[MITgcm-support] Re: question about seaice coupling to ocean velocities

[Jinlun Zhang]

Yes, we can change reference velocity at different level. The water drag on ice
is determined (from McPhee 1975) based on geostrophic velocity below Ekman layer
or mixed layer, which is generally around/below 50m under ice in the Arctic. A
common practice is to pick velocities at certain level to  approximate 'top'
geostrophic velocities. In Martin's case, velocities at 2nd level would be
appropriate if the 1st level is 50-70m thick.
I am not in favor of using velocities at variable depth, just to avoid possible
sudden change.
Jinlun


Dimitris Menemenlis wrote:

> Hi Jinlun, Martin just sent in following question.  Can we change
> reference velocity to some fixed depth, could also be a variable
> depth, right below KPPhbl.  What do you think?
>
> > Hi,
> > I may have asked this before, but couldn't find the answers (if there
> > where any) in the email archives:
> > What's the reason for using the third level (k=3) velocities
> > u/vVel(i,j,3,bi,bj) as "proxy for geostrophic velocities" in
> > seaice_model.F? It is reasonalbe to assume that only the 3rd level
> > velocities excert stress on the sea ice from below? If you are willing
> > to accept that the surface velocities are too noisy to be used,
> > shouldn't the k-level be connected to a certain depth and not to a hard
> > coded k-level? I mean, in a coarse model with, say, 50-70m layer
> > thickeness (as in global_ocean_90x40x15 and derivatives) this will have
> > a completely different effect from a model with resolves the surface
> > layers rather well (say 10m in the upper 100m). What's the rationale
> > behind the choice of k=3?
> >
> > Martin
>
> Martin, my take on this is that it is just a quick-and-dirty estimate of
> surface geostrophic velocity and that it would make more sense to
> connect the k-level to a certain depth and not to a hard-code the
> k-level.  What's needed is velocity right below or near the bottom of
> the Ekman layer.
>
> D.
>
> --
> Dimitris Menemenlis <menemenlis at jpl.nasa.gov>
> Jet Propulsion Lab, California Insitute of Technology
> MS 300-323, 4800 Oak Grove Dr, Pasadena CA 91109-8099
> tel: 818-354-1656;  fax: 818-393-6720

--

-------------------------------------------------------------
Jinlun Zhang
Polar Science Center, Applied Physics Laboratory
University of Washington      Phone: (206)-543-5569
1013 NE 40th St               Fax:   (206)-616-3142
Seattle, WA 98105-6698        Email: zhang at apl.washington.edu
-------------------------------------------------------------


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mitgcm.org/pipermail/mitgcm-support/attachments/20040422/925d1d45/attachment.htm>