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SUMMARY:Scalar mixing patterns in forced simulations of stratified turbule
 nce:  the importance of extreme events - Miles Couchman - Miles Couchman (
 University of Cambridge)
DTSTART:20220511T150000Z
DTEND:20220511T160000Z
UID:TALK174017@talks.cam.ac.uk
DESCRIPTION:Understanding the degree to which turbulence enhances the irre
 versible mixing of scalars in density-stratified fluids is a central probl
 em in geophysical fluid dynamics. An outstanding area of uncertainty invol
 ves determining whether local properties of the velocity field\, such as t
 he turbulent dissipation rate of kinetic energy $\\epsilon$\, may be relia
 bly used to predict local mixing rates\, as measured by the dissipation ra
 te of scalar variance $\\chi$. In parametrizations of ocean turbulence\, f
 or example\, distributions of $\\epsilon$ and $\\chi$ are assumed to be di
 rectly correlated\, yielding a constant mixing coefficient $\\Gamma = \\ch
 i / \\epsilon$\, although such an assumption has been heavily questioned. 
 We here probe local correlations between $\\epsilon$ and $\\chi$ by consid
 ering direct numerical simulations of stratified turbulence in a triply-pe
 riodic domain with an initially linear density gradient and bulk propertie
 s held in steady state by a large-scale background forcing\, similar to si
 mulations considered by Portwood et al (JFM 2016) and Taylor et al (JPO 20
 19).\nWe first demonstrate that the forcing gives rise to a previously unr
 eported vortex structure that induces shearing currents in the surrounding
  flow These shearing layers are found to be correlated with sharp interfac
 es in the perturbed density field\, characterized by a significantly eleva
 ted mixing coefficient $\\Gamma$ and contributing substantially to the tot
 al amount of mixing within the domain\, despite being under-represented in
  volume. Thus\, while the majority of the domain is indeed characterized b
 y the canonical mixing coefficient $\\Gamma = 0.2$ often assumed in oceano
 graphic models\, it is relatively rare\, but extreme mixing events that do
 minate the bulk mixing rates\, a phenomenon also recently identified in ob
 servational oceanographic measurements &nbsp\;by Couchman et al. (GRL\, 20
 21). Our findings emphasize that $\\chi$ and $\\epsilon$ contain independe
 nt information about mixing processes and should thus be considered in tan
 dem\, and suggest that current parametrizations of oceanic heat transport 
 may be skewed by undersampling\, capturing the most common but not necessa
 rily the most important mixing events.
LOCATION:Seminar Room 1\, Newton Institute
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