Smoothing
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The post-processor, which converts data from sigma levels to pressure levels,
automatically smoothes the following pressure-level data: heights (HGT),
winds (UGRD, VGRD), temperatures (TMP), and dP/dT (omega, VVEL).  The
smoothing is done by transforming the data to spectral space, and truncating
the spectrum (T36) and then transforming back to grid.