" Users should recognize that the PX LSM was primarily developed for retrospective simulation, where surface-based observations are available to inform the indirect soil nudging. While soil nudging can be disabled using the FDDA namelist.input setting “pxlsm_soil_nudge,” little testing has been done in this mode, although some users have reported reasonable results. Gilliam and Pleim (2010) discuss the implementation in the WRF model and provide typical configurations for retrospective applications. If soil nudging is activated, modelers must use the Obsgrid objective re-analysis utility to produce a surface nudging file with the naming convention “wrfsfdda_d0*.” Obsgrid takes WPS “met_em*” files and LittleR observation files and produces the “wrfsfdda_d0*” file. The PX LSM uses 2-m temperature and mixing ratio re-analyses from this file for the deep soil moisture and temperature nudging." from http://www2.mmm.ucar.edu/wrf/users/docs/user_guide_V3.8/users_guide_chap5.htm#Phys
I’m new to WRF, wanting to run analysis nudging for soil conditions. What are LittleR observation files? We are working with grib2 files from NOAA if that helps.