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

[mitgcm-support at dev.mitgcm.org]

Tom, Happy New Year to you also and thank you for testing the sea-ice code.

>> I've been exploring your new seaice package (r1_p10) with the exf option
>> selected (SEAICE_EXTERNAL_FORCING) in my 1deg N. Atlantic model. The code
>> diverges after a few 100 steps.
>> 
>> A likely problem is the longwave radiation. In SEAICE_FFIELDS.h, lwflux is
>> defined to be downward, hence we should expect negative values. But in
>> seaice_readparms.F the longwave radiation has a minimum value of 60 Wm^-2.
>> This seems contradictory.

For pkg/seaice bulk formulae you need to provide the "downward longwave
radiation flux", that is, the part of longwave solar radiation that filters
through the clouds, added to that absorbed and re-emitted downwards by the
atmosphere.  That number is always positive, on average 300 W/m^2.

>> Also, your flo.labsea1979 data file has positive values in the range
>> 100-300 Wm^-2. How did you get these numbers?  The Jan1979 NCEP net
>> longwave radiation is in the range 20-90 Wm^-2 which seems more
>> reasonable (attached).
>> 
>> I'm having other problems with the freshwater forcing, but they might be
>> related to the radiative fluxes.

The flo.labsea1979 file contains a mean monthly climatology derived from the
1979-1999 NCEP "downward longwave radiation flux" fields, not from the "net
longwave radiation" fields.  The later, as you point out, are usually negative
because they include ocean-surface reflection as well as the black-body
radiative contribution of SST and sea-ice.

>> Finally, the ALLOW_RUNOFF option doesn't work anymore when it's undef'ed.
>> There's a trivial problem in growth.F

As configured, pkg/seaice requires that ALLOW_RUNOFF be defined.  Actually
there is a check for this in pkg/seaice/seaice_check.F, which will abort the
program if "ALLOW_RUNOFF" is undefined.  If runoff is unknown, both pkg/exf
and pkg/seaice default to zero runoff, as is the case for all the
verification/lab_sea test experiments.

Your questions do bring up the issue of sign convention and unit consistency
between pkg/exf and pkg/seaice bulk formulae, which has been set aside, and
which I need to sort out with Patrick and co.

Cheers, 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