I get numerous questions about the new SOM in the data ocean model of CESM. So, I thought it appropriate to add a sort of FAQ here.
1. Why does the SOM mode no longer run out of the box?
Earlier versions were set up with a test SOM forcing file (pop_frc.1x1d.090130.nc) This forcing file was not intended for science purposes and should not be used except in testing. Hence we made the decision to remove a default SOM forcing file from CESM1.1 and onward.
2. How do I set the SOM forcing file?
As long as the forcing file you wish to use is in the standard input directory, i.e. $DIN_LOC_ROOT/ocn/docn7/SOM, then as of CESM 1.1., one simply modifies DOCN_SOM_FILENAME in env_run.xml. The default value of this variable is UNSET. In older versions of the code, this had to be modified in the docn.buildnml.csh script or the docn.stream.txt file.
3. What if my SOM forcing file is in a different directory?
Here you will need to modify the docn.streams.txt file to manually point to the directory you wish. As of CESM 1.1, you can copy $CASEROOT/CaseDocs/docn.streams.txt to $CASEROOT/user_docn.streams.txt and edit it.
4. Do you have some scientifically valid SOM forcing files available?
Yes! These are in the input data repository at:
For the most part, they are documented as to the run they came from by doing an 'ncdump -h' on the file.
5. Can I create my own SOM forcing file?
Yes! The tools for this are available from the release code repository at:
We are striving to keep these up to date, but there may still be issues with them. There is a simple README file here with a description of the tools.
6. Can I run the old CAM3 SOM method with gensom?
The short answer is no. This mode is no longer supported in CESM. If you wish to use this technique to generate SOM forcing, we cannot help you.
7. Why can't I run the old CAM3 SOM?
We believe that it is scientifically better to use the new SOM method. We recommend reading the following paper that discusses the old SOM versus the new SOM at:
Bitz et al. 2012, J. Climate, http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-11-00290.1
Also, there is a white paper on the subject under the CESM DOCN user's guide at:
8. What compsets is the DOCN-SOM used in?
The E compset with fully active atmosphere, land, and sea ice components uses the DOCN-SOM component as does the active ice only compset (D).
9. Why do I have to run fully active ice with the SOM?
This is more of a scientific question, but we have found that when computing the sea surface temperature (SST) prognostically as is done in the DOCN-SOM, it is better to use fully active ice with it. One could potentially turn on thermodynamic-only ice by turning off the sea ice dynamics (kdyn = 0), but we do not routinely run that way. Much work has been done over the years stressing the importance of the sea ice dynamics. We do not support prescribed ice mode with the DOCN-SOM. When specifying the sea ice fraction, one must also use specified SST (in an F compset).
10. These did not answer my question:
If you have issues with parts of this, or further questions, you can post something to this forum and we will attempt to address it as soon as possible.