WMS vector field layer is not created from GRIB2 dataset with u/v components

When configuring the TDS 5 catalog to expose the WMS service on a GRIB2 dataset containing u/v vector component variables, ncWMS2 does not generate a combined/derivative vector field layer as it should, per ncWMS2 documentation, even though the variable names conform to the expected convention.

I have reproduced this problem using both a simple <dataset>:

<datasetRoot path="gfs" location="content/gfs/" />
<dataset name="GFS Test" ID="gfstest"
    serviceName="wms" urlPath="gfs/gfs.0p25.2017021212.000.grib2" dataType="Grid"/>

as well as with a <featureCollection>:

<featureCollection name="NCEP GFS 2.5 Degree" featureType="GRIB2" path="gfs2p5">
  <metadata inherited="true">
    <serviceName>all</serviceName>
    <documentation type="summary">NCEP GFS 2.5 degree</documentation>
  </metadata>
  <collection spec="content/gfs/.*grib2$"
          name="GFS_2P5"
          timePartition="file"
          olderThan="5 min" />
  <update startup="nocheck" />
  <tdm rewrite="test" rescan="0 0/15 * * * ? *" />
</featureCollection>

To my knowledge, this issue only seems to affect GRIB2 datasets. TDS 5 + ncWMS2 was successful in visualizing similar u/v component variables into a vector field when a CF-compliant NetCDF was used with a <featureCollection featureType="FMRC">.

The sample GRIB2 files I used were from NOAA/NWS/NCEP Global Forecast System (GFS), which can be downloaded from the NCEP FTP server in subdirectories of [ftp://ftp.ncep.noaa.gov/pub/data/nccf/com/gfs/prod](), e.g. [ftp://ftp.ncep.noaa.gov/pub/data/nccf/com/gfs/prod/gfs.2017022800/gfs.t00z.pgrb2.0p25.f048]().

Unidata / thredds

WMS vector field layer is not created from GRIB2 dataset with u/v components #763