ismip7_calibration

The landice/ismip7_calibration test group calibrates MALI’s sub-shelf melt parameterization against the Ice Sheet Model Intercomparison for CMIP7 (ISMIP7) Antarctic ice-ocean protocol (Reese et al., Sect. 4.2), and produces the 5th, 50th and 95th percentiles of the melt parameter that the ISMIP7 MALI projections need.

The protocol fits the free parameter of the melt module against four terms:

term	what it constrains
J1	present-day melt integrated over each IMBIE2 drainage basin
J2	present-day melt integrated over bins of equal buttressing importance
J3	the warm-minus-cold basin-mean melt difference of ocean models
J4	observed melt of Pine Island and Dotson, per observation year

It then draws 100,000 random samples of the term weights and of the targets within their uncertainties, minimising the objective for each draw. The distribution of the minimisers gives the percentiles.

The test group includes two test cases.

replication reproduces the published 8 km calibration through compass’s own code path. It runs in about a minute on one core, needs no MALI run, and needs only the ISMIP7 datasets. Use it to check that everything is wired up before running the full calibration.
ais is the calibration itself on an Antarctic MALI mesh: it remaps the ISMIP7 masks and forcing, runs one single-timestep MALI melt diagnostic per ocean state, aggregates the melt, selects the parameter and fits the per-basin thermal-forcing correction dT_b. Its deliverable is three MALI parameter files per melt form, fit_delta_t/melt_params_<form>_p05.nc, _p50.nc and _p95.nc, each carrying the basins, the fitted dT_b and the calibrated parameter at that percentile. A projection points its parameter input stream at the one it wants.

Two melt forms can be calibrated, and by default both are:

ismip7: the Burgard et al. (2022) local quadratic recommended for ISMIP7, calibrating K
ismip6: the non-local quadratic MALI already had, calibrating gamma0. With a constant salinity this is algebraically the same as the Burgard semi-local form, so this is how that form is calibrated.

A note on salinity

The calibration runs MALI with a constant ocean salinity. MALI can read a 3-D salinity field, but the ISMIP7 ocean forcing already processed for the MALI projections carries thermal forcing only, so a projection has no salinity field to read. Calibrating against a spatially varying salinity would tune the melt parameter for physics the projections cannot run.

Melt is linear in salinity, and basin-mean salinity spans about 34.1 to 34.7 against the default constant of 34.5, so this shifts the calibrated parameter by roughly 1%. The published K was obtained with a locally varying salinity, so comparison with it is approximate at about that level.

Usage

The test group needs the ISMIP7 AIS datasets and a MALI mesh. Supply them in a user config file:

[ismip7_calibration]
base_path_ismip7 = /path/to/ISMIP7/data/AIS
base_path_mali = /path/to/inputdata/glc/mpasli/mpas.ais4to20km
mali_mesh_file = ais_4to20km.20250625.nc
mali_mesh_name = ais_4to20km
# optional; set to None if no ISMIP6-era region mask exists on this
# mesh, which skips the basin cross-check
region_mask_file = ais_4to20km_region_mask.20230105.nc

Then set up and run, for example:

compass setup -t landice/ismip7_calibration/replication \
    -f my.cfg -w $WORKDIR
cd $WORKDIR/landice/ismip7_calibration/replication
sbatch job_script.sh

Setting up needs nothing but the config file above; every part of the calibration is a step of the test case, so compass run from the test case directory does the whole thing in order, and the job script compass setup writes runs exactly that.

The ais test case creates one step per ocean state and melt form – 56 with the default settings – plus two more for the linearity check. Each is a short MALI run, but compass runs steps one after another, so the ensemble takes a few hours of wall clock. The runs do not depend on each other, so if that matters, run compass run from each step directory in separate jobs instead, and then run the verify_melt, aggregate, calibrate, fit_delta_t and report steps in that order.

Config options

The default config options are:

[ismip7_calibration]

# Which ocean states to use. Options:
#   minimal     - 7 states, those marked X in protocol Table 2
#   recommended - 11 states, adding the regional pairs marked +
#   all         - 28 states, everything ISMIP7 distributes
ocean_state_subset = all

# Which melt forms to calibrate, comma separated
melt_forms = ismip7, ismip6

# Number of MPI tasks for ESMF_RegridWeightGen
esmf_ntasks = 128

# Number of MPI tasks for each MALI run
ntasks = 128

# Length of the single timestep the melt diagnostic takes
timestep = 0000-00-01_00:00:00

[ismip7_calibration_melt]

# Practical salinity at the ice draft, PSU
salinity = 34.5

# The sin(theta) factor of the ISMIP7 quadratic
sin_slope = 0.0051117

# Magnitude of the Coriolis parameter, s^-1
coriolis = 1.4e-4

[ismip7_calibration_objective]

# Number of random draws of the term weights and the targets
sample_size = 100000

# Seed for the random draws, so the percentiles are reproducible
seed = 0

# Which summands carry non-zero weight: 'published' or 'all'
t3_models = all
t4_regions = all
t4_years = all

See compass/landice/tests/ismip7_calibration/ismip7_calibration.cfg for the full set with comments.

The t4_regions option is worth understanding. The published weighting uses Pine Island alone, which is 2 of the 18 available observations. So weighted, J4 constrains a single amplitude that either melt form can match by rescaling its parameter, so it cannot discriminate between them. Including Dotson makes J4 a relative constraint between two shelves, which no rescaling can satisfy if the ratio is wrong. The default is therefore all; set published to reproduce the published weighting.

Similarly, the reduced ocean-state subsets leave J4 structurally under-powered, which is why ocean_state_subset defaults to all.