C*******************************************************************************
C                            POM (C-Grid)   
C                          ================
C
C    The diagram below and some of the text was provided by D.-S. Ko. 
C    It is for the case where U and V are the primary boundary conditions
C    together with T and S (co-located with E). E = EL itself is rather
C    unimportant and is substituted from an adjacent interior point.
C    
C    Inside ....... indicate the interior (non-boundary) grid points.
C    In general only those variables in the interior are computed and
C    variables at open boundary have to be specified.
C    All interpolations are centered in space except those at lateral
C    open boundary where an upstream scheme is usually used.
C
C    Horizontal locations of E(EL), T and S (etc.) are coincident.
C    NU = Not Used is indicated by *. However, for attractive output
C    adjacent interior values may be filled in at these points.   
C
C    I have been asked many times what kind of B.C. along the side wall
C    POM uses from people not acquainted with sigma coordinates. Although
C    the issue is not important as it might be for z-level grids, a direct
C    answer is "half slip" which, of course, is between free slip and
C    non-slip B.C.s.
C*******************************************************************************
C                            POM (C-Grid)   
C
C-------------------------------------------------------------------------------
C     |                               N O R T H
C     |
C     |    1     2     3           I-1   I    I+1         IM-2  IM-1   IM 
C-----+-------------------------------------------------------------------------
C     |   NU BC BC                                                    BC BC
C     |    v  v  v                                                     v  v
C     |
C     |BC> U* E  U  E  U  E  .  .  U  E  U  E  U  E  .  .  U  E  U  E  U  E  <BC
C     |    |     |     |           |     |     |           |     |     |      
C  JM |BC> +--V--+--V--+--V--   .  +--V--+--V--+--V--   .  +--V--+--V--+--V- <BC
C     |    |     | ....|...........|.....|.....|...........|.....|.... |      
C     |    U* E  U :E  U  E  .  .  U  E  U  E  U  E  .  .  U  E  U  E: U  E 
C     |    |     | :   |           |     |     |           |     |   : |      
C JM-1|    +--V--+--V--+--V--   .  +--V--+--V--+--V--   .  +--V--+--V--+--V-
C     |    |     | :   |           |     |     |           |     |   : |
C     |    U* E  U :E  U  E  .  .  U  E  U  E  U  E  .  .  U  E  U  E: U  E
C     |    |     | :   |           |     |     |           |     |   : |
C JM-2|    +--V--+--V--+--V--   .  +--V--+--V--+--V--   .  +--V--+--V--+--V-
C     |            :                                                 :          
C W   |       .  . :.  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .: .  .    E
C E   |            :                    Interior                     :         A
C S   |       .    :.     .     .     .     .     .     .     .     .:    .    S
C T   |            :                                                 :         T
C     |    |     | :   |           |     |     |           |     |   : |
C     |    U* E  U :E  U  E  .  .  U  E  U  E  U  E  .  .  U  E  U  E: U  E
C     |    |     | :   |           |     |     |           |     |   : |
C   3 |    +--V--+--V--+--V--   .  +--V--+--V--+--V--   .  +--V--+--V--+--V-
C     |    |     | :   |           |     |     |           |     |   : |      
C     |    U* E  U :E  U  E  .  .  U  E  U  E  U  E  .  .  U  E  U  E: U  E
C     |    |     | ....|...........|.....|.....|...........|.....|.... |      
C   2 |BC> +--V--+--V--+--V--   .  +--V--+--V--+--V--   .  +--V--+--V--+--V- <BC
C     |    |     |     |           |     |     |           |     |     |      
C     |BC> U* E  U  E  U  E  .  .  U  E  U  E  U  E  .  .  U  E  U  E  U  E  <BC
C     |    |     |     |           |     |     |           |     |     |      --
C   1 |NU> +--V*-+--V*-+--V*-   .  +--V*-+--V*-+--V*-   .  +--V*-+--V*-+--V* <NU
C     |
C     |    ^  ^  ^                                                     ^  ^
C     |   NU BC BC                                                    BC BC
C-----+-------------------------------------------------------------------------
C     |    1     2     3           I-1   I     I+1         IM-2  IM-1  IM
C     |
C     |                                S O U T H
C
C*******************************************************************************
C                            POM (C-Grid)   
c                           Mode Split
c                          ============
c
c e.g., ISPLIT = 5         External Mode
c
c             <----- UTB ----->UT <----- UTF ----->
c             <----- VTB ----->VT <----- VTF ----->
c             <----- EGB ----->   <----- EGF ----->
c         <ETB>       |       <ET >       |       <ETF> 
c           |         |        DT         |         |   
c           |         v         |         v         |
c           v                   v                   v
c
c                            ELB EL  ELF 
c                            UAB UA  UAF
c                            VAB VA  VAF
c
c     |   |   |   |   |   |   |   |   |   |   |   | ->|DTE|<- |   |   |   |
c    -------+-------------------+-------------------+-------------------+-->
c           |                   |                   |<---    DTI    --->|
c
c         TB/SB                T/S                TF/SF
c         UB/VB                U/V                UF/VF
c                               W                      
c                              RHO        ^
c                                         |
c                               |         |
c                               v |------------------>|
c                       DRX2D,ADX2D,WUBOT
c                       DRY2D,ADY2D,WVBOT
c                             AAM2D 
c
c                          Internal Mode

C  There is no "UT" (nor VT) in POM - UT is the average of UTB and UTF
C  and is used (divided by DT) for adjusting the internal velocity.   UTs
C  and VTs are averages of UA*D and VA*D at each external time steps over
C  one internal time step.
C
C  EGB/EGF are used to compute the barotropic part of internal velocity.
C  Internal velocity is the full 3D velocity, not the so called "baroclinic"
C  velocity.
C 
C  ETB/ETF are used for computing W, the velocity normal to sigma levels
C  and are the average of EL at the two time steps centered at the internal
C  time step.   However, due to the time filter, three time steps of EL
C  are used.
C
C  DRX2D,ADX2D,WUBOT (DRY2D,ADY2D,WVBOT) and AAM2D are depth-averages of 3D
C  variables computed using internal velocities.   They are hold constant and
C  used in the following internal time step for external mode calaculations.
C
C*******************************************************************************
c
c                   Leap Frog Time Stepping (CT)
c                  ==============================
c
c  dTime = 0. (i.c., Time = Tim0)  Initialize FAB,FA & FB,F

c                      FAB FA (FAF)
c   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |    
c   --+-------------------+-------------------+-------------------+---->
c     |                   |                   |                   |
c     FB                  F                  (FF)                

c  dTime = DTE  (at the end of first external mode)

c                         FAB FA (FAF)
c   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |    
c   --+-------------------+-------------------+-------------------+---->
c     |                   |                   |                   |
c     FB                  F                  (FF)                


c  dTime = DTE*ISPIT  (at the end of external mode and
c                      at the beginning of internal mode)

c                                         FAB FA (FAF)
c   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |    
c   --+-------------------+-------------------+-------------------+---->
c     |                   |                   |                   |
c     FB                  F                  (FF)                

      
c  dTime = DTI  (at the end of internal mode)

c                                         FAB FA  FAF
c   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   
c   --+-------------------+-------------------+-------------------+---->
c     |                   |                   |                   |
c                         FB                  F                  (FF)

C*******************************************************************************