Velocity Potential (VPOT) and Stream Function (STRM)

The velocity potential and stream function were calculated by converting
the UGRD and VGRD on the Gaussian grid to spectral space and doing the
spectral calculation for VPOT and STRM.  The divergence and relative
vorticity could have been calculated at the same time but these quanties
are rarely used.

The program that does the calculation is core_uv2sv (UGRD/VGRD to STRM/VPOT).
This program uses splib (spectral) and wgrib2lib (grib).