Experimental Protocol for Transcom 3 Level 3 (T3L3)


Aim

This page provides detailed information for participants of T3L3. For background information on this project see the TRANSCOM  home page and the introduction to the TRANSCOM 3 experiment. So far, the Transcom experiments have had a strong focus on differences between transport models and the causes and consequences of that (which, after all, was the goal of the project).  The analyses conducted in T3L1 and T3L2 allow a rigorous indepth assessment of the estimated source/sink distributions and their sensitivity with respect to the different transport models. It is clear, however, that, besides transport, the results of inversions are also sensitive to many other aspects of the inversion set-up, such as data selection and errors, definition of priors and their covariances, and the mathematical procedures to solve the inversion problem. This sensitivity has not yet been adressed in the level 1 and 2 excercises, and the results to some extent give a misleading picture of the constraints on carbon cycle fluxes that inverse modelling can provide. Transcom 3 level 3 aims at a more realistic assessment of this constraint, by allowing not only the transport to be different accross the simulations, but also various aspects of the optimization procedure. It is hoped, that the outcome of this exercise will provide a more realistic assessment of surface fluxes and their uncertainties which clearly will be extremely useful for the wider carbon cycle research community.

Overview of the Simulations

The participants are asked to perform the following inversions:

1) Reproduce T3 Level 2 results with own optimization software ( details).

2) as 1) but using personally preferred optimization procedure ( details).

3) as 2) but, in addition, using personally preferred boundary conditions ( details ).

Inversion 3) is the final goal of the exercise and should reflect the range of estimates that individual groups consider the most realistic they can provide. Inversion 1) is meant is a consistency check of the various codes that are used for optimization. Inversion 2) is meant to quantify what part of the differences in Inversion 3) are due to the different regularization techniques that are used.

Each participant is asked to submit simulated fluxes and corresponding uncertainties for each inversion as well as a detailed specification of the applied inversion set-up. Note that the range of estimates themselves will be the main outcome. Given the large degrees of freedom in the experiment, the interpretation of specific details in the results will likely be difficult (which, however, is not the primary goal). The strength of this experiment (relative to comparing just a bunch of publications) is that the relevant differences between the set-ups are well documented, and that the results refer to the same fluxes that can therefore straightforwardly be compared (and be compared with other Transcom 3 levels).

Details of Inversion 1

Repeat the simulation as specified in the experimental outline of the interannual inversion of Transcom 3 level 2 by David Baker (download here, see also the example of questionnaire 1 - under submissions - for a summary) for your own model. As the target period of the inversion we take 1988-2001 (excluding spin-up). The number of measurement stations is 76, which are to be used throughout the target period. The results that were computed in Transcom 3 level 2 can be downloaded (here - NB: these posterior fluxes are WITHOUT fossil fuel). These results were obtained without any SVD/Fourier truncation or other regularization tricks except for a prior term in the cost function (just the plain Bayesian optimum). Try and get a best as possible match with these results. You may want to try the basefunctions of your model as they were reconstructed from the level 2 output (to be downloaded here). In the meanwhile we found out that a different region assignment of the presubtracted fields may explain some of the differences. You can check this using the T3L2 posterior fluxes without postadding any of the presubtraction fields (to be downloaded here)  Send in the most accurate match that you obtained, and a documentation of any findings/hints that you think may explain the remaining differences (for details of the submission see below ).
Note, this experiment assumes that participants use the Bayesian concept for their inversion. If you use a different method, it may be that to carry out this experiment would require substantial reprogramming. In this case you may skip this part.

Details of Inversion 2

Start with the same set-up as described in Inversion 1). In particular the target time period and the observations and their uncertainties should remain the same as in Inversion 1). Change the optimization method to one that you consider the best you can do. This includes changes in the number and the geometry of the regions (in space and time); and you may want to use different internal base func tion structures and a priori covariances (variances, however, should remain the same). Integrate/interpolate the estimated fluxes and uncertainties over/to monthly Transcom 3 regions, and document all changes as compared with Inversion 1 (see  submissions).

Details of Inversion 3

Start with the same set-up as described in Inversion 2). Change all boundary conditions to what you consider the best you can do. By boundary conditions we mean every aspect of the inversion, other than the target period. This includes the number of observations, selection method, uncertainties, and the use of interannually varying meteorology etc. to drive your transport model. However, the optimization method and region geometry should be the same is used in Inversion 2).  Integrate/interpolate the estimated fluxes and uncertainties over/to monthly Transcom 3 regions, and document all changes as compared with Inversion 2) (see  submissions).

Submissions

For every simulation one results file and one documentation file are submitted. The file name convention is:

                                                model-id.inversion-id.format-id

where

model-id: as used in previous Transcom experiments, or new (e.g. TM3.Houweling)
inversion-id: inversion that the file contents refer to (inv1, inv2 or inv3)
format-id: 'txt' for documentation, 'nc' for inversion results in netcdf format.

The .txt files contain a completed questionnaire in ascii format (download examples here) which is to contain a detailed and quantitative documentation of all changes that have been introduced as compared with the previous inversion (such that someone else could repeat the inversion on the basis of this documentation only). Because of the various approaches employed by the different participants this documentation is very important, and possibly we will get back to you for even more information.

The .txt file for inversion 1 is optional (just in case there are comments in connection with efforts to make the two inversions match).

The .nc file is a netcdf formatted file and contains the following results for each inversion:

Fluxes and 1 sigma uncertainties should refer to monthly Transcom regions (see map ). The unit is PgC/month with respect to the atmosphere (positive means source). Note that fluxes and uncertainties refer to the sum of ALL processes (INcluding e.g. fossil fuel use) Please make use of our standardized fortran output subroutine to create this file ( output_nc.f ).
Ftp all files to the a nonymous ftp-server ftp.bgc-jena.mpg.de; cd pub/incoming/; bin; put *.*; bye.

Send a notification e-mail of your submission to all three of us:

Sander Houweling (s.houweling@phys.uu.nl),
Martin Heimann (martin.heimann@bgc-jena.mpg.de),
Stefan Körner (stefan.koerner@bgc-jena.mpg.de).

Analysis Workshop

The Jena group has organized a workshop in spring of 2003 (12-15 May) where the first results were presented and discussed. Information on the workshop program, attendee list, pictures and links to the powerpoint presentations can be found here. For the T3L3 intercomparison the following has been proposed:
  • Sept 1: New version of the results that were submitted by March 31 (distributed by e-mail)
  • Oct 1: Extended deadline for new submissions  
  • Dec 2003: New version of results including Oct 1 submissions (meeting to be announced)
  • June 2003: Update of T3L3 analyses at the next TRANSCOM meeting (..?)
Based on our experience so far the protocol has undergone some minor changes (see the correction list for details).

Please send an e-mail to s.houweling@phys.uu.nl if you encounter any problems.
Last update: June 4, 2003 by Sander Houweling.
Corrections since the first release (9-12-2002) are listed here