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Hi,

I am trying to model a large area using Telemac 2D in spherical coordinates and with tidal forcing from TPXO (including the tidal body force). I am entering the grid in UTM 41 N coordinates and I've entered SPATIAL PROJECTION TYPE : 2. The model results (e.g. M2 tidal amplitude) are very different compared to both the TPXO tide results and tide gauge measurements.

I then tried to run the same model but in cartesian, by setting SPHERICAL COORDINATES: NO, TIDE GENERATING FORCE : NO (since this is not supported in cartesian mode) and choosing a suitable Coriolis coefficient. The results of the cartesian model are then in good agreement with measurements but are strongly different from the spherical model results.

Any idea why the spherical model and cartesian model give very different results? I know that some differences are expected because the cartesian model uses a fixed Coriolis coefficient, no tidal body force and different equations, but the differences seem too big to be explained by that.

Many thanks!

I believe I have figured out what the problem was.

If you set SPATIAL PROJECTION SYSTEM to 2 (Mercator projection) and GEOGRAPHIC SYSTEM to 2 (WGS84 northern UTM), then Telemac v6.3 only assumes the UTM projection for the tidal boundary (and initial) condition, and it uses the 'Mercator for Telemac' projection for the actual flow model. Mercator for Telemac is not the same as UTM (even though UTM is also a Mercator projection).

I was only able to run the model correctly by defining my grid coordinates as Mercator for Telemac (formulas to generate these are given in readgeo3.f) and then setting SPATIAL PROJECTION SYSTEM to 2 (Mercator projection) and GEOGRAPHIC SYSTEM to 5 (Mercator for Telemac).

I did not run the model with grid coordinates in lat/long because it appears that the flow model accepts coordinates in radians and the tidal boundary conditions (and the tidal body force) accepts them in (decimal) degrees.

Perhaps it would be useful to clarify in the manual what the difference is between Mercator for Telemac and UTM, and which commands pertain to the tidal boundary conditions and which ones to the flow model. It might also be useful to throw an error if the user specifies different projection systems, or to make the coordinate system options more consistent between the tidal and flow parts of the model. Hope this is useful.

Hello thijslan!
Thanks for your post: I've your same problem and it is very useful.
My mesh is in cartesian coordinates in WGS84 UTM 33N and I see that the tide generating force option in T2D is not supported for my case, if I well understood.
So, actually, how did you proceed?
Did you firstly convert your coordinates at Mercator projection for Telemac by using the fortran file you mentioned (readgeo3.f )? Could you include this fortran file in the cas file? And then did you use the new defined geo.slf file for your runs, setting all the options you said?
What about the origin point coordinates?
Thanks in advance for your help!


Gabriella

Gabriella,

The first question is: how big is your model domain. If your model domain is relatively small, then you probably do not need to use the tide generating force, provided that the tide is properly defined at the boundaries.

If your domain is relatively large, then you should work in spherical coordinates because then the effect of the grid projection becomes important. Last time I checked (which was Telemac version v6p3r2), entering coordinates in latitude/longitude gave some problems. It was planned to resolve these problems in Telemac 7.0 but I am not sure if the problems are fully resolved (perhaps the developers can weigh in here).
So the best way to run the model in spherical coordinates is to use the Mercator for Telemac projection (setting SPATIAL PROJECTION SYSTEM to 2 (Mercator projection) and GEOGRAPHIC SYSTEM to 5 (Mercator for Telemac)). The origin point coordinates should be somewhere in the middle of your domain.

If you already have a nice mesh made in WGS84 UTM 33N, you don't need to start from scratch. If you're handy with Matlab, you can load the selafin file in Matlab using the Telemac Tools (www.mathworks.com/matlabcentral/fileexch.../25021-telemac-tools), re-project the grid points, and then save the selafin file again.

Then you can turn on the tide generating force. However, there is an error in the formulation of the tide generating force that will only be corrected in the next version of Telemac v7p0r1, which hasn't been released yet. Essentially, you need to add a minus sign to the x component of the tide generating force. You can do this in the file marast.f, the line
00191 FXL = F0L * COS(DL) * SIN(AHL) *
should be replaced by
00191 FXL = -1.D0 * F0L * COS(DL) * SIN(AHL) *
and line
00195 FXS = F0S * COS(DS) * SIN(AHS) *
should be replaced by
00195 FXS = -1.D0 * F0S * COS(DS) * SIN(AHS) *