[MITgcm-devel] another bug in growth.F ?

[Dimitris Menemenlis]

Martin, I also notice increased ice thickness around Antarctica in the high-res 
cubed sphere integration and increased sea-ice extent, much more summer ice 
extent than observed.

What is you run34?  In terms of CVS repository,
http://mitgcm.org/cgi-bin/viewcvs.cgi/MITgcm/pkg/seaice/growth.F
which version of growth.F did you use: pre 1.29, 1.29 with flooding and 
advection turned on, 1.30, or 1.34?  So far as I have looked in my own tests, 
results from 1.30 and 1.34 are not that different.  But then your run34 does not 
have snow over open water, so it cannot be pre 1.29?

Optimistically, the too-much ice in run38 may be a bug in the NCEP/CORE forcing 
fields rather than in growth.F.  That is, too much precipitation is converted 
into snow, which extracts heat from ocean when it melts.  The effect would be 
highly non-linear since more ice/snow extent means higher albedo, which leads to 
cooler ice/ocean surface temperature, which in turn leads to more precipitation 
being converted to snow, since in present treatment of precipitation, rain to 
snow conversion depends on thermodynamic ice growth (rain) or melt (snow).

Optimistically again, the high tangent linear sensitivity noted by Patrick and 
also in the verification experiments that you report is also due to above 
effect.  Incidentally, with growth.F prior to 1.30, the verification/lab_sea 
domain is at all times 100% covered with snow (but not ice).  So low forward 
sensitivities prior to 1.30 are almost certainly for wrong reason.

One possible way of reducing the forward sensitivity (I have not yet tried it) 
would be to remove the snow/rain dependence on ice growth rate and instead make 
it depend on forcing field, e.g., surface air temperature and fresh water 
freezing point, as is done in pkg/thice.  Should I give this a try?

Dimitris