[MITgcm-support] numerical issues with exf_interp_uv?

[Martin Losch]

Hi all,

Longjiang and I found a potential instability in exf_interp_uv:
when the last latitude < 90degN of the data grid is very close to 90degN, e.g. 89.999… then the interpolated velocity fields can exceed 1e6, because in the function LAGRAN, the variable denom becomes very small (a = py_ind, x = yG):
          denom = denom*(a(i) - a(k))
          numer = numer*(x    - a(k))
so that numer/denom becomes very large. The problem can be fixed by making sure the that last grid point is far enough from 90degN, but maybe we can find a way of issuing a warning or even of stopping the model when abs(py_ind-90deg)<single precicsion or similar.

This only happens for the vector field but not the scalar fields (in our case), we are not quite sure why.

Martin and Longjiang

Martin Losch // Alfred-Wegener-Institut, Helmholtz Zentrum für Polar- und Meeresforschung
Postfach 120161, 27515 Bremerhaven, Germany
mailto:Martin.Losch at awi.de // Tel./Fax: ++49(471)4831-1872/1797
http://www.awi.de/ueber-uns/organisation/mitarbeiter/martin-losch.html

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