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FAQ: Data ocean slab mode (DOCN-SOM)


CSEG and Liaisons
Staff member
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.

*Note this has changed slightly for CESM2*

Here you will need to modify the docn.streams.txt.som file to manually point to the directory you wish. As of CESM 1.1, you can copy $CASEROOT/CaseDocs/docn.streams.txt.som to $CASEROOT/user_docn.streams.txt.som 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, Climate Sensitivity of the Community Climate System Model, Version 4 | Journal of Climate | American Meteorological Society

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.
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Thank you for posting instruction of DOCN-SOM. That is very helpful.

I have a question about formulation of qflux calculation.
I found in the white paper qflux is Fnet minus SST tendency term.
However, in Bitz et al. 2012 paper, qflux is calculated as SST tendency term minus Fnet.
So there will be a sign difference from these two calculations. 
I wonder which one we should use when calculating qflux for DOCN-SOM runs?
THank you so much for the reply.
We are trying to run E_1850_CN at f19_g16 with CESM 1.2.0, and would like to know which SOM forcing file to use. It looks like the file pop_frc.b.c40.B1850CN.f19_g16.100105.nc might be appropritate, but that file looks to be from an older version of CESM since it was generated back in 2010. Is it appropriate to use a SOM forcing file from a previous version of CESM? ThanksSean
Hi Dave,I had tried to run the CESM1.1.1 E_1850_CAM5_CN compset. I would like to check the SST variable, but I found no output in ".../ocn/hist/" directory. I changed the  "f_sst" in ice_in file to "mxxxx", and find SST variable in the output file in "../ice/hist".
Did this way make sense?Thanks!


CSEG and Liaisons
Staff member
The SST comes out in the CICE history. Have a look in ice/hist. A good way to check the variables as well is to write out coupler history files. Have a look at the user's guide to see how to do this.Dave
hiiii,I want to run cam5 with slab ocean model using source codeCESM1.2My commands are as follow./create_newcase -case som_f19_g16 -res f19_g16 -compset E_2000_CN_GLC_CISM1 -mach chandra./cesm_setupthese 2 commands run successfully./som_f19_g16.buildI have attatched the content of above command (after execution).Please go through it.suggest me what should be my next step.(I mean how should I approach  step by step to get the necessary files required to run the model in slab ocean mode)Thanks in advance 
hi dbaileythanks for this info.I just tried one file athttps://svn-ccsm-inputdata.cgd.ucar.edu/trunk/inputdata/ocn/docn7/SOM/
and I see some bands. For example, see west of South America in the attachment.They all seem to be created a few years ago. Is there anything from new simulations.Having the SOM prep files for this would be great for me:https://www.earthsystemgrid.org/dataset/cmip5.output2.NCAR.CCSM4.historicalGHG.mon.ocean.Omon.r1i1p1.htmld


CSEG and Liaisons
Staff member
Hello,We do have one new SOM forcing file that is not on the repo yet. It is from the Large Ensemble control run. However, I'm not sure the banding will be any better. This is largely a function of the surface fluxes and you are seeing the imprinting of the atmospheric grid on the ocean. I will try to make the other one available soon.Dave
Hi Dave,this sounds promising. I am looking forward to getting this data.For the banding I might end up applying some smoothing just for the very sharp gradients.Daniele


CSEG and Liaisons
Staff member
I just checked in the file pop_frc.b.e11.B1850C5CN.f09_g16.005.082914.nc. Give that a try and let me know. Dave
Hi dbailey,I would like to run SOM with the E_2000_CN_GLC_CISM1 compset that is avaialble for CESM_1_2. However, it seems to me that the Q-flux data for the 1990 control run under the input directory (e.g. pop_frc.1x1d.090130.nc from b40.999 and pop_frc.gx1v3.051202.nc from b30.004) were generated using the CCSM4 model with a different compset (B_2000?). Since the new compset uses a different sea ice component (CISM) from the one used in CCSM4, I am wondering whether it is possible to use these files in our SOM simulation. Could you please let me know if you see a problem in using these data? Or do you have new Q-flux data for this particular compset (E_2000_CN_GLC_CISM1)? We prefer to use the Q-flux data that were generated from a present-day simulation.Thank you!- Kai


CSEG and Liaisons
Staff member
I'm afraid we have not run a present day control run with the new code. All of the newer control runs are for 1850.Dave
Dave,(I am back on this)I am running f09_g16 and I am planning to use this file pop_frc.b.e11.B1850C5CN.f09_g16.005.082914.nc for the SOM.What do I want in these fields?



CSEG and Liaisons
Staff member
The SOM configuration (E compset) uses fully-active sea ice and prognostic SST, so these are not used.Dave
Dave,(I was making my own compset but I'll take a look at the E compsets)Where does SOM get the initial ice amount even if it is prognostic?
It's not on pop_frc.b.e11.B1850C5CN.f09_g16.005.082914.nc.Daniele


CSEG and Liaisons
Staff member
In the CICE namelist there is a variable called ice_ic. This the file name from where the initial ice state variables are set. If ice_ic = 'default', this means it sets it to a uniform ice cover wherever it is "cold". If ice_ic = 'none' it is initialized with no ice at all.Dave