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

[Jean-Michel Campin]

Dimitris,

I am confused about your comments (i) to (v) below.
For me, the actual snow thickness is more natural that 
the effective one. Can do in-situ measurements. 
If it does not exist in some parts of seaice_growth but still exists
in seaice_solve4temp, it's going to be quite chalenging to explain 
to new users how seaice pkg works.
Point (iii) to (v) are even more confusing to me.

But to come back to my example of a good snow fall (> 1.cm/h) over 
small fraction of seaice, which could be as low as 10^-5 for default
SEAICE_area_reg=siEPS, within the same grid cell, the SST can 
still be warm enough to be cooled by the latent heat of the snow 
and to remain above freezing (no new ice formed). But instead of 
doing this, the falling snow will increase the actual snow thickness 
over seaice at a rate ~10^5 larger than the real accumulation rate.
Can easily get 1.km of fresh snow (ye, fresh powder !) in an hour or so.

Cheers,
Jean-Michel

On Mon, Apr 23, 2012 at 09:29:33PM +0000, Menemenlis, Dimitris (3248) wrote:
> 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
> 
> 
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