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meeting minutes



OCMIP devotees:

US OCMIP had a meeting recently and I have included the minutes below.  If you
have thoughts on anything we discussed, please discuss them on this mailing
list.

Ray Najjar



US OCMIP meeting minutes
May 19, 1998

Attending:  Ray Najjar (minutes taker), Jim Orr, Mick Follows, Ken Caldeira,
Jorge Sarmiento, Scott Doney, Matt Hecht, Chris Sabine, Bob Key and
John Bullister.

ANALYSIS/SOFTWARE

Jim gave us an update on the development of graphical and analysis software
packages being developed by him and Patrick Brockmann at LSCE.  Data storage
will likely be in the form of NetCDF.  Graphical packages could vary. 
Currently LSCE uses TecPlot, which has the advantage of handling a variety of
grids.  One must purchase this, however.  Ferret, on the other hand, is free,
but cannot handle any grid.

SAVING MODEL OUTPUT

There was some discussion as to what aspects of the model simulations should
be saved.  We reached no firm conclusion on this.  A proposal will be made by
Jim and Ray and sent to OCMIP participants.  For the steady-state runs, the
choice of what to save is not critical because the model can be run out a 
little longer and additional information saved.

EQUILIBRATION TIME

An issue that came up repeatedly was defining how long runs should be in order
to be considered at a steady state.  Jorge showed an interesting figure of
a tracer study in which an ocean GCM was initialized at zero and restored 
towards unity at the surface with a 30-day time scale.  Even after 6000 years
of integration, there was a 6% north-south difference in the deep Pacific.
The only part of the model that was above 0.99 was the thermocline.  These
results suggest that it may take longer than previously thought to reach
equilibrium.  It was decided that criteria need to be developed to define
"equilibrium," even if some models, because of computational cost, can not
be run out to equilibrium.  It is therefore critical that some information
about temporal trends in the models be recorded so that these could be taken
into account when the models are analyzed and compared.

It was suggested that some effort be put into looking at the equilibration
issue.  Jorge's group will continue the simulation described above to see how
long convergence takes.  Ken and Scott agreed to look at simple models to try
to understand the problem.  (Ken and Rick Murnane have already started an
electronic dialogue about this on the ocmip-all mailing list.)  Ray suggested
that one group do very long runs of the equilibrium runs in OCMIP: the solu-
bility pump, natural C-14 and the biological model.

Ways to speed up models are to use different timesteps for momentum and
tracers, as well as increasing the timestep with depth for some tracers.
Matt has also experimented with using larger timesteps for passive vs active
tracers.  In any event, it is advisable to run the model towards the end of
the run with the same timestep for all prognostics for a fixed amount of time.
Scott's experience with the NCAR ocean model is that only 20-30 years are
needed for running synchronously.

Jim also mentioned a scheme that he and others (Aumont et al., 1998, Climate
Dynamics) developed to accelerate offline model runs.  The scheme is called
DEGINT.  This can amount to large savings in computer time.  An issue here
is whether offline models can give the same results as online models.  Jorge's
group has essentially abandoned the offline model because of this.
 
GAS EXCHANGE

We agreed that model SSTs were the appropriate choice for computing gas
solubilities and Schmidt numbers for gas exchange.  It was also agreed to
take into account climatological sea level pressure and surface humidity
variations in relating the dry air mixing ratio to the saturation
concentration.

One interesting point brought up by Jim is that a CFL-like criterion can be
violated in the computation of air-sea gas fluxes, particularly for non-CO2
gases, which have relatively short equilibration times.  He presented a
algorithm for adding in a correction term that relieves this instability.

EQUILIBRIUM CONSTANTS

We all agreed on adopting the recommendation of Chris and Ray that the
Dickson and Millero refit of the Mehrbach constants K1 and K2 are the best
choice for OCMIP.  Many studies are converging on the opinion that these
constants are the most accurate for computing pCO2 from DIC and Alk, which all
OCMIP models need to do for solubility pump, biological pump and CO2 uptake 
simulations.  All other aspects of the equilibrium constants protocol were
also agreed upon.  Chris and Bob agreed to make a subroutine based on this
protocol that computes the CO2 aqueous concentration given, as inputs, the
DIC, Alk, phosphate, silicate, temperature and salinity.  It was also noted
that very useful information about the carbonate system is available from the
"DOE Handbook" edited by Dickson and Goyet.  The web sites for this document is 
www-mpl.ucsd.edu/people/adickson/CO2_QC

CHLOROFLUOROCARBONS

We agreed to adopt the CFC protocols designed by Ray and John, with one
exception: an alternative formulation describing the meridional variation of
CFCs should be considered.  In the protocol, a straight line fit between two
points was proposed; a piecewise linear fit, or something else, may be
preferable. John and Scott agreed to look at observations and determine what
would be the best shape to pass through the two "observations."

John also gave an update on the WOCE CFC data.  The global survey is now
almost complete and the spatial resolution is remarkable, with zonal and
meridional transects approximately every 20-30 degrees.  It is clear that this
is going to be an extremely valuable data set for OCMIP.  John and others
(Rana Fine, Bill Smethie, Mark Warner and Ray Weiss) have a proposal
submitted to work on the synthesis of the data.  A number of issues
need to be addressed in the synthesis, such as quality control and
the non-synoptic nature of the data--it was collected over a 12-year period.
By the end of this year, John felt that OCMIP should be in a position to make
some initial evaluations of the models with WOCE CFC data.

HOW-TOs

Jim suggested, in addition to the protocols that Ray has been sending out, 
that we describe in more detail the step-by-step process of incorporating
the protocols into the models.  We agreed that this was a good idea, and
Jim passed out an example for the CFC simulations.

NUTRIENT-RESTORING MODEL

Most aspects of the nutrient-restoring model proposed by Ray were accepted.
Some exceptions are:
  (1) To eliminate nitrogen fixation (set it to zero).  This means that if
  there is any denitrification, the ocean is a net source of oxygen to the
  atmosphere, but it will likely be very small.
  (2) To more thoroughly look at the literature to make sure the time constant
  for DOP remineralization (0.5 years) is the most appropriate.
  (3) To improve the nutrient maps.  Ray presented the monthly phosphate maps
  that Ferial Louanchi and he have been working on creating from the NODC data.
  There is a clear seasonal signal in the data, particularly at high latitudes,
  but there is also a lot of additional spatial and temporal variability that
  is probably not truly representative of the mean annual cycle.  It was
  suggested that Ferial and Ray attempt to produce smoother maps without 
  weakening the seasonal cycle.  Some ideas that were tossed around were
  fitting the data to an annual harmonic, zonally smooting the data, and using
  different nutrient-temperature regressions (which are used for default 
  fields) for different months.  It was also suggested that the spatial
  distribution of the observations be made available.
Jorge volunteered to have his group do a "trial" run of the nutrient-restoring
model to check to see if the model is properly formulated and gives reasonable
results.  It was decided that, so long as this could be done quickly, that the
other groups would hold off on doing the nutrient-restoring run until we are
satisfied with the Princeton results.  We did not decide what was meant by
quickly.  Ray will clarify this.

RADIOCARBON

Ken showed that there are significant differences in D14C in simulations using
the method of Toggweiler, which carries the 14C/12C ratio as the only variable,
in comparison to a "full-blown" method which carries, 12C, 13C and 14C
separately.  Pre-bomb distributions show a consistent offset of 15 permil and
post-bomb distributions show a 6 permil offset.  Ken also showed some box-
model studies that qualitatively supported these results.  These results 
suggest that OCMIP should not simulate D14C by carrying the 14C/12C ratio
as a tracer.

Jim then showed some 3D model studies by Bacastow and Maier-Reimer (Climate
Dynamics, 1990) in which it was clear that biological processes have a
negligible impact on the the distribution of D14C.  In their model, a 
solubility pump model was run for both 12C and 14C and the ratio was computed.

The above suggests the following protocol for simulating C-14:
   (1) Run 12C and 14C in a solubility pump mode until steady state is reached,
   fixing atmospheric 12CO2 and 14CO2 at preindustrial values.  Assume the
   steady state represents conditions in 1765.
   (2) Continue to run both tracers starting at 1765 and increase 12CO2 as 
   given by observations from 1765 to 1950.  Atmospheric 14CO2 remains the
   same.  This is the "Suess effect" part of the run.
   (3) Continue to run both tracers starting at 1950 and increase 12CO2 and
   14CO2 as given by observations from 1950 to 1995.  This is the "bomb" part
   of the run
We decided to adopt this protocol for simulations of radiocarbon.  Jim agreed
to write up this protocol and distribute it to ocmip-all for final approval.

Bob Key gave an overview of the WOCE 14C data, showing the variety of ways
the data could be used to evaluate the models.  For example, there are useful
comparisons of WOCE data with earlier (e.g., GEOSECS) data, showing the
evolution of the bomb signal.  There is also a way to extract the pre-bomb
signal with greater accuracy than previous efforts.  Bob noted that Broecker's
method of using the silicate distribution to isolate pre-industrial 14C has
serious problems.  A better solution appears to be the use of "potential
alkalinity" (salinity- and nitrate-corrected alkalinity).

CO2 SURVEY DATA

Chris gave us an update of the CO2 Survey data.  The Indian Ocean QC and
preliminary analysis is essentially done:  there are two north-south sections
and three east-west sections.  Chris has made estimates for the spatial
distibution of natural and anthropogenic CO2 in the Indian Ocean.  He expects
that the Pacific should be done by the end of this year.

Chris and Bob will not only provide analyses to the modelers, but will
also contribute to OCMIP model evaluation, as they have been doing for the
Princeton/GFDL model.  As was argued in our proposal, we will produce much
better science if we have close collaboration between modelers and measurers.