4 research outputs found
Transition to turbulence in wind-drift layers
A light breeze rising over calm water initiates an intricate chain of events
that culminates in a centimeters-deep turbulent shear layer capped by
gravity-capillary ripples. At first, viscous stress accelerates a laminar
wind-drift layer until small surface ripples appear. Then a second
"wave-catalyzed" instability grows in the wind-drift layer, before sharpening
into along-wind jets and downwelling plumes, and finally devolving into
three-dimensional turbulence. This paper elucidates the evolution of wind-drift
layers after ripple inception using wave-averaged numerical simulations with a
random initial condition and a constant-amplitude representation of the
incipient surface ripples. Our model reproduces qualitative aspects of
laboratory measurements similar those reported by Veron & Melville (2001),
validating the wave-averaged approach. But we also find that our results are
disturbingly sensitive to the amplitude of the prescribed surface wave field,
raising the question whether wave-averaged models are truly "predictive" if
they do not also describe the evolution of the coupled evolution of the surface
waves together with the flow beneath
CATKE: a turbulent-kinetic-energy-based parameterization for ocean microturbulence with dynamic convective adjustment
We describe CATKE, a parameterization for ocean microturbulence with scales
between 1 and 100 meters. CATKE is a one-equation model that predicts diffusive
turbulent vertical fluxes a prognostic turbulent kinetic energy (TKE) and a
diagnostic mixing length that features a dynamic model for convective
adjustment (CA). With its convective mixing length, CATKE predicts not just the
depth range where microturbulence acts but also the timescale over which mixing
occurs, an important aspect of turbulent convection not captured by convective
adjustment schemes. As a result, CATKE can describe the competition between
convection and other processes such as baroclinic restractification or
biogeochemical production-destruction. We estimate CATKE's free parameters with
a posteriori calibration to eighteen large eddy simulations of the ocean
surface boundary layer, and validate CATKE against twelve additional large eddy
simulations with stronger and weaker forcing than used during calibration. We
find that a CATKE-parameterized single column model accurately predicts the
depth structure of buoyancy and momentum at vertical resolutions between 2 and
16 meters and with time steps of 10-20 minutes. We propose directions for
future model development, and future efforts to recalibrate CATKE's parameters
against more comprehensive and realistic datasets.Comment: submitted to J. Adv. Model. Earth Sy., 24 pages, 8 figure
On the coupled evolution of oceanic internal waves and quasi-geostrophic flow
Oceanic motion outside thin boundary layers is primarily a mixture of quasi-geostrophic flow and internal waves with either near-inertial frequencies or the frequency of the semidiurnal lunar tide. This dissertation seeks a deeper understanding of waves and flow through reduced models that isolate their nonlinear and coupled evolution from the Boussinesq equations. Three physical-space models are developed: an equation that describes quasi-geostrophic evolution in an arbitrary and prescribed field of hydrostatic internal waves; a three-component model that couples quasi-geostrophic flow to both near-inertial waves and the near-inertial second harmonic; and a model for the slow evolution of hydrostatic internal tides in quasi-geostrophic flow of near-arbitrary scale. This slow internal tide equation opens the path to a coupled model for the energetic interaction of quasi-geostrophic flow and oceanic internal tides. Four results emerge. First, the wave-averaged quasi-geostrophic equation reveals that finite-amplitude waves give rise to a mean flow that advects quasi-geostrophic potential vorticity. Second is the definition of a new material invariant: Available Potential Vorticity, or APV. APV isolates the part of Ertel potential vorticity available for balanced-flow evolution in Eulerian frames and proves necessary in the separating waves and quasi-geostrophic flow. The third result, hashed out for near-inertial waves and quasi-geostrophic flow, is that wave-flow interaction leads to energy exchange even under conditions of weak nonlinearity. For storm-forced oceanic near-inertial waves the interaction often energizes waves at the expense of flow. We call this extraction of balanced quasi-geostrophic energy 'stimulated generation' since it requires externally-forced rather than spontaneously-generated waves. The fourth result is that quasi-geostrophic flow can encourage or 'catalyze' a nonlinear interaction between a near-inertial wave field and its second harmonic that transfers energy to the small near-inertial vertical scales of wave breaking and mixing
OceanBioME.jl: A flexible environment for modelling the coupled interactions between ocean biogeochemistry and physics
<h2>OceanBioME v0.9.1</h2>
<p><a href="https://github.com/OceanBioME/OceanBioME.jl/compare/v0.9.0...v0.9.1">Diff since v0.9.0</a></p>
<p><strong>Merged pull requests:</strong></p>
<ul>
<li>Fixes a typo: No idea how we've missed this one (#158) (@jagoosw)</li>
<li>(0.9.1) Move testing and docs to buildkite (+ minor GPU bug fix) (#159) (@jagoosw)</li>
</ul>If you use this software, please cite our article in the Journal of Open Source Software