Set default time_integrator for Omega

[xylar]

In the manufactured_solution test, this needs to be RungeKutte4. (I'm not worrying about other tests yet.)

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

This PR addresses the error found in E3SM-Project/Omega#137.

[xylar]

Testing

I am able to run the 200km forward run from manufactured_solution with this change on Chrysalis, whereas it failed without this change. (The check for output.nc then fails as expected.)

[xylar]

@hyungyukang, would you be willing to review this fix as well? Perhaps you can test along with other testing you're doing anyway.

[hyungyukang]

@hyungyukang, would you be willing to review this fix as well? Perhaps you can test along with other testing you're doing anyway.

Yes. I will test this PR as well!

[hyungyukang]

@xylar , Thanks for updating this. I was able to create the manufactured solution test with Omega model (--model=omega). In omega.yml in the test directory, the TimeStepper is correctly configured as RungeKutta4.

I'm approving this PR based on my testing and @xylar 's as well as visual inspection. Thanks @xylar !

[xylar]

Thanks, @hyungyukang!