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Timestep stability criteria

As part of the POP diagnostics, the Courant number for various processes can be output. The Courant number should always be less than one and often should be kept under 0.5 for stability. If such a CFL condition is exceeded, reducing the timestep may be a solution. However, it should be noted that often other underlying problems may be causing these limits to be exceeded (e.g. unrealistically high velocities) and reducing the timestep will only prolong the agony. In such cases, the particular CFL condition that is violated can help pinpoint the source of the problem.