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Help Understanding Observation Nudging

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Hello All,

I am running WRF 3.9 for a single domain at 3 km resolution, the initial and boundary conditions are being provided by the HRRR, and have observation nudging enabled. I am using standard METAR and Upper Air observations for nudging, but I also have some aircraft data that I am using as well. I do not appear to be having any issues with the process from formatting the observations in little-r, or in the OBSGRID process, and WRF completes just fine without any errors. What I do want to know, and don't have a great understanding of, is how the observations are handled.

So if we have use_p_tolerance_one_level set to True, the tolerance set to 7 mb, and I have a single above ground observation that does not match up exactly to an initial model pressure surface but is within the tolerance level (i.e. observation is at 847 mb instead of right at 850 mb), is there any modification made to any elements of that observation? I have been trying to examine the observation nudging sections in the rsl.out/rsl.error file and I guess what I am looking for and can't find is, the observation was read in from OBS_DOMAIN101 as this, and after observation nudging the observation was used by WRF looked like this, and I can't seem to find what I am looking for. And maybe with the use_p_tolerance_one_level set to true there is no change to the observation, but I just can't seem to find any documentation that directly states that.

Any help to better understand this process would be much appreciated!

Thanks and happy holidays,

My understanding is that, OBSGRID will conduct quality control of the ob data at 847 mb as if it were at 850 mb and so it will compare the observation against the 2D 850 mb level field for the background check. However, OBSGRID will not change the pressure of the ob to 850 mb, and if this ob is nudged, it is still taken as a 847 mb observation.
I am wondering if you wouldn't mind sharing a little more information about your simulation setup. I am trying to run a nudged simulation with upper air data and am having no luck with WRF applying the upper air data for nudging. If I only include the first point from the sounding, WRF will apply nudging but if I add the rest of the upper air data, no nudging is applied. I've looked at all of the thresholds and other fdda namelist settings and can't get the model to do anything with the upper air data. Would you be willing to share your namelist.input, rsl.out, and OBS_DOMAIN files?

Any help would be greatly appreciated.