restoring timescale for OGCM

[climater]

Hi all, I'm setting up a tropical Pacific domain multilayer OGCM simulation run and using restoring surface conditions at this point. The model will be restored to monthly SST/SSS climatology of World Ocean Atlas (WOA). What's the proper restoring timescales that I can choose for my run?

Thanks.

[vinu]

Hi Wang,

I think, you can set a 30 day restoring time scale for SST and
SSS (if you are not concerned with interannual simulations).
A range of time scale settings can be found in literature (for
example Song et al. (2004) used a 30 day for both SST and SSS, while
Miyama et al. (2003), used 6-days. Also surface salinity is found to have
restored with time scales of 17-days (Rao et al (2002) , and 40 days
Hirst and Godfrey (1993).

If you concerned with interannual simulations (it is unlikely to
simulate long term, without specifying/estimating heat flux as boundary
condition), the advective "SST anomalies" will be "reabsorved" within
a short distance into the climatology, if the relaxation
is too strong. (i.e. short time scale). Also, see various papers of
Seager et al (1988, 1994) for restoration in heat flux.

Also, if you use MOM (3 or ealrier version), there is a depth scale
over which the restoring SST is converted to a "flux". So inorder to
get the desired time scale of restoration, the damping depth should
match with the model uppermost layer thinckness.

You can find the above references in the Bibliography section of our
web-site. Please search through the forum index.


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[climater]

Thanks very much, vinu. This is pretty much helpful esp you mentioned me the various journal articles to refer to, which I often failed to find by myself effectively... I actually use mitgcm, not mom. btw, I'm also confused about specifying the right values of horizontal/vertical viscosity/diffusivity and various parameters in GM, KPP parameterizations applicable to my grid resolution and time step. any suggested reading/papers plz?

Thanks

[vinu]

Hi wang,

Your query seems much specific about mitgcm settings,
but I use ACOM2 (a version of MOM2), for my simulations.

Horizontal viscosity/diffusion.
I think mitgcm is eddy permitting OGCM, and the coefficients
are being parameterized in the model. Griffies (GFDL, Princeton, NJ)
discussed about the flow dependent viscosity according to
Smagorinsky schemes (refer his web-page and please find the
reference therein).
Now, if you use constant coefficients (suppose for a 1x1 degree resolution)
a value of (2 to 4)x10^3 m^2.s^-1 is reasonable for horizontal
viscosity and diffusion.

Now, with the Gent-McWilliams isopycnal mixing, i am not used to
set it for my case.

Vertical mixing
In k-profile parameterization, i suppose you are looking for
background values for viscosity and diffusion coefficients (as
the coefficients are being parameterized in the model),
in my case, i set it to (0.2 and 0.01) x10^-4 m^2.s^-1, respectively. But
that was to comply with a hybrid vertical mixing scheme where
the KPP is coupled with Kraus-Turner bulk vertical mixing scheme
and shear dependent dynamic instability mixed layer deepening.
Refer Chen et al., (1994), JPO, 24, 2146-2179, for a revision
of existing vertical mixing parameterization.


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