Land Masks
Land Masks Documentation
Version
DREV64441

Mask Generation:

This page is up-to-date with respect to the version: DREV64441.

Additions from (j) 20160406 ...
The first step, masks from the ocean/ice models are interpolated to the atmosphere grid using a local area conservation scheme in the mediator. The result is a fractional land mask file, eg., field_med_atm_a_land_mask.nc. Grid cells with values equal to 1 can be considered "all land". Grid cells with values between 0 and 1 will eventually be part land, part ocean/ice. (NB: the NetCDF file, field_med_atm_a_land_mask.nc so far, does not have complete metadata beyond the IMAX, JMAX grid extent and may reverse the grid cell values, for which there is a manual switch - see below) Currently, the GSM atmosphere cannot handle cells that are part land, part ocean/ice and must create a binary mask representation using some cutoff parameter. Any cells with a value greater than the cutoff parameter will be land.
A second step involves running chgres and orography utilities in order to get appropriate input files for GSM. The inputs generated by chgres are siganl and sfcanl, which are initial conditions for the atmospheric model. The siganl file contains the sigma level spherical harmonic spectral coefficients in a binary file of the surface pressure, orography, and remaining model 3D dependent variables and tracers. The sfcanl file contains surface and land surface files in physical space (Gaussian grid) used by the physics component.
In the directory of the orog_maker files is an ieee file and a GrADS control file that reads and plots the ieee file. The second record of the ieee file is the sea-land mask made by the code.
The utility chgres uses the sea-land mask (slm) grib file output made by the orography maker code, if it is present, with the chgres SLMASK exported variable pointing to the Gaussian grib slm file which in turn was made from landmast.txt input to the orography code from the field_med_atm_a_land_mask.nc netcdf file. In the absence of the export variable SLMASK being set explicitly, the sea-land mask Gaussian file will attempt to be found by chgres in the model fix fields directory and thus be made from the USGS 30" elevations, and the UMD 30" ocean/lake/land mask file. The same can be expected for the orography file if the export variable OROGRAPHY is not set to a new orography file or the file is not present. The cutoff rule parameter may be changed with recompilation of the orography code.
The sea land mask cutoff parameter used to create the set of mask files received on 10/5/15 is believed to be 0, so any cells with value greater than 0 were considered land. This is what has been used to run DREV64441.
After generating the 10/5/15 files, the sea land mask cutoff parameter was set to 0.5, so any cells with a value greater than 0.5 in the fractional netcdf ocean file would be considered land. A new set of mask files was received on 11/17/15 corresponding to the 0.5 cutoff parameter.
Additional files generated by the orography utility and chgres are:

File Format:

The headers for the signanl/sfcanl file inputs are of the form:

sighdr siganl.gfs.2015040100
jcap
126
lonb
384
latb
190
idate
2015041500
stop
global_sighdr ending ...

and

sfchdr sfcanl.gfs.2015040100
lonb
384
latb
190
idate
2015041500

Mask Generation:

In DREV64441, the mediator sends the land mask field_med_atm_a_land_mask.nc to the GSM atmosphere. There is a parameter in GSM that sets a cutoff point for land. In the version of GSM associated with DREV64441, any point greater than 0.01 is considered a land point. This is approximately consistent with the 0 cutoff value that was used with the chgres and orography utilities. In GSM, the points that are associated with ocean and ice following the binary land mask and cutoff parameter receive and use ocean and ice exchange field data from those components.

Longer Term:

Consistency Improvement. It would be preferable if the atmosphere did not choose or determine mask values. The mediator would determine ocean/ice and land grid cells, and send appropriate data. Ideally the mask used by the mediator would be automatically consistent with the mask used for chgres and orography file generation.
Fractional Cells. Eventually the atmospheric model should be able to handle grid cells that are divided into ice/ocean/land portions.

History:

Date 2016 September 2
Author V.Kunkel
Note Updated the documentation structure. Added the history section.