PXLSM Soil Nudging


I am trying to get the PX soil nudging working for WRF according to this document

It seems like there is a lot of out of date practices on it since it was set up for WRF3.7.1 is there a better document of how to setup and run this. Is this still even a practice in WRF4

Nothing is out of date. We use the same practices. For someone that is not familiar with WRF, PX or especially the long-used LittleR observation format it can be very confusing though because of the number of steps. Much like any data assimilation methodology. Be advised, these are for retrospective simulations. If you are running a forecast and think this will just help with the soil properties, it is not applicable.

The document above provides two links. One to Obsgrid (http://www2.mmm.ucar.edu/wrf/src/OBSGRID.tar.gz) and one to get a utility that converts MADIS obs files to LittleR format.

I would simplify things to start and not even worry about the LittleR files. You can run Obsgrid to get a wrfsfdda_d01 file for example that is used by the PX LSM for the soil nudging.

So to be more explicit,

  1. Run WPS geogrid.exe - ungrib.exe - metgrid.exe.

This results in met_em* files for each analysis time (e.g., met_em.d01.2008-02-29_12:00:00.nc)

  1. Compile Obsgrid via link above. Link all of these met_em* files into an Obsgrid run directory. Copy the namelist.oa file from the Obsgrid code and modify to reflect your time period of met_em* files.

  2. Create empty LittleR obs files with name convention like empty:2016-04-19_15 where the 2016-04-19_15 reflect the date and time of day (UTC). Again, this step will just create the soil nudging file that is based on your NOAA Grib file analysis dataset. This will allow the PX LSM soil nudging to work well. You will just have the ability to refine these soil nudging analyses if you provide the full LittleR files.

Once you run Obsgrid and then real.exe for WRF inputs you will have input files wrfbdy_d01 (from real.exe), wrfinput_d01 (from real.exe), wrffdda_d01 (from real.exe), wrflowinp_d01 (from real.exe) and wrfsfdda_d01 (from Obsgrid).

Then you can run WRF. If you want to email me directly I can help refine your namelist.input file. That will be easier if you supply that namelist from your real.exe run.

But these are some of the settings we use for 3D grid nudging (FDDA) and the indirect PX soil nudging.

grid_fdda = 1,
grid_sfdda = 1,
pxlsm_soil_nudge = 1,
sgfdda_inname = “wrfsfdda_d01”,
sgfdda_end_h = 9999999,
sgfdda_interval_m = 180,
sgfdda_interval = 10800,
gfdda_inname = “wrffdda_d”,
gfdda_end_h = 999999,
gfdda_interval_m = 180,
fgdt = 0,
if_no_pbl_nudging_uv = 1,
if_no_pbl_nudging_t = 1,
if_no_pbl_nudging_q = 1,
if_zfac_uv = 0,
k_zfac_uv = 13,
if_zfac_t = 0,
k_zfac_t = 13,
if_zfac_q = 0,
k_zfac_q = 13,
guv = 0.0001,
gt = 0.0001,
gq = 0.00001,
guv_sfc = 0.0000,
gt_sfc = 0.0000,
gq_sfc = 0.0000,
if_ramping = 0,
dtramp_min = 60.0,
io_form_gfdda = 2,
rinblw = 250.0

I can help walk through creating LittleR file, but would like to make sure the process above is in place. As said, using the NOAA analyses like GFS or NAM12 will work just find in PX with no point observations provided in the Obsgrid step. It is just that the reintroduction of surface point obs in the Obsgrid step will lead to a soil nudging file that is a little more tight with respect to the fit to observations so your statistical analysis of model performance would improve some. But just using the NAM12 alone will still give very good results.

Your response was very helpful! We are running retrospective simulations so we are hoping the nudging will help our results. Obsgrid and real.exe ran (seemingly) successful but WRF spit out, “Error trying to read metadata.” I believe the file it listed is from metgrid (I’ve attached a photo). Not sure if it is related to having empty data files that will be fixed when we use LittleR or if there’s a different issue.

Since our scaling is comparatively small (less than 10 km) I think observational nudging, as opposed to 3D grid nudging, is what we need to do for our simulations.