[MITgcm-devel] Add sea ice surface forcing to pkg/seaice

[Menemenlis, Dimitris (3248)]

Jean-Michel - sorry for mixing topics.

> My question 2.b is not stupid, because snowPrecip (when specified
> from a file) can fall over open ocean (whereas with empty snowPrecipFile,
> it only snows when some ice is present).


You are correct.  This is a key distinction.
I am assuming that if/when someone specifies "snowPrecip", they will do
so because (at a minimum) they explicitly want to take into account the
negative latent heat of freezing that the snow contains.  So one needs to
keep track of all the snow, including that falling over open ocean.

> And regarding 2.a: I think we know how snow accumulates:
> If snow is falling at a rate of 1.cm per hour, I expect afer 1 hour
> to have the actual snow thickness to have increased by 1.cm
> (and not by 1000 time more in the case where the area is 10^-3).

I agree that this is not very realistic but a few comments:

(i) I think that it is a little misleading to think of HSNOW/AREA as
"actual snow thickness", especially in the extreme case when
AREA=10^-3. I prefer to think of HSNOW*RAC as a volume reservoir
of frozen water available to cool and freshen the surface ocean
and/or to create sea ice in that grid.

(ii) Where the "actual snow thickness" and AREA will matter
is in setting albedo. For the specific problem that I have in mind,
albedo is secondary to the release of freshwater and
negative latent heat.

(iii) One way to get around this problem may be to set AREA=1
as soon as sea ice starts growing due to snowPrecip
(and I can implement this, if you prefer - although I am
a little scared about possible unintended consequences).

(iv) However, there are many other modeling errors and uncertainties
that are also problematic, for example, the salinity, freezing point,
and albedo of slushy ice that results form fresh snow falling on salty
water, and wind and wave interactions that may cause
AREA<1 during a snow storm.

(v) In actual tests of code as it is right now, if there is sufficient
continuous snow over open ocean, the sea ice concentration
quickly goes from AREA=0 to AREA=1, and much of the
excess snow gets converted to freshwater ice - especially since
any excess heat is used to melt the snow first, and then the ice.

Dimitris Menemenlis