Date: Sat, 1 Oct 94 09:24:52 BST
Message-Id: <788.9410010824@met.ed.ac.uk>
From: P Mote <mote@meteorology.edinburgh.ac.uk>
Subject: Re: Courant limits
To: ccm-users@ncar.ucar.edu
Bob Turner writes:
>
> To all ccm2 users:
>
> I ran ccm2 for a 120-day simulation with a delta t of 30
> minutes. Everything runs fine until the 5653th step and
> then it says "original Courant limit exceeded at k, lat= 1
> 37 (estimate= 1.001), solution has been truncated to
> wavenumber 29. What does this mean ? Is there an error or
> should I disregard it ?
>
The Courant limit is a number which depends on the resolution, the time
step and the maximum wind speed. The Courant limit is exceeded when the
wind speed is sufficient to carry parcels from one grid point to the
next in one time step (the analog in spectral models is a bit less
intuitive but the idea is the same). This is bad from a numerical
standpoint, so the CCM2 tests for such a situation and if it occurs,
truncates the solution at that level; this is called the Courant
limiter. Unless you've changed namelist parameter KMXHDC, the Courant
limiter only operates in the top model level. All this is explained on
page 35 of the "Description of the NCAR Community Climate Model (CCM2)."
If it only happens occasionally it's nothing to worry about. It is
relatively common when running the CCM2 in a stratospheric configuration
(model top in the mesosphere) because wind speeds in the southern
hemisphere polar stratospheric jet can exceed 200 m/s locally, and under
those conditions sometimes the model crashes despite the best efforts of
the Courant limiter. I would be very surprised if the CCM2 crashed at
L18 resolution as the winds don't get that big, but if you've turned off
the gravity wave drag that might happen.
---------------------------------------------------------------------
Philip Mote Internet: mote@met.ed.ac.uk
Dept. of Meteorology Phone: +44 (31) 650-8743
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