27.1.4 Recommendation for tuning the polar filter

Tests have been carried out using the tuning method given below and the explicit free surface option. Similar tests have not been carried out with the stream function option. Use of the explicit free surface method is recommended. The following steps are recommended for tuning the polar filter. For reasons given above, the finite impulse response filter (-Dfirfil) is the recommended filter of choice.

• Make a short integration of a few months using the intended model configuration except shorten the density, baroclinic, and barotropic time steps so that no filtering is needed. Use the same time step for density and baroclinic velocity. Do not use any polar filtering and save a snapshot at the end of the integration.

• Set the filtering latitudes as far poleward as possible. In the test case, the polar filtering latitudes start at $70^\circ$. This is for the purpose of studying the interaction of filtering on shelf processes and is for testing purposes only. Based on testing, it may be desirable to move the filtering poleward of $80^\circ$. This will eliminate filtering in the southern hemisphere and keep the filter away from coastal shelf areas in the Arctic.

• Turn on filtering with the finite impulse filter (-Dfirfil) and repeat the run using the same small timesteps. Examine the differences in snapshot files. Change the filtering latitudes and number of filter applications (numflt and numflu in setocn.F) until results are acceptable. Testing in this way shows what damage is done by the filtering. If there is alot of small scale information in the initial density stratification, then the initial T and S will have to be filtered more to remove it. It is counter intuitive but if small scales are left in the initial conditions, then increasing the amount of polar filtering in the integration will cause the model to blow up faster. The reason is that when filtering tendencies, increasing the number of filtering passes kills off the small scales in the tendencies which prevents them from wiping out small scales in the initial conditions. These small scales persist and eventually contaminate the solution.

• Increase the time steps to the expected values consistant with filtering latitudes. (The model output will give an analysis of what time steps should be ok). Repeat the run. If it blows up, increase the filtering on tracers first (numflt). If it still blows up, increase the filtering on velocity (numflu).