SUBROUTINE EXTRAP1D_SURFACE_X(numInterpSets, srcNumLevels, 
destNumLevels, src_ijDim, dst_ijDim, vLevelSource, stateSource, stateDerivSource, 
posnDestInSrc, vLevelDestn, stateDestn, stateDerivDestn, extrapEnableDown, 
extrapEnableUp, extrapGuideDown, extrapGuideUp, m_slStateValue, numExtArraysIn,
numExtArraysOut, ExtArraysIn, ExtArraysOut)

integer, intent(in) :: numInterpSets
integer, intent(in) :: srcNumLevels
integer, intent(in) :: destNumLevels

integer, intent(in) :: src_ijDim
integer, intent(in) :: dst_ijDim

real, dimension(src_ijDim, srcNumLevels), intent(in) :: vLevelSource
real, dimension(src_ijDim, srcNumLevels), intent(in) :: stateSource
real, dimension(src_ijDim, srcNumLevels), intent(in) :: stateDerivSource

integer, dimension(dst_ijDim, destNumLevels), intent(in) :: posnDestInSrc
real, dimension(dst_ijDim, destNumLevels), intent(in) :: vLevelDestn
real, dimension(dst_ijDim, destNumLevels), intent(out) :: stateDestn
real, dimension(dst_ijDim, destNumLevels), intent(out) :: stateDerivDestn

logical, intent(in) :: extrapEnableDown, extrapEnableUp
real, intent(in) :: extrapGuideDown, extrapGuideUp

external m_slStateValue

integer, intent(in) :: numExtArraysIn
integer, intent(in) :: numExtArraysOut
real, dimension(src_ijDim, numExtArraysIn), intent(in)  :: ExtArraysIn
real, dimension(dst_ijDim, numExtArraysOut), intent(out) :: ExtArraysOut
DESCRIPTION 
For each destination level that is between the two lowest source levels, a state
value (but not a derivative) is extrapolated.  It is presumed that the lowest level
supplied represents the Earth's surface, and that that value is a true value, not
the one that is obtained from GEM.  Technically, that makes this an interpolation,
or perhaps a guided extrapolation of the results from the forecast model.  Other
values that would be interpolated, or extrapolated upward, are untouched by the
routine.

Through the routine that is passed as an argument, this extrapolation takes into
account the physics that governs diffusion near the Earth's surface.  Thus, it is
not applied to extrapolation above the highest vertical level.  Moreover, it should
not be used for a general mathematical extrapolation; the algorithm makes sense
only when considering the surface of the Earth.

N.B.:  It must be noted that the routine passed as an argument probably (This cannot
       be stated absolutely, because the user is free to supply whatever routine he
       sees fit, provided that it respects the interface.) presumes that
       the location of each vertical level is given as z (meters above the Earth's
       surface).  If this is the case, vLevelSource and vLevelDestn must be in z.

       Furthermore, that same routine presumes that the lowest level represents the
       Earth's surface.  The stateSource values given for that level should be true
       values and not those given by the model (GEM).

       The 'extrapolation' performed by this routine is thus an interpolation
       over the interval between the Earth's surface and the next vertical level.

Author: Jeff Blezius - revised Oct 2003
ARGUMENTS

It can be noted that the arguments to all of the Interp1D_* and Extrap1D_* routines are identical, for ease of switching a call from one to another of them. You should examine the generic description of the arguments. The following are uses of the generic arguments that are specific to this routine:

Input:
extrapGuideDown	unused
extrapGuideUp	unused
ExtArraysIn	only the first 4 entries are used
Output:
stateDerivDestn(dst_ijDim, dstNumLevels)	unused
ExtArraysOut	unused

SEE ALSO