Default time step is too long for high RHi cases

[sdeastham]

The default transport and ice growth time steps are currently 10 minutes, which is too long for high RHi cases. See below - when ice crystals are able to grow large, they settle through many grid cells in a single step. As a result, water in the intervening cells is not taken up, resulting in "pulse-like" behaviour:

Reducing the time step to one minute improves (but does not fully fix) this issue:

Not only is this an imperfect fix, it increases run times significantly (~4x in the given test case, going from 30 seconds to 2 minutes). It would be better to find other fixes (e.g. include simplified ice growth in transport sub-cycling).

[sdeastham]

It looks like 5 minutes may be a reasonable compromise. Going even lower dramatically increases run times but does not significantly change the total ice mass as a function of time, at least for this case. More testing is needed.

[sdeastham]

This issue appears to be (somewhat) less severe at lower RHi; in this case, at 110% RHi and 10 K lower (the loRH cases):

[Calebsakhtar]

Just to follow up on this. I have conducted my own tests on a 125% RHi ISSL case (as opposed to the 140% RHi ISSL performed by @sdeastham above). Here are the results:

Ice mass (kg / m):

Integrated Optical Depth (m):

Number of ice crystals (#/m):

Variation of the time-integrated intOD with timestep:

Observations
With increasing the timestep from 0.5 mins, the behaviour "converges". From about 5 mins upwards, the behaviour "diverges".

The convergence is likely numerical diffusion, as @sdeastham mentioned. I might choose to manually verify this by checking APCEMM's transport equation at a later date.

This competes with the pulse-like behaviour induced divergence, which becomes increasingly problematic at higher timesteps.

Conclusion
4-5 mins seems to be when these effects cancel each other out. This concurs with @sdeastham's analysis at 140% RHi.

I recommend the use of a 5 minute timestep for both ice growth and transport