With the single observation test, you may get some ideas of how the background and observation error statistics work in the model variable space. A single observation test is done in WRFDA by setting num_pseudo=1, along with other pre-specified values in record &wrfvar15 and &wrfvar19 of namelist.input.
With the settings shown below, WRFDA generates a single observation with a pre-specified innovation (Observation – First Guess) value at the desired location; e.g. at (in terms of grid coordinate) 23x23, level 14 for “U” observation with error characteristics 1 m/s, and innovation size = 1.0 m/s.