[an error occurred while processing this directive] [an error occurred while processing this directive]

Report of the CCSM Isotope Group Meeting

Breckenridge, CO

22 June 2005





1. Discussion of CAM3 water isotope scheme by D. Noone


Water isotopes have been implemented in CAM. There is a single isotope microphysics sub-model for deep convection, shallow convection and large-scale condensation. The current formulation has a significant performance hit (doubles time for standard CAM). This is likely due to lots of checks and lots of iterations to isotope routine. Results are generally good (relative to observations of either d18O in precipitation or HDO from aircraft, satellites). The Meteoric water line not as well reproduced.


Next steps are migration from LSM to CLM. Continued validation, need a collaborator for the ocean.


Papers in progress: Document scheme and compare to observations (Noone). Other scientific papers (Payne). Also paleoclimate runs (NCAR)


Questions:  Can we ‘tack this on’ to an existing LGM run of the model, using LGM as initialization? Yes, troposphere equilibrates in about a year (up to 50 yrs for land), so can run 10 or 20 years from LGM conditions. Can also tag ocean basins.


How do we ‘bless’ code?  Answer: depends on working group. No formal process for doing so. See discussion below, but once code has been compared to observations it should be ready as a first version.


Discussion of a need to know what it contains. What are the details of how it treats convection (what temperature is used for condensation? Details of high frequency interactions). Important questions include.  What does convection do? Can we reproduce the amount effect?


How do isotopes affect the atmospheric codes?  Can we use isotopes to constrain the development of moist processes in the atmosphere.  It was discussed that we can try to implement at least water tracers in the atmosphere as a step forward.


2. Other efforts


Suits:  Carbon isotopes into CLM2.1, now adding to CN code of CLM. Snag: need a reservoir. Hope to have it soon. Focus on 13C, could add 14C. 


Cameron-Smith: interested in 14C in atmosphere. New measurement technique at Lvermore. Where should we sample? Easy to get CMDL site samples.


Fung: What about 14C in ocean (Lindsay says untested in POP, but exists). Also, Jun-Eung Lee has a CAM2 isotope scheme that can do the amount effect. Scheme is consistent with and complementary to Noone effort.


Randerson: 14C and D14C is important biosphere tracer. Gives turnover times in biosphere.  Also working on land and water isotopes


Baker: Working with Sam Levis on revision of CLM code. Prognostic canopy airspace. “Don’t waste time putting isotopes into CLM”: but, maybe physics won’t change much.


3. Discussion of future work/priorities and path forward


There was a good deal of discussion of the fact that you can’t wait forever for the perfect model, and you have to go with what you have. CLM is being rewritten, CAM as well will likely be rewritten soon, but still have 3-4 years of work.


It was also discussed that we have to be careful of software engineering so we don’t spend too much effort on version dependent codes. Really what is needed is a strong statement (probably from the SSC) that codes should be able not just to transport tracers, but to trace water and carbon through all processes. An analogue in the ocean is salinity. This way all future codes could be compatible with isotopes of water and carbon. It would also help unify treatments of deposition and aqueous chemistry.


This is slightly different from using the isotopes to constrain the physics. It is the first step that makes this possible, and allows functionality to be easily added (or subtracted) for any individual problem.


The key for the atmosphere to make certain that isotope codes are not specific to one model version is to use tendencies rather than tracers. Can we do what we need with tendencies (yes, sort of for most problems).


The path forward is not exactly clear, but for all functionality added to the model need an evaluation test: either a ‘sanity check’ (i.e. conservation of isotopes, or preservation of ratios with fractionation off) or ‘verification/evaluation’ against observations. Would be good to try to generalize scheme for all water / carbon tracers.  So we also need to bring observations to the table for evaluation.


There was also some discussion of starting to use the coupler to pass isotopes around. This was considered to be a trivial implementation with new modifications to the coupler. Contact CSEG for details.


The meeting finished with an agreement to continue discussions, and designate a ‘point of contact’ on isotopes for each component model.


Points of contact for each model:

            Atmosphere: Gettelman

            Ice Model: Noone

            Ocean: Lindsay

            Land: Thornton*

            Ice Sheets: Lipscomb*










David Noone

U. Colorado


Water, carbon, ozone

Naomi Levine



18O, oceans

Ulli Seibt

Carnegie Inst


13C, 18O

Sheri Conner Gausepohl



CO2, H2O, 18O

Helen He



CO2, O3, 18O, 13C

Inez Fung

UC Berkeley



Phil Rasch




Bette Otto-Bliesner



18O, 13C

Jim Randerson



14C, 18O, 13C

Keith Lindsay




Neil Suits



13C, 18O

Phillip Cameron-Smith




Ian Baker




Brian Eaton




Andrew Gettelman




Natalie Mahowald