WRF Vtable Carpentry
Sept. 28, 2016
Posted by: RDA Team
Note: This page was originally sourced from our Blogger page: http://ncarrda.blogspot.com/2016/09/wrf-vtable-carpentry.htmlWhen we announced the introduction of WRF-able data sets (Vtables auto-generated from RDA-collected metadata) in January 2016, WRF Preprocessing System (WPS) component, ungrib, was able to use the RDA Vtables without any hiccups.
Around the time of the release of WRF 3.8.0 in April 2016, firstname.lastname@example.org began receiving reports of error messages when running ungrib with RDA-provided Vtables. If the Vtable includes lines describing fields that are not contained in that particular input GRIB file, ungrib stops instead of ignoring that field (as it had done previously.)
WPS V3.8 Updates
The simple solution is for the user to edit the RDA-provided Vtable to remove the extraneous line(s) referring to fields not contained in the input GRIB file.
After investigation and consideration, wrfhelp and rdahelp decided not to change either the WPS code or the auto-generated Vtables to return to plug and play. Here's why.
The RDA provides archives of analysis, forecast and reanalysis data sets. Reanalysis data sets contain the same variables/parameters throughout the entire time series. Operational analysis and forecast systems vary over time. Parameters or vertical levels may (and should!) change over the time series.
With advances in computational power and modeling of physical processes, newer fields and levels can be added. Bandwidth is limited. Dropping vestigial parameters when they are replaced with better ones is necessary.
RDA-provided Vtables are complete. They include lines describing every WRF-usable parameter found in the data set time series. To see how parameters and vertical levels changed over time, click on the "Variables by dataset product" or "detailed metadata for level information" links from the data set home pages.
Consider the long-running NCEP analysis series, FNL, ds083.2. When it began in 1999, it was in GRIB (aka GRIB1) format. Beginning in 2007, it is offered in GRIB2 format. Several times, the model was changed and fields were added (and sometimes subtracted.)
|View the vertical level information of long-running NCEP analysis series, FNL, ds083.2. Select "detailed metadata" to view GRIB1 details; select "GRIB2 level table" to view GRIB2 details.
|Defaults to "Parameter View". Also explore "Vertical Level View."
|Notice that the soil layers expanded from a single layer 10-200 cm below the surface to three layers in 2005
|Delete the lines that refer to variables not found in the file you are using.
|These soil layers are found only in FNL files 2005-05-31 and later. Delete for earlier dates.
To learn more about GRIB data sets, read our GRIB blog series, particularly Setting up to work with GRIB2. Use wgrib and wgrib2 to explore the contents of your GRIB or GRIB2 files. You might find a parameter you aren't already using but might be helpful. Or, you may see a lot of parameters and vertical layers that you don't need. In that case, may I suggest you Subset to save time and bandwidth?