CALL SEQUENCE...
       mslpr  GPTEMP GPPHI GPMSL OUTPUT
 
 
      DESCRIPTION...
       mslpr  - COMPUTES MEAN-SEA-LEVEL-PRESSURE FROM T,PHI PRESSURE FILES
 
 
      AUTHOR  - J.D.Henderson, NOV 27/80.
      LAST REVISION: $Header
 
 
      PURPOSE - COMPUTES A GRID FILE OF MEAN-SEA-LEVEL PRESSURE FROM THE
                LOWEST AVAILABLE ATMOSPHERIC LEVEL OF TEMPERATURE AND PHI.
                THE LAPSE RATE FROM THE LOWEST TEMPERATURE LEVEL TO
                1000 MB IS .6 TIMES THE DRY ADIABATIC LAPSE RATE.
                THE LAYER BETWEEN 1000 MB AND MSL IS ISOTHERMAL.
 
 
      INPUT FILES...
       GPTEMP = PRESSURE MULTI-LEVEL GRID SETS OF TEMPERATURE.
       GPPHI  = PRESSURE MULTI-LEVEL GRID SETS OF GEOPOTENTIAL.
 
      OUTPUT FILE...
       GPMSL  = GRIDS OF MEAN-SEA-LEVEL PRESSURE (MB)
 
 
      EXIT CONDITIONS...
        0 NO PROBLEMS
        1 FIRST FIELDS NOT THE SAME SIZE
        2 PREMATURE EOF ON GEOPOTENTIAL FILE 
        3 MONTAINS AND GEOPOTENTIAL NOT THE SAME SIZE
        4 UNABLE TO READ GPTEMP
        5 UNABLE TO READ GPPHI
        6 LEVELS DO NOT VARY MONOTONICALLY
        7 FOUND WRONG VARIABLES
        8 VERTICAL COORDINATE IS NOT PRESSURE