ESCOMP
COSP-RTTOV integration with CAM
What is the feature/what would you like to discuss?
I have coupled the COSP-RTTOV satellite emulator with CAM and would like to pull it into the main code repository. This update to COSP significantly extends emulation capabilities and is described here: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-169/
I have separately coupled COSP-RTTOV to CAM/CESM in CESM2.1.5 and CESM2.3 versions in CAM forks: https://github.com/jshaw35/CAM/tree/cam_cesm2_1_rel_60_rttov https://github.com/jshaw35/CAM/tree/cam6_3_144_cosprttov
An important note: Integrating with COSP-RTTOV with CAM requires linking an external build of RTTOV to CESM during the build step and changes to CIME. This is further complicated by the fact that the RTTOV source code is free but only available to registered users and creating a new build is slow relative to a normal CESM build, so linking an existing RTTOV build during CESM build appears to be the best solution.
CIME fork here: https://github.com/jshaw35/cime/tree/cime6.0.217_httpsbranch02_cosprttov
Is there anyone in particular you want to be part of this conversation?
@cacraigucar @dustinswales
Will this change (regression test) answers?
No
Will you be implementing this enhancement yourself?
Yes, but I will need some help
@jshaw35 - It was my understanding that the COSP external library would be updated with this additional code and that we would be accessing this new feature directly in the same way that we currently access COSP. Is that not the case?
@cacraigucar - The changes to COSP that add RTTOV are still pulled in via the COSP external library (I am also working with the COSP team to official pull that code into the COSP trunk). Linking RTTOV to CESM/CAM, however, required me to make changes to how COSP was built in CAM. If there are ways to do this that avoid the deeper model changes I am happy to pursue them.
I'd separate the code changes into 3 categories:
- Changes to the interface between CAM and COSP (effectively just cospsimulator_intr.F90).
- Changes that allow CAM/CESM users to use new features in COSP-RTTOV (both RTTOV itself and satellite swathing of COSP outputs). (largely the addition of namelist options)
- Changes to CAM that allow RTTOV to be (optionally) linked during build.
@jshaw35 - The biggest concern is that users must register to get RTTOV. Currently CESM/CAM is available with simply a git clone and all externals are brought in automatically. Do you know if the requirement to register for RRTOV will be required or once it is brought into COSP, will the "registration" already have occurred?
@cacraigucar - This is a really good question. To clarify re: the COSP-RTTOV integration, COSP users will still need to register as RTTOV users if they wish to access the RTTOV source code. So I expect that running COSP-RTTOV within CESM will have the same requirement. I know this is more complex than the standard CESM instructions, and this motivated the option to build and run CESM without any knowledge of RTTOV since most users will not want to use it. So the task of getting RTTOV code is only needed if you are a CESM user interested in running the RTTOV functionality within COSP-RTTOV.
For users running on NCAR machines, having a shared build of RTTOV would also allow users to use this functionality without additional steps.
Overall, I totally understand that the additional steps of this integration are a barrier to use. I'm hopeful that CESM users interested in this functionality are either running on the NCAR systems or willing to work through the additional integration steps.
Would the external users who want to use RRTOV, need to go to the following website to register and download? https://nwp-saf.eumetsat.int/site/software/rttov/ Is that what you are envisioning? If so, then I believe CESM will need to discuss if this is something we would want to do.
@cacraigucar - Yes, I don't think there is another way to access the RTTOV source code. I asked the NWP-SAF developers about distributing RTTOV with COSP a few years ago and they were pretty clear that they wanted users to go through the official registration process. I can always ask again in the specific context of COSP-RTTOV's integration with CESM.
@jshaw35 - Asking them if they would make a special accomodation for COSP-RTTOV being included in CESM would be great. In the meantime, I will bring this up for discussion at our next meeting with the CESM SEs. Since we are a community model, when we moved to git, we conciously made the decision to have everything available without requiring any registration. Since the current understanding will be a deviation from this, we will need to discuss this. I will be sure to mention that this code will be optional.
@cacraigucar - I just submitted a ticket to NWP-SAF with that request! 🤞 Thank you for raising this with the CESM SEs and let me know if I can be of any help.
Update: Here is the response from NWP SAF:
Hi Jonah,
Unfortunately our hands are tied by the licence under which RTTOV is distributed. The licence is dictated by EUMETSAT, not by us (the developers or the NWP SAF project team). I will ask Fabien Carminati, the NWP SAF manager, but I would be a bit surprised if an exception would be made I'm afraid.
We have raised the idea with EUMETSAT of changing the licence to something less restrictive that would allow redistribution of the RTTOV code precisely for cases like this as we believe it would be beneficial. The response from EUMETSAT has been positive, but progress on actually effecting the change has been slow so I can't offer a timescale on which this is likely to occur.
Furthermore, my understanding is that the new licence will apply to future releases and will not be applied retrospectively to extant versions - if this is the case it will not help you with RTTOV v13.2. I will of course clarify this.
Fabien is out of the office for a week or so, but I will get back to you when I have an update.
Best wishes, James
@cacraigucar another update from the RTTOV developers at EUMETSAT:
"We have heard back that this exception would require approval (not guaranteed!) by EUMETSAT council. The next council meeting is in December, so unfortunately there will be no decision before then."
Separately, I will be updating the COSP-RTTOV changes to an up-to-date version of CAM this week.