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two things



US OCMIP participants:

Two of you mentioned you had difficulty reading the attached postscript file
describing my proposal for CFC modeling protocols.  You can try accessing this
file, or an ADOBE PDF version, by anonymous ftp to ftp.essc.psu.edu; cd to the
directory /pub/meteo/najjar.  Let me know if you still have trouble or if you
have other ideas about how I can get such files to you.  I use FrameMaker for
my word processing, so it would be easy for me to make a FrameMaker file
available.  I don't think I can easily make a Microsoft Word file available--
sorry.

A second point that I would like to open up for discussion is gas exchange in
OCMIP.  Niki Gruber found the use of model SST for computing solubility and
observed SST for computing the Schmidt number inconsistent.  Jim Orr thinks
we should stick with OCMIP-1 protocols for gas exchange.  If I understand
correctly, everyone in OCMIP-1 used the same gas transfer *coefficient* (K)
for CO2 gas exchange.  Thus the flux was parameterized by

F = K(pw-pa)

where F is the upward CO2 flux, pw is the partial pressure of CO2 in water and
pa is the partial pressure of CO2 in the atmosphere.  Everyone used the same
fields of K, computed with observed SSTs and winds.  (Recall that K = alpha*kw
where alpha is the solubility of CO2 and kw is the gas transfer velocity for
CO2.  kw was taken from Wanninkhof, 1992.)  Everyone also used the
same value of pa, but, of course, values of pw varied from model to model.  pw
is a diagnostic quantity that must be computed from prognostic variables in
the model (DIC and Alk), but it is not clear to me what SSTs were used to
compute pw.  Either way, I see problems.

If model SSTs were used for computing pw, then essentially two different SST
data sets were used to compute the CO2 solubility:  model SSTs for computing
alpha for pw and observed SSTs for computing alpha for K.

If observed SSTs were used for computing pw, then there is inconsistency
between the model circulation field and the temperatures used for computing pw. This is more important than it may seem.  It's reasonable to assume that deep
water will form in regions with very specific T-S characteristics, and
therefore, to the extent that surface waters equilibrate, very specific gas
concentrations.  It seems to me that you want consistency here so that, for
example, if deep water sources are very cold that they be characterized by very
high gas concentrations.  Because of the long equilibration time for CO2, this
may not be a big problem, but I think it could be disastrous for oxygen and
CFCs.

My suggestion, therefore, is that only model SSTs are used for computing
gas solubilities.  In other words, I propose that all OCMIP-2 gas exchange 
(CO2, O2 and CFCs) is parameterized using

F = kw(C - alpha*pa),

where C is the surface gas concentration computed by your model, alpha is
computed with model SSTs and pa is specified to be the same for all groups.
The question, which Niki brought up, is whether the Schmidt number (imbedded
in kw) should be computed with model or observed SSTs.  I am ambivalent on this
issue.  On the one hand, we might want everyone to use the same value of kw;
on the other we might want internal model consistency (only one set of SSTs
are used, period).  I guess I am more inclined to agree with Niki.

Let me know your thoughts so we can come to some agreement soon.

Ray