isomip_plus

The ocean/isomip_plus test group includes variants of the Ice Sheet-Ocean Model Intercomparison Project, second phase (ISOMIP+) experiments from Asay-Davis et al. (2016). These experiments use idealized ice-shelf geometry from the Marine Ice SheetModel Intercomparison Project, third phase (MISMIP+; see Cornford et al. 2020) performed with the BISICLES ice-sheet model.

Currently, only the Ocean0 experiment is supported but the plan is to add the Ocean1 and Ocean2 experiments in the next few months, and the Ocean3 and Ocean4 experiments at a later date, once MPAS-Ocean supports moving grounding lines.

By default, the test case is available at 2 km and 5 km horizontal resolution with a z-star Vertical coordinate. The test case has 36 vertical layers, each of 20-m thickness outside of the ice-shelf cavity.

The initial temperature for the whole domain is constant (1 degree Celsius), while salinity varies linearly with depth from 34.5 PSU at the sea surface to 34.7 PSU at the sea floor, which is at a constant at 2000 m depth. The conceptual overlying ice shelf depresses the sea surface height buy as much as 1990 m (leaving a 10-m water column) for the first 30 km in y. Over the next 30 km, it rises to 1490 m, then fairly abruptly to zero over the next 15 km, where it remains for the second half of the domain. The ice shelf occupies these first 75 km of the domain: fluxes from ice-shelf melting are only applied in this region.

A cross section through the center (y = 40 km) of the ISOMIP+ Ocean0 test case at 5 km resolution, showing potential temperature averaged over month 9 of the simulation.

The isomip_plus test cases are composed of 3 steps that run by default: initial_state, which defines the mesh, interpolates the ice geometry, and computes the initial conditions for the model; ssh_adjustment, which modifies the landIcePressure field to balance the ssh field, see Sea surface height adjustment; and performance, which performs a 1-hour time integration of the model and compares the results with a baseline if one is provided.

Four additional steps can optionally be run: simulation, which performs one month of simulation, then updates the “evaporative” fluxes used in the test case to prevent sea level from rising significantly due to meltwater inflow at the ice-shelf base; streamfunction, which computes the barotropic (vertically integrated) and overturning streamfunctions; viz, which plots time series and movies of various variables of interest; and misomip, which interpolates the results to the MISOMIP comparison grid.

shared config options

The isomip_plus test cases share the following config options:

# Options related to the vertical grid
[vertical_grid]

# the type of vertical grid
grid_type = uniform

# Number of vertical levels
vert_levels = 36

# Depth of the bottom of the ocean
bottom_depth = 720.0

# The type of vertical coordinate (e.g. z-level, z-star)
coord_type = z-star

# Whether to use "partial" or "full", or "None" to not alter the topography
partial_cell_type = None

# The minimum fraction of a layer for partial cells
min_pc_fraction = 0.1

# Options relate to adjusting the sea-surface height or land-ice pressure
# below ice shelves to they are dynamically consistent with one another
[ssh_adjustment]

# the number of iterations of ssh adjustment to perform
iterations = 10


# config options for ISOMIP+ test cases
[isomip_plus]

# number of cells over which to smooth topography
topo_smoothing = 1.0

# minimum thickness of the ice shelf, below which it is removed ("calved")
min_ice_thickness = 100.0

# a scalar by which the ice draft will be scaled (squashed).  This is
# convenient for testing vertical coordinates
draft_scaling = 1.0

# Minimum number of vertical levels in a column
minimum_levels = 3

# Minimum thickness of the initial ocean column (to prevent 'drying')
min_column_thickness = 10.0

# Minimum fraction of a cell that contains ocean (as opposed to land or
# grounded land ice) in order for it to be an active ocean cell.
min_ocean_fraction = 0.5

# Threshold used to determine how far from the ice-shelf the sea-surface height
# can be adjusted to keep the Haney number under control
min_smoothed_draft_mask = 0.01

# Minimum fraction of a cell that contains land ice in order for it to be
# considered a land-ice cell by MPAS-Ocean (landIceMask == 1).
min_land_ice_fraction = 0.5

# the initial temperature at the sea surface
init_top_temp = -1.9
# the initial temperature at the sea floor
init_bot_temp = 1.0
# the initial salinity at the sea surface
init_top_sal = 33.8
# the initial salinity at the sea floor
init_bot_sal = 34.7

# the restoring temperature at the sea surface
restore_top_temp = -1.9
# the restoring temperature at the sea floor
restore_bot_temp = 1.0
# the restoring salinity at the sea surface
restore_top_sal = 33.8
# the restoring salinity at the sea floor
restore_bot_sal = 34.7

# restoring rate (1/days) at the open-ocean boundary
restore_rate = 10.0

# the "evaporation" rate  (m/yr) near the open-ocean boundary used to keep sea
# level from rising
restore_evap_rate = 200.0

# southern boundary of restoring region (m)
restore_xmin = 790e3
# northern boundary of restoring region (m)
restore_xmax = 800e3

# Coriolis parameter (1/s) for entire domain
coriolis_parameter = -1.409e-4

# initial value for the effective density (kg/m^3) of seawater for entire
# domain
effective_density = 1026.

# config options for ISOMIP+ time-varying land-ice forcing
[isomip_plus_forcing]

# the forcing dates
dates = 0001-01-01_00:00:00, 0002-01-01_00:00:00, 0003-01-01_00:00:00

# the amount by which the initial landIcePressure and landIceDraft are scaled
# at each date
scales = 0.1, 1.0, 1.0

# config options for computing ISOMIP+ streamfunctions
[isomip_plus_streamfunction]

# the resolution of the overturning streamfunction in m
osf_dx = 2e3
osf_dz = 5.

# config options for visualizing ISOMIP+ ouptut
[isomip_plus_viz]

# whether to plot the Haney number
plot_haney = True

# whether to plot the barotropic and overturning streamfunctions
plot_streamfunctions = True

# frames per second for movies
frames_per_second = 30

# movie format
movie_format = mp4

# the y value at which a cross-section is plotted (in m)
section_y = 40e3

You can modify the horizontal mesh, vertical grid, geometry, and initial temperature and salinity of the test case by altering these options.

Ocean0

ocean/isomip_plus/2km/z-star/Ocean0 and ocean/isomip_plus/5km/z-star/Ocean0

This test case is initialized with “warm” ocean conditions: 1 degree C at the sea floor, decreasing to -1.9 degrees C at the ocean surface. These conditions are approximately similar to those in the warmest waters on the Antarctic continental shelf in the Amundsen and Bellingshausen Seas. At the northern boundary, the temperature is restored to the same warm profile, leading to a vigorous circulation under the ice shelf that continually supplies heat and produces relatively high melt rates. Because of the rigorous flow, the simulation reaches a quasi-steady state in 2-3 years.

Ocean1

ocean/isomip_plus/2km/z-star/Ocean1 and ocean/isomip_plus/5km/z-star/Ocean1

This test case is initialized with “cold” ocean conditions: -1.9 degree C throughout the water column. These conditions are similar to cold-shelf regions such as the Antarctic continental shelf in the Weddell and Ross Seas. At the northern boundary, the temperature is restored to the same warm profile as in Ocean0. The initially cold cavity has low melt rates and a weak flow, so that warm water from the northern boundary may take about a decade to reach the ice-shelf base. At this point, the melting and flow rapidly increase, eventually (in the coarse of ~20 years) leading to the same quasi-steady-state as in Ocean0. The ISOMIP+ protocol suggests running this simulation for 20 years.

Ocean2

ocean/isomip_plus/2km/z-star/Ocean2 and ocean/isomip_plus/5km/z-star/Ocean2

This test case is initialized with “warm” ocean conditions as in Ocean0. At the northern boundary, the temperature is restored to the cold profile used for the initial condition in Ocean1: -1.9 degree C throughout the water column. Thus, where Ocean1 transitions from cold to warm cavity conditions, Ocean2 makes the opposite transition from warm to cold. The geometry is also taken from a different stage of the BISICLES MISIMP+ run than Ocean0 and Ocean1 in which the ice shelf has undergone significant thinning and retreat. The initially warm cavity has high melt rates and a strong flow, so that cold water water from the northern boundary will reach the ice-shelf base within a few years. At this point, the melting and flow exponentially decrease, approaching a new quasi-steady state. The ISOMIP+ protocol suggests running this simulation for 20 years, which is not long enough to reach quasi-steady state.

time_varying_Ocean0

ocean/isomip_plus/2km/z-star/time_varying_Ocean0 and ocean/isomip_plus/5km/z-star/time_varying_Ocean0

This test case is identical to Ocean0 except that the land-ice pressure and land-ice draft are prescribed to evolve in a very simple way in time. By default, the these 2 fields start out at year 0001 with 10% of their normal value (so the ice shelf is 10% of its thickness in a normal Ocean0 run). Then, over the course of a year, both fields increase to 100% of their normal value and stay there for another year. This test case is a simple way of exploring changing ice thickness without the need to support a changing grounding line (which remains fixed in time).

Users can modify the test case by adding or modifying entries in these config options before running the test case:

# config options for ISOMIP+ time-varying land-ice forcing
[isomip_plus_forcing]

# the forcing dates
dates = 0001-01-01_00:00:00, 0002-01-01_00:00:00, 0003-01-01_00:00:00

# the amount by which the initial landIcePressure and landIceDraft are scaled
# at each date
scales = 0.1, 1.0, 1.0

Dates do not have to be the beginnings of years, they could be any list that is monotonic in time. Scales can be any fraction between 0.0 and 1.0.

Performance run

By default, isomip_plus test cases are configured for “performance” runs. The initial condition is created, the the sea surface height and ice-shelf pressure are adjusted to be in balance. Then, a simulation is performed for only 1 simulated hour (appropriate for regression testing). Finally, potential temperature and salinity are plotted at the top and bottom of the ocean and along a cross section of through the middle (y = 40 km) of the domain.

Simulation run

isomip_plus test cases can be manually configured for longer simulation runs. First, do a performance run as described above (the default when you just do compass run in the test case work directory).

Then, edit the config file in the work directory (e.g. Ocean0.cfg) to set setup_to_run = simulation streamfunction viz in the [test_case] section at the very top. With this setting, one month of simulation will be performed, then the streamfunctions will be computed based on the latest results in the streamfunction step and time series plots and movies will be updated in the viz step. You can manually keep running compass run in the test case directory to run a month at a time, or you can create a job script to run compass run repeatedly (say 240 times for a 20-year simulation) inside a for-loop.