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What is the COURLIM: *** Courant limit exceeded message?

pjr

Member
The spectral form of the equations of motion in CAM can become unstable when wind speeds become very strong. This situation is quite similar to the instability present in many finite difference schemes. The first analysis of the instability was performed by Courant Friedrichs and Levy in 1928. CAM will attempt to remove the instability when the winds would cause the model to go unstable. The algorithm is described in the users guide in the section on horizontal diffusion.

A "Courant limit exceeded" message is issued whenever the algorithm is employed.

There are two reasons I am aware of that trigger the limiter:

1) the wind fields can get extremely strong in the middle atmosphere at the winter pole. This is a natural phenoma and occurs in the real world as well as the model. The limiter will kick in under these circumstances and reduce the wind speed to maintain stability. This is a perfectly normal occurance, and nothing to worry about.

2) If there is an instability generated by any other aspect of the model (for instance you might have introduced a bug), then it can amplify, and occasionally the Courant limiter will begin firing, The model will soon halt (something will go terribly wrong) You may see other manifestations of such an instability (ie other warning messages will begin appearing).

Phil Rasch
 
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