High-resolution simulations of interactions between surface ocean dynamics and frazil ice

Abstract

Abstract. Frazil and grease ice forms in the ocean mixed layer (OML) during highly turbulent conditions (strong wind, large waves) accompanied by intense heat loss to the atmosphere. Three main velocity scales that shape the complex, three-dimensional OML dynamics under those conditions are: the friction velocity u* at the ocean--atmosphere interface, the vertical velocity w* associated with convective motion, and the vertical velocity w*,L associated with Langmuir turbulence. The fate of buoyant particles, e.g. frazil crystals, in that dynamic environment depends primarily on their floatability, i.e., the ratio of their rising velocity wt to the characteristic vertical velocity, dependent on w* and w*,L. In this work, dynamics of frazil ice is investigated numerically with a high-resolution, non-hydrostatic hydrodynamic model CROCO (Coastal and Regional Ocean COmmunity Model), extended to account for frazil transport and its interactions with surrounding water. An idealized model setup is used (a square computational domain with periodic lateral boundaries; spatially uniform atmospheric and wave forcing). The model reproduces the main features of buoyancy- and wave-forced OML circulation, including the preferential concentration of frazil particles in elongated patches at the sea surface. Two spatial patterns are identified in the distribution of frazil volume fraction at the surface, one related to individual surface convergence zones, very narrow and oriented approximately parallel to the wind/wave direction, and one in the form of wide streaks with separation distance of a few hundreds meters, oriented obliquely to the direction of the forcing. Several series of simulations are performed, differing in terms of the level of coupling between the frazil and hydrodynamic processes: from a situation when frazil has no influence on hydrodynamics (as in most models of material transport in the OML) to a situation when frazil modifies the net density, effective viscosity, transfer coefficients at the ocean--atmosphere interface and exerts a net drag force on the surrounding water. The role of each of those effects in shaping the bulk OML characteristics and frazil transport is assessed, and the density of the ice–water mixture is found to have the strongest influence on those characteristics.

Tridimensional nonhydrostatic transient rip currents in a wave-resolving model

“…It benefits from capabilities long developed in oceanic models -high-performance computing; high-order discretization; coupling with biogeochemistry and sediment models; pre-processing tools for rapid generation of model input; various online and offline diagnostics. The nonhydrostatic model version can thus be ap-6 plied without much effort to realistic, highly nonlinear regimes, e.g., large internal solitons and hydraulic jumps (Hilt et al, 2020), Kelvin-Helmholtz instabilities (Penney et al, 2020), Langmuir turbulence (Herman et al, 2020) or wave-induced nearshore circulation as in the present study. It is naturally suited for bridging ocean and coastal sciences, e.g., addressing surf-shelf exchange processes in a 3D, rotating and stratified framework.…”

Wave-in-ice: theoretical bases and field observations

“…Floating sheets manufactured with specific viscoelastic properties have been used to test some theories (as mentioned in [11]). In addition, wave–ice interactions have also been studied using remote sensing data [10] and numerical simulations [60–62]. The discussion below is limited to field work.…”