map factors from WRF

[ChristinaHood]

I'm looking at EMEP's calculations of cell area for a global run, driven by WRF v4.6.

In GridValues_mod.f90 the map factors at the centre of each cell are interpolated from boundary values of MAPFAC_UY and MAPFAC_VX. Is there a reason for using this approach rather than reading the additional WRF variable(s) MAPFAC_MX and MAPFAC_MY which are already defined at cell centres?

[gitpeterwind]

The MAPFAC_UY and MAPFAC_VX (xm_i and xm_j in the emep model) are actually the important ones: they steer the advection of pollutants through the gridcell boundaries.
The other mapping factors "xm" in the code are used to determine the area of the gridcells and are less important. In addition the mapping factors are very smooth (except for near poles in lat lon projection), and the interpolation method will have negligible effects.
So in short, we could use directly the wrf values, but we do not expect that it will give any significant change in results.

Did you notice any problem related to this? We do not use wrf at our institute, so we completely rely on feedback like yours to improve the wrf interface!

[ChristinaHood]

Thanks for the helpful clarifications Peter. I agree that this difference of approach is unlikely to cause any significant change.

I was looking at values of GridArea_m2 written out from within the code and the Area_Grid_km2 output variable in the _fullrun.nc file, also the AREA2D variable from WRF (which is calculated using the internal equivalents of MAPFAC_MX), and trying to understand the differences. The WRF output variable had an associated bug, after fixing WRF and making use of the EMEP 'coord_in_processor' function to obtain relevant values of GridArea_m2, all three of these are now only very slightly different (0.01% - rounding error level?) so I think the confusion is resolved.