[Mitgcm-support] Re: pkg/seaice and pkg/exf

[mitgcm-support at dev.mitgcm.org]

>> (i) Which shortwave flux should I use if both ALLOW_KPP and ALLOW_ATM_TEMP
>> are defined?  It would help if these subtleties were documented somewhere.

I agree this is confusing.  I have sent a message to Patrick about this and
hope to make some changes to pkg/exf, which will simplify and clarify things a
bit in near future.  Your feedback really helps for this.  In the meantime the
situation is as follows:

1) KPP package requires net shortwave, that which actually penetrates
   into the ocean.

2) Sea-ice bulk formulae require the downward shortwave, that which arrives at
   the surface of the ocean.  Part of it will be reflected due to albedo.
   pkg/seaice will then generate the correct shortwave flux, which is passed
   on to the KPP package.

3) I am not sure what pkg/exf bulk formulae require or do.  I am waiting to
   hear from Patrick about this.

In the meantime, if you use pkg/exf and pkg/seaice with
SEAICE_EXTERNAL_FORCING defined and SEAICE_EXTERNAL_FLUXES undefined, that is,
if you use pkg/exf to read-in atmospheric state but pkg/seaice bulk formulae,
as you are, then you need to provide "downward", not "net" shortwave flux.

>> (ii) I get a compile-time error in growth.F if ALLOW_RUNOFF is undef'ed,
>> not a runtime warning.

OK thanks.  I'll fix that.

>> (iii) I'm using SEAICE_EXTERNAL_FORCING and EXF_NO_BULK_COMPUTATIONS so as
>> to use the pkg/seaice bulk formulae code. My evaporation field looks wrong,
>> however (it's O(10^-5) m/s and has both signs - I expect O(10^-8), and only
>> positive values). With EXF_READ_EVAP undef'ed and no evaporation file
>> defined I was expecting pkg/seaice to calculate the evap. In fact,
>> exf_getffields.F seems to be used to get it. I'm confused!

Oops!  You are really giving the seaice package a run for its money!
pkg/seaice does not compute evaporation.  It expects evaporation to be
provided.  With EXF_READ_EVAP undefined you are relying on the evaporation
field computed by exf_getffields.F, which from the sounds of it is not working
too well.  Problem is probably one of units/sign incosistency between pkg/exf
and pkg/seaice, which I need to sort out with Patrick and will be easy to fix.
But even beyond that, there will be some question mark regarding application
of exf_getffields.F evaporation fields over ice-covered areas.  This requires
a little more thought and hacking.  In the meantime, your best option is to
explicitly provide evaporation files from the NCEP reanalysis.

Dimitris

-- 
Dimitris Menemenlis                      menemenlis at jpl.nasa.gov
Jet Propulsion Lab, MS 300-323           tel: 818-354-1656
4800 Oak Grove Dr, Pasadena CA 91109     fax: 818-393-6720