The types of observations that WRFDA gets to use actually depend on what is included in the observation file and the WRFDA namelist settings. For example, if you have SYNOP data in the observation file, you can suppress its usage in WRFDA by setting use_synopobs=false in record &wrfvar4 of namelist.input. It is OK if there are no SYNOP data in the observation file and use_synopobs=true.