The option -submsg_uv is like the option
-ncep_uv. The latter option puts UGRD and VGRD into
the same grib message. The former combines all the vector fields into
their own grib message.
The option -submsg_uv may be important for
parallel processing. For example, you want to regrid from a Gaussian
grid to a polar stereographic grid that covers the North Pole.
For scalar fields, the grid can be regridded independently which
allows a possibility of parallel processing. Vector fields have
to processed together (U and V). One approach is to put the
vector fields into their own grib message (combine the corresponding
U/V fields). Then all the grib messages can be regridded independently.
The vector fields are the same as used by the
-new_grid option. The vector list can
be changed using the -new_grid_vectors option.
$ wgrib2 test.grb2 -match ":500 mb:" -submsg_uv output.grb