Underground Values
                           ----------- ------

It is common for analyses to show pressure level data for regions that 
are underground.  It makes life easier.  Can you imagine a synoptic 
class trying to plot the mean-sea-level pressure when you don't 
extrapolate the pressure to sea level?


----------------- from code ---------------------------------------

C ABSTRACT: INTERPOLATES WINDS, OMEGA, HEIGHT, TEMPERATURE AND HUMIDITY
C   FROM THE SIGMA COORDINATE SYSTEM TO THE MANDATORY PRESSURE LEVELS.
C   ASSUMES THAT RELATIVE HUMIDITY, TEMPERATURE, GEOPOTENTIAL HEIGHTS,
C   WIND COMPONENTS AND VERTICAL VELOCITY VARY LINEARLY IN THE VERTICAL
C   WITH THE LOG OF PRESSURE.  UNDERGROUND HEIGHTS ARE OBTAINED USING
C   THE SHUELL METHOD AND UNDERGROUND TEMPERATURES ARE OBTAINED USING
C   A CONSTANT MOIST ADIABATIC LAPSE RATE.  HEIGHTS ABOVE THE TOP SIGMA
C   LEVEL ARE INTEGRATED HYDROSTATICALLY.  OTHERWISE FIELDS ARE HELD
C   CONSTANT OUTSIDE THE SIGMA STRUCTURE AND NO EXTRAPOLATION IS DONE.
C
C PROGRAM HISTORY LOG:
C   92-10-31  SELA,NEWELL,GERRITY,BALLISH,DEAVEN,IREDELL
C

...

C  BELOW GROUND USE SHUELL METHOD TO OBTAIN HEIGHT, CONSTANT LAPSE RATE
C  TO OBTAIN TEMPERATURE, AND HOLD OTHER FIELDS CONSTANT


The Shuell method dates back to the 6 level model.  There was never
any good reason to change.

RH, winds are fixed while the temperature has a fixed lapse rate.
Note: d(wind)/dz and d(RH)/dz is usually non-zero underground as
there is some spectral smoothing applied to the pressure-level data.