Relative humidity in met_em files not equal to the input data (modified ERA5)

[koszutam]

I am trying to modify ERA5 data to test different assumptions to amplify precipitation using the WRF model. I have done some work to modify the input data I am using to initialize WRF (ERA5). In the extreme case, I tried modifying the relative humidity ERA5 data to be 100% in all vertical layers over some footprint. The modified input data looks fine for all vertical layers but the resulting met_em files appear to impose some limit on RH when the pressure level is less than 700 hPa (e.g., appears limited to ~80% at 300 hPa).

Is there some check in the metgrid processes that adjusts the data to be consistent with other variables (e.g., temperature or geopotential)? If so, where is this done in the code? Would there be a similar issue if I modify temperature by adding some perturbation temperature to all levels?

Thanks for any insights in advance!

 

[Ming Chen]

Did you run with ERA5 pressure level data or model level data?
Can you post RH plots from an intermeidate file and from a met_em file that demonstrate the issue you raised? Thanks.

 

[koszutam]

I am running it with ERA5 pressure level data from RDA using GRIB files downloaded early last year/late 2023.

So, this is a plot of the 250 hPa pressure level RH at Dec. 22, 1964 00:00UTC. Trying to better understand this, I grabbed the original ERA5 and the resulting met_em file, which also shows this reduction in RH too. So my question seems to apply more generally to if and how metgrid.exe modifies the input data (RH).

From the top left, clockwise: ERA5 data downloaded from RDA (E5), modified ERA5 where RH along the AR is set to 100% (E5-mod), RH from the met_em file derived from the modified ERA5 data (met_em-mod), and met_em file from the original ERA5 data (met_em).

 

[Ming Chen]

I don't think metgrid does anything to adjust RH. However, pressure level RH is recalculated in rrpr.F, in which you can find the codes below. I believe this is the reason why you saw difference between original ERA5 and met_em.

! Repair GFS and ECMWF pressure-level RH


        if (index(map%source,'NCEP GFS') .ne. 0 .or.  &


            index(map%source,'NCEP GEFS') .ne. 0 .or. &


            index(map%source,'NCEP CDAS CFSV2') .ne. 0 .or.  &


            index(map%source,'ECMWF') .ne. 0 ) then


          call mprintf(.true.,DEBUG, &


             "RRPR:   Adjusting RH values ")


          do k = 1, nlvl


            if ( is_there(nint(plvl(k)),'RH') .and. &


                 is_there(nint(plvl(k)),'TT') ) then


              call fix_gfs_rh (map%nx, map%ny, plvl(k))


            endif


          enddo


        endif