[MITgcm-devel] tidal forcing

[Ryan Abernathey]

I think this is a great suggestion.

I wonder if there is a way to directly calculate the tidal potentials
within MITgcm itself. In my experience, the tidal forcing fields can be
huge (hundreds of GB!). But they are essentially deterministic functions of
a few orbital parameters. Calculating them "online" would save a lot of
disk space and eliminate potential errors related to timing, coordinate
systems, etc.

Joern Callies has a python code to calculate the potentials:
https://naif.jpl.nasa.gov/naif/documentation.html
This is based on the NASA SPICE toolkit, which has a Fortran API.
https://naif.jpl.nasa.gov/naif/index.html

An MITgcm tides package would depend on the SPICE library, in the same way
mnc depends on the netCDF library.

It is clearly a significant amount of work. But it seems like the best
long-term solution. I just thought I would raise the idea.







On Tue, Oct 3, 2017 at 1:06 PM, Menemenlis, Dimitris (329C) <
Dimitris.Menemenlis at jpl.nasa.gov> wrote:

> Jean-Michel, this sounds very useful!
> Is Rui on MITgcm Devel list?
> It would be good to run your decisions listed below past him.
> They are definitely beyond my field of expertise.
>
> (Rui I owe you a response about tides soon.  Sorry right now I am a little
> overwhelmed.)
>
> D.
>
> > On Oct 3, 2017, at 11:47 AM, Jean-Michel Campin <jmc at mit.edu> wrote:
> >
> > Hi Dimitris and other,
> >
> > Until now, when you run some global ocean with tides, the tidal forcing
> > was specified as an equivalent atmospheric pressure, presumably through
> pkg/exf.
> >
> > There are few limitations with this approach:
> > 1) the exf atmospheric pressure field cannot be used for something else,
> > such as a more precise estimation of air density (#define
> EXF_CALC_ATMRHO).
> > 2) the tide geopotential is added to the ocean pressure field (totPhiHyd)
> > which prevent to use it inside the EOS (selectP_inEOS_Zc=2, default
> > with EOS: JMD95P, UNESCO, MDJWF and TEOS10).
> > 3) if one want to specify both an atmospheric loading and a tidal
> forcing,
> > the forcing input field become a mix, not practical nor very natural.
> > And this is also the case when coupled to atmospheric GCM.
> >
> > Therefore, I propose to add an other 2-D forcing field to account for
> > tidal forcing. The most natural way would be a tidal geopotential (or
> > any time-dependent geopotential anomaly), units: m^2/s^2.
> >
> > An other advantage is that it would be available in the same form for
> > Ocean in p-coords set-up as well as for atmospheric set-up.
> >
> > The gradient of it would be added to horizontal momentum tendency as
> > part of the forcing (in S/R EXTERNAL_FORCING_U & EXTERNAL_FORCING_V)
> > which is different form current atmospheric loading, for instance
> regarding
> > the Adams-Bashforth (pressure gradient in or out AB depending on
> staggerTimeStep,
> > whereas forcing has it's own parameter: momForcingOutAB).
> >
> > Any suggestion or comments ?
> >
> > Cheers,
> > Jean-Michel
> >
> > On Fri, Sep 29, 2017 at 04:41:26PM +0000, Menemenlis, Dimitris (329C)
> wrote:
> >> Jean-Michel and Atanas, two small coupling details that worry me:
> >>
> >> 1. How do we include tides?  Right now for ocean only simulations
> >> we add hourly tidal potential forcing to atmospheric pressure.
> >> What is best way to do this in coupled model?
> >>
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