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| Titel |
The role of tracer advection formulation on diapycnal mixing in geopotential coordinate eddying ocean models. |
| VerfasserIn |
Chris Hill, David Ferreira, Jean-Michel Campin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043467
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| Zusammenfassung |
| We report on an examination of how advection operator formulation for active tracers can
significantly reduce spurious mixing across isopycnals for eddying ocean models which use
height as their vertical coordinate. Simulations in which the ocean mesoscale eddy field is
partially resolved are becoming increasingly prevalent due to increases in available computer
resources. Numerical studies [e.g. Griffies et al 2000] find that using a vertical coordinate that
does not align with isopycnals in an eddying model introduces an artificial background
diffusion across isopycnal surfaces as high as, 6 Ã 10-4m2s-1; a factor of sixty greater than
the actual background diapycnal mixing measured in the open-ocean during deliberate
tracer release field experiments [Ledwell et al. 1993]. Surprisingly the numerical
studies found excess diapycnal mixing initially increases with higher resolution in
eddying regimes. Eventually spurious mixing does fall, but at resolutions in the
kilometer range which still remain computationally challenging for large scale
models.
The work we present examines whether improved formulation of advection of active
tracers can address the spurious mixing issue for eddying models at reasonable resolutions.
We consider idealized eddying flow in a periodic zonal channel forced by an along channel
wind stress and a cross channel heat flux gradient with a linear equation of state in
temperature (so that temperature and density surfaces are equivalent). The system is
integrated to a statistical equilibrium in which an overturning cell counters the differential
heating with eddies fluxing heat laterally. For this equilibrated state we examine (i) the
overturning cell in temperature coordinates and (ii) effective diapyncnal diffusivity diagnosed
from the rate of change of temperature distribution and from diapycnal mixing of an injected
tracer. We diagnose these quantities for a range of spatial resolutions and with two different
advection schemes (1) a high-order flux limited scheme [Darau and Tenaud, 2004] and (2) a
finite-volume, general orthogonal curvilinear coordinate implementation of the second-order
moment scheme of Prather [Prather, 1986]. We find that the third order scheme
behaves in accord with previous studies i.e the temperature coordinate overturning
cell shows interior fluxes across isotherms and the diagnosed diapycnal mixing is
large for moderately resolved eddying solutions. In contrast, the Prather scheme
produces an equlibrated solution in which temperature coordinate overturning is along
isotherms in the interior and the diagnosed diapycnal mixing is close to observed
values.
The Prather scheme requires more computation per timestep and requires more
information to be carried at each model grid cell. However, it can reduce spurious
mixing by more than an order of magnitude. For a practical eddying resolution it
lowers background numerical diapycnal mixing to levels that are comparble to
observed background mixing. We believe that, in many situations, this more than
compensates for the enhanced computational cost that the second-order moment
scheme entails. To illustrate this point we will show results from a preliminary
application of the Prather scheme to a fully global eddying ocean circulation model. |
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