Make a_min and m_min adjustable

[eclare108213]

CICE/cicecore/cicedyn/dynamics/ice_dyn_shared.F90

Line 68 in b109fa4

a_min = p001 , & ! minimum ice area

CICE/cicecore/cicedyn/dynamics/ice_dyn_shared.F90

Line 69 in b109fa4

m_min = p01 ! minimum ice mass (kg/m^2)

a_min and m_min are fixed parameters in ice_dyn_shared.F90, which were originally intended to prevent the dynamics code from blowing up when the ice area or volume are very small. Tests in E3SM indicate that these can be as small as 1.e-11 and 1.e-10, respectively (although that might be a function of the driver code in E3SM), and that reducing them improves the simulation results. I recommend adding a_min to namelist and setting m_min=10*a_min for easier testing.

I'm curious whether any model blows up with such small values, or if E3SM's stability is unique and worth a closer look.

[dabail10]

Finally had a chance to run this case. I believe the change to a_min and m_min is within variability in the CESM. I have run for 50 years so far, but likely will do 100 just to be safe.

https://webext.cgd.ucar.edu/BLT1850/b.e30_beta04.BLT1850.ne30_t232_wgx3.121min/ice/html/ice/Hemis_seaice_visual_compare_obs_lens.html