WRF Polar Modifications (Released V3.1)
A set of modifications to better represent processes and conditions over ice sheets was added to WRF and is first released in WRF V3.1. The modifications described here change treatments and code in the Noah LSM. A capability for the model to handle fractional sea-ice, applicable to polar regions and high latitudes, is described in separate documentation.
1) Snow thermal diffusivity usage
If the surface is largely snow covered (more than 97%), a value of thermal diffusivity for snow is used rather than the default of a thermal diffusivity value for soil.
2) Latent heat of sublimation/vaporization usage
In subroutine PENMAN (potential evaporation and transpiration) the mods provide for a scaling of the latent heat constant used between snow-covered regions (latent heat of sublimation) and snow-free regions (latent heat of vaporization). For glacial land or sea-ice regions, the latent heat of vaporization is used if the skin temperature (T1) is greater than freezing.
3) Limit on depth of snow layer in soil heat flux calculation
Over sea-ice points and glacial land points, the depth of the snow layer is limited in computing soil heat flux. The limited depth is determined relative to the depth of the top soil layer.
5) Runoff treatment
Runoff is updated over non-glacial land points. This affects the RUNOFF3 and RUNOFF2 setting in subroutine SFLX (surface fluxes). Over glacial land or sea-ice, the runoff goes directly to surface runoff RUNOFF1.
6) Glacial ice treatment and characteristics
a) The meaning of the ICE flag variable in SFLX has been changed. Now, the settings are ICE=1 for a sea-ice point, and ICE=-1 for a glacial land point, and ICE=0 for a non-glacial land point.
b) Glacial land points and sea-ice points have the soil moisture value set to 1.0.
c) At glacial land points and sea-ice points, snow density is set to 0.2 (g/cc) for cold permanent ice or new dry snow.
d) Snow-cover fraction is unlimited over glacial land points.
e) The albedo of sea-ice is set to 0.80. The emissivity of sea-ice is set to 0.98. Ultimately it may be better to have this as a namelist option, as the Arctic and the Antarctic behave differently, and each has its own seasonal traits.
f) Thermal conductivity over sea-ice or glacial land points is set to the snow conductivity value.
g) Subroutine HRTICE (heat transfer through ice) is modified to manage the subsurface temperature tendency for both sea-ice and glacial land points. Diffusivities and heat capacities are adjusted depending on whether it is a glacial point or a sea-ice point.
h) The call to subroutine SMFLX (soil moisture flux) is skipped for sea-ice points or glacial land points.
WRF Fractional Sea-Ice Capability (Released V3.1)
A set of modifications to better represent processes and conditions over the high latitudes and a capability to allow for fractional sea-ice coverage were added to WRF and are first released in WRF V3.1. This description covers the fractional sea-ice option. Other polar modifications, involving changes in the Noah LSM, as described in separate documentation.
Fractional Sea-Ice Option
The option to interpret the sea-ice array as a fractional field was added. The range is 0.0 to 1.0, meaning 0% to 100% coverage of a model grid cell by sea ice. Prior to this release, the sea-ice array was interpreted as a binary flag: 0.0 or 1.0, for open water or full sea-ice coverage, respectively.
If the user selects the fractional sea-ice option, surface layer routines which compute surface exchange coefficients and fluxes are called twice: once for open-water conditions and once for ice-cover conditions. The resulting values are then averaged between open-water and ice-cover results, weighted by the sea-ice fraction.
To use the fractional sea-ice option, a fractional sea-ice field is necessary for input to WRF. This field may come from, for example, NCEP GFS output, or other sources (e.g., various datasets available from the National Snow and Ice Data Center: http://nsidc.org/data/seaice/index.html)
Use of Option
1) The user must provide a fractional sea-ice field XICE for the input files for Metgrid (metgrid.exe) (i.e., the WPS intermediate-format files). This field may come from the following sources:
(i) the usual files created by Ungrib (if your fractional sea-ice data is included in the GRIB dataset which provides your meteorological input); (ii) additional submittals of Ungrib (if your fractional sea-ice data is in GRIB files other than those providing your meteorological input); or (iii) your own data conversion program for other sea-ice
datasets to which you have access. If you are using program Ungrib, make sure that the Vtable entry for the ice field is called "XICE" (indicating a fractional field), rather than the default "SEAICE" (indicating a 0/1 field).
2) Edit the METGRID.TBL to have an entry for XICE, including the option to rename the field to SEAICE on output (output_name=SEAICE).
3) For programs Real (real.exe) and WRF (wrf.exe), set the namelist option "fractional_seaice= 1" (which turns on the fractional ice treatment), and set the namelist option "seaice_threshold= 0.0) (which ensures that WRF will not add sea-ice points based on SST).
