WRF cell area for global run: AREA2D values uniform

[CMHood]

I am running WRF 4.6 in a global configuration, with WPS grid settings of
map_proj = 'lat-lon'
e_we = 361
e_sn = 181

This leads to DX and DY settings of 111177.5 for WRF.

I need to normalise some flux variables by cell area for use in a subsequent modelling process. The description attribute for the AREA2D variable in the wrfout files states that it represents "Horizontal grid cell area, using dx, dy, and map factors", with units attribute "m2". However, the values of this variable are spatially uniform, whereas I would expect the cell area to reduce towards the poles.

Please can anyone advise which (combination of) variable(s) should be used to obtain cell area from a global WRF run? Is there any known issue with AREA2D values for global configurations?

 

[Ming Chen]

Please look at the code phys/module_physics_init.F, and find the following piece of code:

#if 0
     do j = jts, min(jde-1,jte)
        do i = its, min(ide-1,ite)
           area2d(i,j) = dx/msftx(i,j) * dy/msfty(i,j)
           dx2d  (i,j) = sqrt ( area2d(i,j) )
        end do
     end do
#else
     do j = jts, min(jde-1,jte)
        do i = its, min(ide-1,ite)
           area2d(i,j) = dx * dy
           dx2d  (i,j) = dx
        end do
     end do
#endif

Change it to:

     do j = jts, min(jde-1,jte)
        do i = its, min(ide-1,ite)
           area2d(i,j) = dx/msftx(i,j) * dy/msfty(i,j)
           dx2d  (i,j) = sqrt ( area2d(i,j) )
        end do
     end do

Then recompile WRF. Please let me know whether this gives you reasonable results.

 

[CMHood]

Thanks for the response, I have tested this solution and it does lead to varying output Area2D values.
Please could you clarify whether:
- this change is also safe for non-global configurations with projected coordinates; and
- this change is relevant to versions of WRF other than 4.6?
Best wishes,
Christina

 

[Ming Chen]

Please see my answers below:

Thanks for the response, I have tested this solution and it does lead to varying output Area2D values.
Please could you clarify whether:
- this change is also safe for non-global configurations with projected coordinates; and

Yes I believe it works for both global and non-global options.

- this change is relevant to versions of WRF other than 4.6?

I am not sure of the older version. Please stay with V4.6

Best wishes,
Christina