Design solution: Retreat of the ice shelf

Date last modified: 2015/09/22


Contributors: Elizabeth Hunke

The area of retreated land ice becomes open water. During the next time step, sea ice can occupy this open water using its normal dynamics (freezing and/or advection).

Jeremy Fyke (Unlicensed) - can you comment on what, if any, changes need to be made in terms of the coupler to allow this?

Design solution: Advance of the ice shelf

Date last modified: 2015/09/22


Contributors: Elizabeth Hunke

The most fundamental change needed for the sea ice model is the addition of a gridded variable, the land-ice area fraction.  This field is already defined and used for the iceberg model (bice in CICE); it essentially reduces the ocean grid cell area available for sea ice coverage.  It can be similarly defined and used for ice shelves, and it will make future incorporation of the iceberg model simpler.  All of the options below require this field to be passed through the coupler from the land ice model to the sea ice model;  grid remapping will be necessary.

CICE with icebergs currently uses sea ice ridging to make room for bergs when they are added (initialization and calving) and to allow for iceberg motion within the sea ice pack.  For ice shelf motion into sea-ice-covered areas, we only need to utilize the former capability.  This is handled as part of subroutine calve_bergs in the current iceberg code within CICE.

Coupling

The coupler will send to the sea ice model the fraction of an ocean grid cell covered by ice shelf, bice.  The fractions of sea ice area (aice), ice shelf area and open water area within a grid cell effectively satisfy aice + bice + open water area = 1 following ridging, and for bice=0, results should be BFB with the code prior to adding ice shelves.  Note that for compatibility in other code functions, the sea ice model maintains aice0=1-aice elsewhere in the code, i.e. aice0+aice covers 100% of the grid that is available to sea ice.  Instead of using aice0, a locally computed open water fraction is computed where needed for ice shelves.  The sea ice area fraction, aice, will be returned to the coupler as it is now.

Ice shelf area fraction

The ice shelf area fraction, bice, will be used for new ice formation, sea ice transport and ridging.  For new ice formation and sea ice transport, bice effectively decreases the total grid cell area available for sea ice to freeze or for transport.  Ridging serves to remove ice area from the fraction of the grid cell newly covered by the ice shelf, and it should be computed prior to other sea ice calculations.

New ice formation and sea ice ridging due to the imposition of land ice are both column functions that are (or can be) be part of the column package.  Changes for horizontal transport are minimal.

Ridging sea ice

Given the fraction of an ocean grid cell covered by ice shelf, bice, compute the change in that fraction since the last coupling step, dice = bice - bice_old.

If dice <= 0, do nothing.  Sea ice will be able to fill in the remaining area 1-bice through advection or new ice growth, if physical conditions are conducive.

If dice > 0, then call ridge_ice_by_shelves.  This subroutine is very similar to ridge_ice in ice_mechred.F90, but does not include the initial calculation of sea ice plus open water area (asum_ridging), and instead of calling ridge_prep it calculates the area opening and closing based on the change in land ice area in each grid cell. Then the thickness distribution of ridging ice (ridge_itd), redistribution (ridge_shift), area check (asum_ridging) and iteration test (ridge_check) are calculated using standard column package routines.  Conservation checks can also be done using standard routines (column_sum, column_conservation_check).  Once the ridging iteration is complete, coupling fluxes (fresh water, heat associated with snow thrown off during ridging) and diagnostics are computed.  

In CICE, bergs are blocked from entering cells that have no space available for them (so that a cell can not be 100% berg-filled), and a final check initiates an abort procedure if the resulting total area fraction is larger than 1.   In this case, an ice shelf may fill 100% of a grid cell, and when that happens, the excess sea ice area will be melted and added to the ocean through the fresh water, heat and salt fluxes.  If conditions are suitable that may then refreeze onto the bottom of the ice shelf elsewhere in the coupled model (i.e., not in the sea ice code).

After ridging, cleanup_itd must be called to rebin thickness categories as necessary and remove categories with very small areas.

New ice formation

When freezing new ice in the open water area, exclude the fraction of the grid cell occupied by ice shelf.

Horizontal transport

When transporting sea ice to or from a grid cell partially covered by ice shelf, the grid cell area is reduced by the ice shelf fraction.  

When transport arrays are filled (state_to_tracers in CICE), the open water area fraction is set to 0 if bice>0.99.  After making masks (used to prevent tracers in cells without ice from being used to computer tracer gradients), create a temporary land mask that also masks out ice shelves, to use in the transport routines.

Restarts

bice will need to be kept.