ice shelf 2d

description

The ice_shelf_2d tasks describe a series of very simplified ice-shelf test cases where topography and initial conditions only vary in the y direction. The Coriolis parameter f is zero. This makes the test case quasi-two- dimensional, with negligible variability in x.

Polaris includes three test cases, a default case, a restart case, and the default case with visualization. Each case includes an init step to set up the mesh and initial condition, a series of ssh_adjustment steps (see SSH adjustment steps), and a forward step. Some cases include additional steps, described below.

All test cases include a relatively strenuous, iterative process to dynamically adjust landIcePressure or ssh to be compatible with one another in the ssh_adjustment steps. In this test case, we perform 10 iterations of adjustment, enough that changes in pressure should be quite small compared to those in the first iteration. Reducing the number of iterations will make the test case run more quickly at the risk of having longer-lived transients at the beginning of the simulation.

# Options related to ssh adjustment steps
[ssh_adjustment]

# Number of ssh adjustment iterations
iterations = 10

# Output interval for the ssh adjustment phase in hours
output_interval = 1.0

# Run duration of each ssh adjustment phase in hours
run_duration = 1.0

# Whether to adjust land ice pressure or SSH
adjust_variable = land_ice_pressure

# Time integration scheme
time_integrator = split_explicit

# Time step in seconds as a function of resolution
rk4_dt_per_km = 10

# Time step in seconds as a function of resolution
split_dt_per_km = 10

# Time step in seconds as a function of resolution
btr_dt_per_km = 2.5

If a baseline run of the test case was provided for comparison, we perform validation of both the prognostic variables (layer thickness, velocity, temperature and salinity) and a large number of variables associated with freshwater and heat fluxes under ice shelves.

Frazil-ice formation is not included in the ssh_adjustment steps but is included in the forward step of this test case.

mesh

The test case currently supports only 5-km horizontal resolution. The x direction is periodic and only 10 cells wide, whereas the y direction has solid boundaries and is 44 cells long. These dimensions are set in the config file by lx and ly.

vertical grid

The conceptual overlying ice shelf is described by a piecewise linear function. The config options y1, y2, and y3 dictate the inflection points in the piecewise function and y1_water_column_thickness and y2_water_column_thickness dictate the water column thickness at those locations. The water column thickness at y3 is always equal to the bottom depth, indicating the ice shelf front location. By default, the ice shelf depresses the sea surface height by as much as 1040 m (leaving a 10-m water column) for the first 30 km in y. Over the next 30 km, it rises to 200 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.

../../../_images/ice_shelf_2d.png

The geometry does not represent a particularly realistic ice-shelf cavity but it is a quick and useful test of the parameterization of land-ice melt fluxes and of frazil formation below ice shelves.

Two vertical coordinates, z-star and z-level, are available. In each case, there are 50 vertical levels given by the config option vert_levels. In the open ocean, each level is 40 m thick.

# Options related to the vertical grid
[vertical_grid]

# the type of vertical grid
grid_type = uniform

# Number of vertical levels
vert_levels = 50

# The minimum number of vertical levels
min_vert_levels = 3

# Depth of the bottom of the ocean
bottom_depth = 2000.0

# 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 = partial

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

# The minimum layer thickness in m
min_layer_thickness = 0.0

initial conditions

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. These initial conditions can be modified with config options temperature, surface_salinity, and bottom_salinity

forcing

N/A

time step and run duration

The time step is determined by the config options dt_per_km so that the time step is proportional to the resolution. By default, a 10 km-resolution test has a time step of 5 min. Run duration will be specified for each test case. Run duration will be discussed for individual test cases.

config options

# config options for 2D ice-shelf testcases
[ice_shelf_2d]

# width of domain in km
lx = 50

# length of domain in km
ly = 190

# How the land ice pressure at y<y1 is determined
y0_land_ice_height_above_floatation = 0.

# Temperature of the surface in the northern half of the domain
temperature = 1.0

# Salinity of the water in the entire domain
surface_salinity = 34.5

# Salinity of the water in the entire domain
bottom_salinity = 34.7

# Coriolis parameter
coriolis_parameter = 0.

# GL location in y in km
y1 = 30.0

# ice shelf inflection point in y in km
y2 = 90.0

# ice shelf front location in y in km
y3 = 90.0

# Vertical thickness of ocean sub-ice cavity at GL
y1_water_column_thickness = 10.0

# Vertical thickness of water column thickness at y2
y2_water_column_thickness = 1050.0

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

default

description

ocean/planar/ice_shelf_2d/${RES}/default is the default version of the ice shelf 2-d test case for a short (10 min) test run and validation of prognostic variables for regression testing.

mesh

See ice shelf 2d.

vertical grid

See ice shelf 2d.

initial conditions

See ice shelf 2d.

forcing

See ice shelf 2d.

time step and run duration

The time step is configured by ice_shelf_2d_default:rk4_dt_per_km or ice_shelf_2d_default:split_dt_per_km and ice_shelf_2d_default:btr_dt_per_km depending on the time integration scheme given by ice_shelf_2d:time_integrator. The run duration is determined by the config option forward_run_duration and is 10 minutes by default.

config options

See ice shelf 2d for config options used by all ice_shelf_2d test cases.

# Options specific to the ice_shelf_2d/default case
[ice_shelf_2d_default]

# Time integration scheme
time_integrator = split_explicit

# Run duration of the forward step in minutes
forward_run_duration = 10.0

# Time step in seconds as a function of resolution
rk4_dt_per_km = 60

# Time step in seconds as a function of resolution
split_dt_per_km = 60

# Time step in seconds as a function of resolution
btr_dt_per_km = 10

cores

The number of processors is hard-coded to be 4 for this case.

default_with_restart

description

ocean/planar/ice_shelf_2d/$RES/default/with_restart runs a short (10 min) integration of the model forward in time (forward step), saving a restart file every time step. Then, a second run (restart step) is performed from the restart file 5 minutes into the simulation. Prognostic variables, variables related to sub-ice-shelf fluxes, and variables related to frazil formation are compared between the “full” and “restart” runs at minute 10 of the simulation to make sure they are bit-for-bit identical.

mesh

See ice shelf 2d.

vertical grid

See ice shelf 2d.

initial conditions

See ice shelf 2d.

forcing

See ice shelf 2d.

time step and run duration

The time step is the same as default. The full run is ten minutes as given by ice_shelf_2d_default:forward_run_duration and the restart run is half the duration of the full run.

config options

See ice shelf 2d and .

cores

See default.

default_tidal_forcing

description

ocean/planar/ice_shelf_2d/5km/default is the default version of the ice shelf 2-d test case for a short (10 min) test run and validation of prognostic variables for regression testing.

mesh

See ice shelf 2d.

vertical grid

See ice shelf 2d.

initial conditions

See ice shelf 2d.

forcing

This test has tidal forcing at the open ocean boundary in the y-dimension. The amplitude and period of the tidal forcing are set at test case set-up to be 1 m and 10 days, respectively, but may be changed by the user in the namelist at runtime.

time step and run duration

Determined by config options in the ice_shelf_2d_default_tidal_forcing section described below.

config options

See ice shelf 2d for config options used by all ice_shelf_2d test cases.

# Options specific to the ice_shelf_2d/default_tidal_forcing case
[ice_shelf_2d_default_tidal_forcing]

_tidal_forcing# Time integration scheme
time_integrator = RK4

# Run duration of the forward step in days
forward_run_duration = 0.01

# Time step in seconds as a function of resolution
rk4_dt_per_km = 5

# Time step in seconds as a function of resolution
split_dt_per_km = 5

# Time step in seconds as a function of resolution
btr_dt_per_km = 1

cores

See default.