Problems with free-drift test cases

[einola]

Investigating some issues in the BBM implementation, I created some free drift test cases, only to find more issues. The test cases re-use the geometry of Mehlmann benchmark, but here we have constant wind and ocean forcing and only a small patch of low concentration ice (with cracks) that's advected under wind and ocean currents. We expect the patch to drift undeformed along a diagonal x=y (if we turn off Coriolis). See the branch freedrift_tests.

Using the mEVP rheology we get strange features along the downwind side of the ice patch while using the BBM rheology we get strange features at the bottom of the patch. This also happens if I comment out the StressUpdateHighOrder functions, so the problem is not there.

[einola]

To clarify: I get the same results even without the stress update in StressUpdateHighOrder. This means that the two functions MEBStep and MEVPStep don't give the same results for either the stress update commented out or with very low concentration (which gives very low stress, as expected).

[pminak]

To get the same results for MEBStep and MEVPStep for MEB time stepping, you need a smaller timestep than MEVP.
I verified it with a function VP, which I will find and commit back.

I'll check free drift and get back to @einola.

[einola]

I didn't think about the time stepping, to be honest. But my feeling is that with very low concentration internal stress is zero and we should get the same results for all time steps shorter than some stability limit - which I expect to be quite large. I'll think about this more though.

[einola]

I've made some progress with this in the freedrift_tests branch. See commit 009058a. This replaces the explicit solver with an exact copy of what I did in the old neXtSIM (see the BBM paper and Hunke and Dukowicz (1997).