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Sea Ice Forcing

A Sea Ice Forcing experiment follows much of the same details as an Sst Forcing experiment, except that Had AM 3 is forced by a sea ice concentration field.

A sea ice concentration field consists of values from 0.0 to 1.0 at all sea grid points for the 12 months of the year. The standard climatological sea ice concentration field is qrclim.ice which, on newton, lives in:

/ohome/lrum/newton/vn4.5/ancil/atmos/cl_9673

In a standard run, the qrclim.ice consists of three fields, sea ice concentration (or sea ice fraction), sea ice thickness, and fractional time. qrclim.ice assumes the sea ice thickness is 2m in the northern hemisphere and 1m in the southern hemisphere. Fractional time is used when the sea ice concentration at a gridpoint changes from non-zero to zero between two sucessive months. The fractional time is the fraction of the month when the sea ice concentration goes to zero. For example:

       |
  Sea  | * 
  Ice  |  \  
       |   \                                                          t'-t0
       |    \                                    fractional time =   -------
       |     \                                                        t1-t0
  Zero |______\_____________________
         +     t'           +
        Jan                Feb
         t0                 t1

However, neither the fractional time field or the sea ice thickness field will be used in the method that follows.

Creating a Sea Ice Concentration file

First, Create a Sea ice concentration file (in netCDF)- perhaps from observational data like HadISST - regrid this to the standard UM 96*73 grid (maybe using xconv.) You can use some of the methods described in Sst Forcing for this.

Creating an Sea Ice Concentration ancillary file

Next, convert this netCDF file to a UN ancil file. To do this, sftp your netCDF Sea Ice Concentration file to wren.cfs.ac.uk . Then copy these files to the same directory as your Sea Ice Concentration file:

/ohome/lrdlrh/SST_ICE/mkseaice/mkseaice.F90
/ohome/lrdlrh/SST_ICE/mkseaice/mkseaice_origin

and do:

chmod +x mkseaice_origin

(ensure you have the line module load netcdf-3.5.1 in your .profile .)

then do:

mkseaice_origin -compile -amip2 -ice seaiceconcentration.nc -iceancil seaiceconcentration.ice

this should convert your seaiceconcentration.nc file to the UM ancil file seaiceconcentration.ice.

Setting up the UM Job

You can specify the location of the Sea ice concentration file in the Um Ui under:

Atmosphere_Ancillary-and-input-data-files_Climatologies-and-potential-climatologies_sea-ice-fields

Also, select the Using AMIP-II method of updating SST and Sea Ice. This allows the model to make assumptions about the sea ice thickness and fractional time - hence we do not need to specify these in the sea ice ancillary file.