CESM2 Configurations (CESM2.2)

The CESM2 system can be configured a number of different ways from both a science and technical perspective. CESM2 supports numerous resolutions, and component sets. In addition, each model component has input options to configure specific model settings and parameterizations.

CESM2 Components

CESM2 consists of seven geophysical model components:

• atmosphere (atm)

• sea-ice (ice)

• land (lnd)

• river (rof)

• ocean (ocn),

• land-ice (glc)

• ocean-wave (wav)

and an external system processing component

• external system processing (esp)

In addition CESM2 is accompanied by a driver/coupler (cpl7) that coordinates the time evolution of geophysical components and periodically permits the components to exchange data. Each component is represented in one of several modes: “active,” “data,” “dead,” or “stub” that permits the whole system to activate and deactive component feedbacks by allowing for a variety of “plug and play” combinations.

During the course of a CESM2 run, the model components integrate forward in time, periodically exchanging information with the coupler. The coupler meanwhile receives fields from the component models, computes, maps, and merges this information, then sends the fields back to the component models. The coupler brokers this sequence of communication interchanges and manages the overall time progression of the coupled system. A CESM2 component set is comprised of eight components: one component from each model (atm, lnd, rof, ocn, ice, glc, wav, and esp) plus the coupler. Model components are written primarily in Fortran.

The active (dynamical) components are generally fully prognostic, and they are state-of-the-art climate prediction and analysis tools. Because the active models are relatively expensive to run, data models that cycle input data are included for testing, spin-up, and model parameterization development. The dead components generate scientifically invalid data and exist only to support technical system testing. The dead components must all be run together and should never be combined with any active or data versions of models. Stub components exist only to satisfy interface requirements when the component is not needed for the model configuration (e.g., the active land component forced with atmospheric data does not need ice, ocn, or glc components, so ice, ocn, and glc stubs are used).

The CESM2 components can be summarized as follows:

“CESM2 model components”

Component Generic Type	Component Generic Name	Component Name	Component Type	Description
atmosphere	atm	cam	active	The Community Atmosphere Model (CAM) is a global atmospheric general circulation model developed from the NCAR CCM3.
atmosphere	atm	datm	data	The data atmosphere component is a pure data component that reads in atmospheric forcing data
atmosphere	atm	xatm	dead	Used only for testing the driver/coupler
atmosphere	atm	satm	stub	Used only to satisy the interface requirements
land	lnd	clm	active	The Community Land Model (CLM) is the result of a collaborative project between scientists in the Terrestrial Sciences Section of the Climate and Global Dynamics Division (CGD) at NCAR and the CESM Land Model Working Group. Other principal working groups that also contribute to the CLM are Biogeochemistry, Paleoclimate, and Climate Change and Assessment.
land	lnd	dlnd	data	The data land component is a purely data-land component (reading in coupler history data for atm/land fluxes and land albedos produced by a previous run, or snow surface mass balance fields) or both.
land	lnd	xlnd	dead	Used only for testing the driver/coupler
land	lnd	slnd	stub	Used only to satisy the interface requirements
river	rof	rtm	active	The river transport model (RTM) was previously part of CLM and was developed to route total runoff from the land surface model to either the active ocean or marginal seas which enables the hydrologic cycle to be closed (Branstetter 2001, Branstetter and Famiglietti 1999). This is needed to model ocean convection and circulation, which is affected by freshwater input.
river	rof	mosart	active	MOdel for Scale Adaptive River Transport (MOSART) , a new large-scale river routing model. MOSART improves the magnitude and timing of river flow simulations.
river	rof	drof	data	The data runoff model was previously part of the data land model and functions as a purely data-runoff model (reading in runoff data).
river	rof	xrof	dead	Used only for testing the driver/coupler
river	rof	srof	stub	Used only to satisy the interface requirements
ocean	ocn	pop	active	The ocean model is an extension of the Parallel Ocean Program (POP) Version 2 from Los Alamos National Laboratory (LANL).
ocean	ocn	mom6	active	Based on the Modular Ocean Model version 6; an early functional release is available starting in CESM2.2. Note that MOM6 is not obtained by default; for instructions on obtaining it, see https://github.com/ESCOMP/MOM_interface/wiki/Detailed-Instructions.
ocean	ocn	docn	data	The data ocean component has two distinct modes of operation. It can run as a pure data model, reading ocean SSTs (normally climatological) from input datasets, interpolating in space and time, and then passing these to the coupler. Alternatively, docn can compute updated SSTs based on a slab ocean model where bottom ocean heat flux convergence and boundary layer depths are read in and used with the atmosphere/ocean and ice/ocean fluxes obtained from the coupler.
ocean	ocn	xocn	dead
ocean	ocn	socn	stub
sea-ice	ice	cice	active	The sea-ice component (CICE) is an extension of the Los Alamos National Laboratory (LANL) sea-ice model and was developed though collaboration within the CESM Polar Climate Working Group (PCWG). In CESM, CICE can run as a fully prognostic component or in prescribed mode where ice coverage (normally climatological) is read in.
sea-ice	ice	dice	data	The data ice component is a partially prognostic model. The model reads in ice coverage and receives atmospheric forcing from the coupler, and then it calculates the ice/atmosphere and ice/ocean fluxes. The data ice component acts very similarly to CICE running in prescribed mode.
sea-ice	ice	xice	dead	Used only for testing the driver/coupler
sea-ice	ice	sice	stub
land-ice	glc	cism	active	The CISM component is an extension of the Glimmer ice sheet model.
land-ice	glc	sglc	stub	Used only to satisy the interface requirements
ocean-wave	wav	wav	ww3	The ww3 component adds prognostic ocean waves to the system
ocean-wave	wav	xwav	dead	Used only for testing the driver/coupler
ocean-wave	wav	swav	stub	Used only to satisy the interface requirements
coupler	cpl	cpl	active	The CESM coupler was built primarily through a collaboration of the NCAR CESM Software Engineering Group and the Argonne National Laboratory (ANL). The MCT coupling library provides much of the infrastructure.

CESM2 Component Sets

The CESM2 components can be combined in numerous ways to carry out various scientific or software experiments. A particular mix of components, along with component-specific configuration and/or namelist settings is called a component set or compset. CESM has a shorthand naming convention (known as an alias) for component sets that are supported out-of-the-box. The compset alias usually has a well-defined first letter followed by some characters that are indicative of the configuration setup.

The first letter in a compset alias generally indicates which of the components are fully active (prognostic), which are data components, and which are completely absent (or stub). For the most part, this first letter refers only to the atmosphere (atm), land (lnd), sea ice (ice) and ocean (ocn) components. The type of component used for river (rof), land ice (glc) and ocean wave (wav) is either specified in some other way in the alias or is not specified explicitly. For example, an evolving land ice (glc) model is denoted by a capital G near the end of the compset alias (e.g., B1850G is similar to B1850 but with an evolving Greenland ice sheet). In some cases, the distinction between prognostic and data components is not clear-cut – for example, when using a data ocean model in slab ocean model (SOM) mode, or when using a prognostic sea ice model (CICE) in prescribed mode.

The following table summarizes these first-letter designations in compset aliases:

Designation	Active Components	Data Components	Notes
A	–	various	All data components; used for software testing
B	atm, lnd, ice, ocn	–	Fully active components
C	ocn	atm, ice, rof
D	ice	atm, ocn, rof	Slab ocean model (SOM)
E	atm, lnd, ice	ocn	Slab ocean model (SOM)
F	atm, lnd	ice, ocn	Sea ice in prescribed mode; some F compsets use fewer surface components
G	ice, ocn	atm, rof
I	lnd	atm
J	lnd, ice, ocn	atm	Can be used to spin up the surface components
P	atm	–	CAM PORT compsets
Q	atm	ocn	Aquaplanet compsets
S	–	–	No components present; used for software testing
T	glc	lnd
X	–	–	Coupler-test components; used for software testing

See supported component sets for a complete list of supported compset options. Running query_config with the --compsets option will also provide a listing of the supported out-of-the-box component sets for the local version of CESM2.

CESM2 Grids

The supported grid resolutions are specified in CESM2 by setting an overall model resolution. Once the overall model resolution is set, components will read in appropriate grid files and the coupler will read in appropriate mapping weights files. Coupler mapping weights are always generated externally in CESM2. The components will send the grid data to the coupler at initialization, and the coupler will check that the component grids are consistent with each other and with the mapping weights files.

In CESM2, the ocean and ice must be on the same grid, but the atmosphere, land, river runoff and land ice can each be on different grids. Each component determines its own unique grid decomposition based upon the total number of pes or processing elements assigned to that component.

CESM2 supports several types of grids out-of-the-box including single point, finite volume, cubed sphere, displaced pole, and tripole. These grids are used internally by the models. Input datasets are usually on the same grid but in some cases, they can be interpolated from regular lon/lat grids in the data models. The finite volume is generally associated with atmosphere and land models but the data ocean and data ice models are also supported on that grid. The cubed sphere grid is used only by the active atmosphere model, cam. The displaced pole and tripole grids are used by the ocean and ice models. Not every grid can be run by every component. The ocean and ice models run on either a Greenland dipole or a tripole grid. The Greenland Pole grid is a latitude/longitude grid, with the North Pole displaced over Greenland to avoid singularity problems in the ocn and ice models. The low-resolution Greenland pole mesh from CCSM3 is illustrated in Yeager et al., “The Low-Resolution CCSM3”, AMS (2006), Figure 1b., Web. Similarly, the Poseidon tripole grid is a latitude/longitude grid with three poles that are all centered over land.

CESM2 Machines

Scripts for supported machines and userdefined machines are provided with the CESM2 release. Supported machines have machine specific files and settings added to the CESM2 scripts and are machines that should run CESM2 cases out-of-the-box. Machines are supported in CESM2 on an individual basis and are usually listed by their common site-specific name. To get a machine ported and functionally supported in CESM2, local batch, run, environment, and compiler information must be configured in the CESM2 scripts. The machine name “userdefined” machines refer to any machine that the user defines and requires that a user edit the resulting xml files to fill in information required for the target platform. This functionality is handy in accelerating the porting process and quickly getting a case running on a new platform. For more information on porting, see the CIME porting guide. The list of available machines are documented in CESM2 supported machines. Running query_config with the --machines option will also show the list of all machines for the current local version of CESM. Supported machines have undergone the full CESM2 porting process. The machines available in each of these categories changes as access to machines change over time.

CESM2 Validation

Although CESM2 can be run out-of-the-box for a variety of resolutions, component combinations, and machines, MOST combinations of component sets, resolutions, and machines have not undergone rigorous scientific climate validation. Control runs accompany scientifically supported component sets and resolutions and are documented on the release page. These control runs should be scientifically reproducible on the original platform or other platforms. Bit-for-bit reproducibility cannot be guaranteed due to variations in compiler or system versions. Users should carry out their own port validations on any platform prior to doing scientific runs or scientific analysis and documentation.