularities. Using video-derived current fields, Marchesiello et al 64 , 65 revisited the processes of surf eddy generation with a new three-dimensional wave resolution model (CROCO) and provided a plausible demonstration of new 3D non-hydrostatic instability and turbulent cascade. Instant fields of surface current were retrieved from shore-based video cameras and unmanned aerial vehicles ( UAV s) by an optical flow (OF) method named " Typhoon " 66 .…”

Improved Gulf Stream separation through Brinkman penalization

“…CROCO is a free-surface, terrain-following coordinate regional ocean model with barotropic-baroclinic mode split-explicit time stepping (Shchepetkin and McWilliams, 2005;Debreu et al, 2012;Soufflet et al, 2016). Here, we use the Boussinesq and hydrostatic approximations, and although a non-Boussinesq and non-hydrostatic solver is also available (Hilt et al, 2020;Marchesiello et al, 2021), it is not required for the mesoscale resolutions used in this paper.…”