Meeting Notes 2018 Science

June 27, 2018

General updates

• First CTSM tag made: ctsm1.0.dev001: merges Bill Sacks's flux separation work with latest CLM master. Moving forward, we'll be using ctsm tags.

• LILAC:

  • Joe Hamman will be pushing forward with the development of the LILAC coupler, to couple CTSM to WRF, etc.

  • Sam Levis (long-time CLM scientist/developer, now consultant) will be working on some aspects to improve usability

• Isotopes, and water tracers in general: Mat Rothstein and Bill Sacks have begun working on the data type rework needed for this.

Data assimilation presentations

(There were three presentations on data assimilation, which I did not take notes on.)

April 25, 2018

Overviews of different modeling systems / applications

Noah-MP user community - discussion

Note that HRLDAS is the offline driver for Noah-MP. The coupling interface between Noah-MP and HRLDAS is the same as between Noah-MP and WRF. Dave L asks if the community using Noah-MP via HRLDAS will present difficulties in terms of switching from Noah-MP to CTSM. Mike's feeling is that LILAC will be key here: if we can couple CTSM to WRF, then we should be able to couple CTSM to HRLDAS.

CLM modeling systems - discussion

Note that most or all of the non-CESM uses of CLM have been one-offs that are not long-term maintainable. This is one of the big motivations of the LILAC project, which should allow for more maintainable couplings moving forward.

Is there a system for parameter estimation / optimization? Not an out-of-the-box solution, but we have been working on pulling parameters out of the code to namelist / parameter files. We'd like to continue more down that path, possibly via LILAC. Fates-CLM has been brought into PECAN, which is a system for parameter sensitivity analyses.

What is the relationship between CLM and the E3SM land model? To a large extent, E3SM started with CLM4.5 and then added the phosphorus cycle. E3SM doesn't have most of the developments in CLM5. There are big divergences in BGC (CLM5 has a lot of new N cycle developments, such as FUN, which are not in E3SM), crop model (both CLM and E3SM have some pieces that the other doesn't have but would like), etc.

WRF-Hydro - discussion

LILAC should probably satisfy a lot of the uses discussed here. However, one thing that could be missing is if LILAC just handles the land-atmosphere coupling: we could also need coupling at the bottom interface, from land to hydrologic models.

Overall thoughts

Dave L: Seems like the biggest challenge could be coupling. He doesn't see any major barriers to our plans for moving forward with CTSM based on what's been discussed.

Noah-MP vs. CLM performance benchmarking

Performance is important, because in applications in which Noah-MP is used, there are tight performance constraints.

Out-of-the-box: CLM4.5 with satellite phenology roughly 15x more expensive than Noah-MP. CLM5.0 with satellite phenology 36x more expensive than Noah-MP.

However, there are some big differences between the two:

• PFT representation: Noah-MP uses the equivalent of 2 PFTs
• Soil layers: Noah-MP generally uses 4 layers
• Number of iterations
• Internal memory allocation inefficiencies in CLM

Able to get about a 90% timing reduction from the CLM5.0 default from addressing the above 4 differences - so CLM 4x more expensive than Noah-MP.

The big remaining culprits are canopy and bare ground fluxes. There's good hope that we could reduce those further with some more work; also note that CLM uses a more complex photosynthesis scheme than Noah-MP.

Note that the Noah-MP driver scales terribly. These numbers just look at the land model itself, not the driver.

These numbers also remove i/o (which accounts for order 10% of CLM runtime).

Dave L: Overall feeling: We are probably close enough to Noah-MP that performance isn't a show-stopper.

Future meetings

We'll plan on monthly meetings, last Wednesday of the month.

March 22, 2018

CTSM review

Martyn gave an overview presentation. The following are notes from the discussion:

One big challenge moving forward will be governance - how to pull together multiple communities.

Ned raises the point that it would be good to have a more robust, easier-to-use system for 1-d land-atmosphere simulations.

Tim Hoar: Will you be able to do ensemble data assimilation via LILAC? e.g., in CESM you can use the multi-instance capabilities; will there be something similar via LILAC? We need to look at this - e.g., look at how this is done for WRF-Hydro, and make sure CTSM-LILAC will fit into this?

Will LILAC be usable for more simple-to-use data atmosphere-forced (land only) runs? Current plan is to continue to use cime/CESM for this, with the data atmosphere-LILAC piece just being used for software testing. In principle, we could use LILAC for this, but the plan is to add more capabilities / simplicity to the cime/CESM infrastructure for this purpose.

Isotopes is a near-term application area where the software redesign for CTSM will be a big benefit.

Besides model structure, another reason why CLM hasn't been used in many application areas in the past has been efficiency. This has driven one of the key focus areas of the recent CTSM work, which Mike will talk about.

Model construction / code management

Some notes from discussion:

How do you select different options? There will be some high-level selection of (e.g.) CLM5 physics, or a certain Noah-MP package. But then you can mix and match - e.g., mainly use CLM5 physics, but parameterization X from Noah-MP.

Request for some API documentation of different parameterizations. e.g., things along the line of what's required of a parameterization - does it need to compute derivatives, for example?

• There is some documentation along these lines within the code, but there is more that can be done here.

What are the near-term steps?

• A high priority is bringing in the Noah-MP parameterizations. Along those lines, SoilHydrology has been the top priority.
• Isotope development is driving some other priorities; first piece there will be CanopyHydrology.

Request for more visibility of the planned course. In part, these monthly meetings will address this.

There are some areas in which we've departed from the original plan:

• Migration to git/GitHub: this took a large amount of time, but has been hugely beneficial
• Work on performance, to make sure CTSM won't be substantially slower than Noah-MP

Model efficiency

(Deferred to the next meeting.)