metgrid.exe adds negative values due to interpolation

Dear Colleges,

I used the SPECCLDL and SPECCLDF variables as additional input from ECMWF HRES (~ 9km) data at model levels (137 model level). The data were successfully processed by ungrib.exe after providing relevant Vtable for the variables. However, after running the metgrid.exe the SPECCLDL and SPECCLDF fields contained very small negative values at some grid-points. I checked both ECMWF input data and the WPS intermediate files, everything was fine, no negative values.

The problem with the metgrid.exe was solved after changing the METGRID.TBL records. First, I changed the definition of missing values assigning to zero, but after that negative values still remained at some grid-points. Finally,I got rid of the negative values by changing the interpolation method (to more simple one). My modified METGRID.TBL records look as follow:

name=SPECCLDL
interp_option=four_pt+average_4pt
fill_missing=0.
missing_value=0.
fill_lev=200100:const(0.)
flag_in_output=FLAG_SPECCLDL
========================================
name=SPECCLDF
interp_option=four_pt+average_4pt
fill_missing=0.
missing_value=0.
fill_lev=200100:const(0.)
flag_in_output=FLAG_SPECCLDF


After these changes I was able to run the wrf.exe.
Do you have any better suggestions on this issue?
Why the default METGRID.TBL produces negative values?

Kind regards,
Artur

Arthur,
What are the two variables of SPECCLDL and SPECCLDF?

Hi,

The two variables of SPECCLDL and SPECCLDF are for Specific Cloud Liquid Water content and Specific Cloud Frozen Water content, respectively.

You can also find those two variables in Vtable.UKMO_ENDGame available in the WPS-4.1/ungrib/Variable_Tables directory.

Kind regards,
Artur

Hi Arthur,

Would you please upload one of your ECMWF datafile for me to take a look? Where did you get this data?
Please also tell me which Vtable you use to ungrib this data and which version of WPS you are running.

Thanks.

Please find attached the original ECMWF file in grib format, the file after running the ungrib.exe and converting to the netcdf format, and my Vtable, I am using the WPS-4.1.

Kind regards,
Artur

I downloaded the data from ECMWF MARS archive.

Arthur,
I am sorry that I have no answer to your question. I talked to our expert and we cannot figure out the reason why negative values appear. Please keep us updated if you know what is wrong. Thank.

Hi Ming,

Thanks for you update. In this case is it fine to use the "interp_option=four_pt+average_4pt" option? It seems this is a coarse interpolation.

Kind regards,
Artur

Arthur,

Personally I think it is fine to use this interpolation option. Please look at met_em files and check whether details are lost, --- I don't think so because the input data is already high-resolution.

Thanks!