# 4. Running the standalone ice sheet model within CESM: T compsets¶

## 4.1. Background¶

The ice sheet model requires much less computational time per simulated year than other CESM components, but often needs to be run for many more model years (e.g., tens of thousands of years rather than a century). Thus, it can be desirable to run CISM in standalone mode, forced by output from a previous CESM run. In this way, you can cycle many times through the forcing data from a previous run (e.g., to spin up the ice sheet model), or run parameter sensitivity analyses much faster than you could within the coupled system.

A run with standalone CISM in the CESM context is known as a T compset. (In the past this was referred to as a TG compset. Now the G appears near the end of the compset name, so compsets have names like T1850Gg, where the final g indicates Greenland.) This compset uses the active ice sheet model for the specified ice sheet(s), forced by a data land model; all other components are stubs. Before running a T compset, you must have coupler history files from a previous run that included CLM (version 4.5 or later). You can run with either existing forcing data (see Section 4.2) or with your own forcing data (see Section 4.3).

## 4.2. Running with existing forcing data¶

There is currently just a single set of out-of-the-box forcing data available for running T compsets. These forcing data were created from the B1850 CMIP6 PI-control run, at resolution f09_g17.

There are three out-of-the-box T compsets that use these forcing data: T1850Gg (Greenland only), T1850Ga (Antarctica only), and T1850Gag (both Antarctica and Greenland). There is no scientific advantage to running both Antarctica and Greenland in the same T compset case, since the two ice sheets won’t interact in this situation (in contrast to a coupled simulation, where it can be scientifically useful to run multiple ice sheets in the same simulation). Thus, the main reasons for using T1850Gag are for convenience or testing. With any of these compsets, you should use f09_g17 resolution — i.e., with the same land resolution and ocean mask used to create the forcing data. You can use any CISM grid(s), although note that you must choose a resolution that specifies a grid for each active ice sheet (see Section 2.1.2).

So a typical create_newcase command when running with the standard 4-km Greenland grid, would look like:

./create_newcase --case my_t_case --compset T1850Gg --res f09_g17_gris4


## 4.3. Creating and running with your own forcing data¶

Currently, T compsets are only able to handle forcing data from a previous CESM run (although, in theory, it should be possible to “fake” CESM output by creating files with the same format as existing T compset forcing files). Thus, performing a T compset run with your own forcing data is a two-step process: (1) Perform a CESM run that includes an active land model (CLM), saving the necessary forcing files, and (2) perform a T compset run using these new forcing data.

### 4.3.1. Performing a run to create forcing data¶

To create the necessary forcing data (surface mass balance and surface temperature in each glacier elevation class, along with surface elevation for the coupler’s vertical downscaling), you need to perform a CESM run using a compset that includes an active land model (CLM). It does not matter whether the glc component is fully-active (e.g., CISM2%GRIS-EVOLVE), active but non-evolving (e.g., CISM2%GRIS-NOEVOLVE) or a stub model (SGLC). If running with CISM, CISM’s domain and resolution also do not matter (because forcing data are saved prior to downscaling to the CISM grid).

However, CLM’s GLACIER_REGION field (on the surface dataset) does matter: Surface mass balance and temperature forcing fields will only be valid in regions whose glacier region behaviors are:

• glacier_region_behavior = virtual: This is needed in order to provide downscaled forcings for all CISM grid cells.

• glacier_region_melt_behavior = replaced_by_ice: This is needed in order to compute SMB throughout the CISM domain.

Thus, you must ensure that there are one or more CLM glacier regions encompassing the full CISM grid(s) that you intend to use in the T compset run, whose glacier region behaviors are those given above.

In order to save the necessary forcing data, you need to set the driver namelist variable, histaux_l2x1yrg. (This variable name should be read as: coupler auxiliary history file (“histaux”) for fields sent from land to coupler (“l2x”) with a 1-year frequency (“1yr”), just focused on glacier-related fields (“g”).) This can be set by adding the following line to user_nl_cpl in your case directory:

histaux_l2x1yrg = .true.


This will cause the coupler to write out annual averages of the forcing fields sent from CLM to CISM (on the CLM grid, separated by elevation class). The files containing these averages will appear in the cpl/hist directory within your archive space, with names like: $CASE.cpl.hl2x1yr_glc.0001-01-01.nc. (For a typical 1-degree land resolution, these files are about 7 MB per year.) A T compset run that later uses these coupler history files as forcing should give nearly identical CISM results as the original run, as long as you ensure that SMB renormalization is done (or not done) in both cases, as described in Section 6. (For example, if the coupler history files were generated from a fully-coupled case with an evolving, two-way-coupled ice sheet, then in the T case you should set glc_renormalize_smb = 'on' in user_nl_cpl in order to reproduce the results of the fully-coupled case.) Small differences arise because these forcing files are written with single precision, leading to roundoff error on the order of 10-7. If you need more precision, you can add the following to user_nl_cpl: histaux_double_precision = .true.  This will give you double precision output, at the expense of roughly doubling the output volume. Short tests have shown that this change allows the T compset run to match the original within double-precision roundoff-level. ### 4.3.2. Performing a T compset run using your own forcing data¶ To perform a standalone CISM run forced by your newly-created forcing data, first create a new case using one of the existing T compsets (e.g., T1850Gg). The land and ocean resolutions of the T compset run (as specified by the --res flag to create_newcase) should match the resolution of the run used to create the forcing data. You can run with a different glc resolution than the one used to create the forcing data. So, for example, if you created the forcing data from an I or B compset with resolution f09_g17_gris4, the T compset run should use resolution f09_g17_xxx, where any value of xxx is acceptable. You can even run the T compset with a different ice sheet than the one(s) used to create the forcing data, as long as CLM’s glacier regions and their behaviors were set up appropriately, as described above. The following variables in env_run.xml should be modified appropriately for your forcing data: • DLND_CPLHIST_DIR: Directory in which your cpl.hl2x1yr_glc files can be found • DLND_CPLHIST_CASE: Name of the case used to create the cpl.hl2x1yr_glc files (files are assumed to be named $DLND_CPLHIST_CASE.cpl.hl2x1yr_glc.yyyy-01-01.nc)

• DLND_CPLHIST_YR_START: First year of forcing data

Note

DLND_CPLHIST_YR_START can be set later than the first existing year of data if you want to use a subset of the available years. However, the data land model expects to find domain information on the first year’s file; typically, this domain information (variable names beginning with doml) only appears on the first coupler history file. So, to start with something later than the first year, you will need to copy all of these doml_xxx fields from the first file onto the file corresponding to DLND_CPLHIST_YR_START. Make sure you copy ALL of the doml_xxx fields (otherwise you may get garbage results).

• DLND_CPLHIST_YR_END: Last year of forcing data (can be set earlier than the last existing year of data if you want to use a subset of the available years)

• RUN_STARTDATE: Determines the model year in which the run starts. This can be set to anything you want, but a good convention is:

• For transient T compset runs forced by output from a transient CESM run, set to the first year of forcing data (this corresponds to the real-world year, in some sense)

• For non-transient T compset runs (forced either by output from a non-transient run, or by cycling through the available forcing data multiple times), set to 0001-01-01 (in this case, there is no real-world meaning to the start year)

• DLND_CPLHIST_YR_ALIGN: The simulation year corresponding to DLND_CPLHIST_YR_START. This will usually be the same as the year in RUN_STARTDATE, but it can be set to a different year to start the simulation with a different year of forcing data.

To confirm you have set up the paths and file names correctly, run:

./preview_namelists


and examine the generated file, CaseDocs/dlnd.streams.txt.sno.cplhist.

## 4.4. Changes to some CESM defaults for T compsets¶

T compsets have much lower computational expense per simulation year and much greater typical run lengths compared to most CESM configurations. Thus, a number of settings are changed automatically when running with a T compset. These include:

• Default run length: 5 years (rather than 5 days)

• Default coupling frequency: annual (rather than daily or more frequent)