I downloaded the data for this Radar Assimilation tutorial. http://box.mmm.ucar.edu/wrf/users/wrfda/Tutorials/2012_July/class/radar.html
What I did:
1 - copy wrfinput_d01 as fg
2 - copy namelist.input, I changed iso_temp to 0 and cv_options to 3
3 - run da_wrfda.exe without ob.radar and ob.ascii in folder
4 - checked statistics and other files to see that no data was assimilated.
5 - subtract fg from wrfvar_output for variable T2
Step 5 gave me this image:
use this link if the image not load(https://drive.google.com/file/d/0B16l6oMrNK5eVWlTWEtSc0VfVW8/view?usp=sharing
I was expecting a zero field, this looks strange to me and I wonder why it is changing the first guess even though there is no data to be assimilated.