4. The dynamic ice sheet model¶
This section gives a brief overview of ice flow modeling and of CISM, the dynamic ice sheet model in CESM. For more details, including a technical description of the model, please see Rutt et al. (2009) for the Glide shallow-ice dycore and Lipscomb et al. (2018) for the Glissade higher-order dycore. (The latter paper will be submitted to Geoscientific Model Development in June 2018, soon after the release of CESM2.0). See also the CISM documentation, which is slightly out of date but still provides a useful description of the Glissade dycore and the climate model interface (Glad).
4.1. The Community Ice Sheet Model¶
CISM is a thermomechanical ice sheet model that solves the equations of ice flow, given suitable approximations and boundary conditions. The source code is written primarily in Fortran 90 and 95. The model resides on the github repository (https://github.com/ESCOMP/cism), where it is under active development. CISM2.1 is currently the default ice sheet model in CESM2.0.
CISM2.1 introduces a new dynamical core, Glissade, which runs in parallel and solves equations for the conservation of mass, momentum, and internal energy. Glissade supports several approximations of the Stokes equations for ice flow: the shallow-ice approximation (SIA), the shallow-shelf approximation (SSA), a depth-integrated viscosity approximation (DIVA), and the Blatter-Pattyn (BP) approximation. DIVA is the default solver for CISM in the CESM framework.
In previous versions of CESM with CISM coupling capabilities, the dynamical core of the model was known as Glide and only solved the shallow-ice approximation in serial. While this option is still available, it is no longer supported.
The surface boundary conditions (e.g., the surface temperature and surface mass balance) are supplied by a climate driver. When CISM is run in CESM, the climate driver is Glad, which receives the temperature and SMB from the coupler and passes them to the ice-sheet grid. The lower boundary conditions are given by a geothermal dataset, which supplies heat fluxes at the lower boundary, and by a basal topography dataset. Optionally, the basal topography can be modified at runtime by an isostasy model; isostasy is turned off by default.
The new version of CISM adds a set of basal sliding and calving law options of varying complexity. The default options and parameters were chosen based on standalone Greenland simulations (with SMB forcing from the RACMO2 regional climate model) to support a stable, reasonably accurate simulation of the Greenland Ice Sheet; see Lipscomb et al. (2018) for details. The default basal sliding option is pseudo-plastic sliding, with parameters based on Aschwanden et al. (2016). The default calving law is “no-float”, with all floating ice calving immediately.
The model currently has simple treatments of basal hydrology, including a local till model used in conjunction with pseudo-plastic sliding. More complex schemes for subglacial water hydrology and evolution of basal till strength are being developed.
4.1.1. Configuring and running the model¶
Timesteps: There are several kinds of timesteps in CISM.
- The forcing timestep is the interval in hours between calls to Glad. Currently, the forcing timestep is the same as the coupling interval at which information is passed from the coupler to GLC. The forcing timestep is determined by the CISM namelist variables dt_option and dt_count. It is 24 hours by default for most compsets, but 1 year for T compsets. Note that these are the only values that have been tested extensively; results should be checked carefully if the forcing timestep is changed from these defaults.
- The mass balance timestep is the interval over which accumulation/ablation forcing data is summed and averaged. This timestep is set in subroutine glad_mbc_init in module glad_mbal_coupling.F90. The current default is one year. With the default settings of the forcing timestep and mass balance timestep, Glad will accumulate forcing data from the coupler over 365 daily forcing timesteps and average the data. The mass balance timestep must be an integer multiple of the forcing timestep.
- The ice sheet timestep is the interval in years between calls to the dynamic ice sheet model, Glissade (or Glide). The ice sheet timestep should divide evenly into the mass balance timestep. The current default is 0.1 year for 4-km, and 0.5 year for 20-km.
Two optional runtime parameters can be used to make the time-stepping more intricate:
- The mass balance accumulation time, mbal_accum_time (in years), is the period over which mass balance information is accumulated before calling Glissade. By default, the mass balance accumulation time is equal to either the ice sheet timestep or the mass balance timestep, whichever is larger. (For current defaults, this means that mbal_accum_time is set equal to the mass balance timestep: 1 year.) But suppose, for example, that the ice sheet timestep is 5 years. If we set mbal_accum_time = 1.0, we accumulate mass balance information for 1 year and use this mass balance to force the ice sheet model, thus avoiding 4 additional years of accumulating mass balance data. Note that this parameter cannot currently be modified via *user_nl_cism*, because it is not recommended that users change it.
- The timestep multiplier, ice_tstep_multiply, is equal to the number of ice sheet timesteps executed for each accumulated mass balance field. Suppose that the mass balance timestep is 1 year, the ice sheet timestep is 1 year, and ice_tstep_multiply = 10. Glad will accumulate and average mass balance information for 1 year, then execute 10 ice sheet model timesteps of 1 year each. In other words, the ice sheet dynamics is accelerated relative to the land and atmosphere. This option may be useful in CESM for multi-millennial ice-sheet simulations where it is impractical to run the atmosphere and ocean models for more than a few centuries.
These time options (apart from the forcing timestep) are set in cism.config. This file contains (or may contain) the following timestep information:
- The ice sheet timestep dt (in years) is set in the section [time] in the ice config file.
- The mass balance time step is not set directly in the config file, but is set to the number of hours in a year (i.e., 8760 hours for a 365-day year).
- The values of ice_tstep_multiply and mbal_accum_time, if present, are listed in the section [GLAD climate].
Note that the total length of the simulation is not determined by CISM, but is set in the file env_run.xml in the case directory.