*           rr_diff_vx.F
*
*           Rick Romea
*           Jan. 24, 2000
*
*     Meridional momemtum diffusive term :  Am v_xx
*     Units :     cm/s/s
*     MOM2 Grid:  U   
*
*******************************************************************

      SUBROUTINE RR_diff_vx_init(id)
	IMPLICIT NONE
      INCLUDE 'ferret_cmn/EF_Util.cmn'
      INTEGER id

      CALL ef_set_desc             (id,
     .'Am(d^2v/dx^2) momentum diffusive term (cm/s^2);MOM2 U Grid')
      CALL ef_set_num_args         (id, 2)
      CALL ef_set_axis_inheritance (id, IMPLIED_BY_ARGS, 
     .                                  IMPLIED_BY_ARGS, 
     .                                  IMPLIED_BY_ARGS, 
     .                                  IMPLIED_BY_ARGS)
      CALL ef_set_piecemeal_ok     (id, NO, NO, YES, YES)

      ! Define first argument (U)

      CALL ef_set_arg_name         (id, ARG1, 'V')
      CALL ef_set_arg_desc         (id, ARG1,
     . 'Zonal velocity, on the MOM2 U Grid. ')
      CALL ef_set_arg_unit         (id, ARG1, 'cm/sec')
      CALL ef_set_arg_type         (id, ARG1, FLOAT_ARG)
      CALL ef_set_axis_influence   (id, ARG1, YES, YES, YES, YES)

      !  Extend the x and y axis one point backward and forward

      CALL ef_set_axis_extend      (id, ARG1, X_AXIS,-1,+1)
      CALL ef_set_axis_extend      (id, ARG1, Y_AXIS,-1,+1)

      ! Define second argument (TEMP)

      CALL ef_set_arg_name         (id, ARG2, 'TEMP')
      CALL ef_set_arg_desc         (id, ARG2,
     . 'Potential temperature, required to access MOM2 T Grid. ')
      CALL ef_set_arg_unit         (id, ARG2, 'deg-C')
      CALL ef_set_arg_type         (id, ARG2, FLOAT_ARG)
      CALL ef_set_axis_influence   (id, ARG2, NO, NO,YES,YES)

      !  Extend the x and y axis one point backward and forward

      CALL ef_set_axis_extend      (id, ARG2, X_AXIS,-1,+1)
      CALL ef_set_axis_extend      (id, ARG2, Y_AXIS,-1,+1)

      END


      SUBROUTINE RR_diff_vx_compute(id, arg_1, arg_2, result)
      IMPLICIT NONE
      INCLUDE 'ferret_cmn/EF_Util.cmn'
      INCLUDE 'ferret_cmn/EF_mem_subsc.cmn'

      INTEGER        id

      REAL           bad_flag(EF_MAX_ARGS)
      REAL           bad_flag_result

      REAL           arg_1 (mem1lox:mem1hix, mem1loy:mem1hiy, 
     .                      mem1loz:mem1hiz, mem1lot:mem1hit)
      REAL           arg_2 (mem2lox:mem2hix, mem2loy:mem2hiy, 
     .                      mem2loz:mem2hiz, mem2lot:mem2hit)
      REAL           result (memreslox:memreshix,memresloy:memreshiy,
     .                       memresloz:memreshiz,memreslot:memreshit)

      INTEGER        res_lo_ss (4)
      INTEGER        res_hi_ss (4)
      INTEGER        res_incr  (4)

      INTEGER        arg_lo_ss (4,EF_MAX_ARGS)
      INTEGER        arg_hi_ss (4,EF_MAX_ARGS)
      INTEGER        arg_incr  (4,EF_MAX_ARGS)

      INTEGER        i,  j,  k,  l
      INTEGER        i1, j1, k1, l1

      REAL*8         yU(1024)
      REAL*8         xU(1024)
      REAL*8         yT(1024)
      REAL*8         xT(1024)
      INTEGER        ilat
      INTEGER        ilon
      CHARACTER *16  ax_name(4)
      CHARACTER *16  ax_units(4)
      LOGICAL        backward(4)
      LOGICAL        modulo(4)
      LOGICAL        regular(4)

      REAL dxt, dxu, dyt
      REAL diff_fe 
      REAL csu,xjt

      INCLUDE 'rr_parameters.h'

C*********************************************************************
C
C     THIS IS THE MOM2 CODE:
C
c     dyt     = latitudinal height of "t" grid box (in cm)
c     dxt     = longitudinal width of "t" grid box at the equator (cm)
c     dxu     = longitudinal width of "u,v" grid box at the equator (cm) 
c     dyu     = latitudinal height of "u,v" grid box (cm)
C
c     dus     = yu(j) - yt(j)
c     dun     = yt(j+1) - yu(j)
c     duw     = xu(i) - xt(i)
c     due     = xt(i+1) - xu(i)
C
c     yu(j)   = latitude of the jth "u,v" point in degrees 
c     yt(j)   = latitude of the jth "t" point in degrees   
c     xt(i)   = longitude of the ith "t" point in degrees  
c     xu(i)   = longitude of the ith "u,v" point in degrees
C

c     csu     = cosine of "u" grid point latitude
c     csur    = reciprocal of "csu"
c     dxu     = longitudinal width of "u,v" grid box at the equator (cm) 
c     dxur    = reciprocal of "dxu"
c     dxt     = longitudinal width of "t" grid box at the equator (cm)
c     dxtr    = reciprocal of "dxt"

c     am      = constant lateral viscosity coeff for momentum (input)

C     DIFF_Ux(i,k,j)  =(diff_fe(i,k,j)-diff_fe(i-1,k,j))*csudxur(i,j)
C     csudxur(i,j)    = csur(j)*dxur(i)
C     diff_fe(i,k,j)  = am_csudxtr(i,j)*(u(i+1,k,j,n)-u(i,k,j,n))
C     am_csudxtr(i,j) = am*xjt(j)*csur(j)*dxtr(i+1)
C     xjt(j)          = 3.166*dyt(j)*dytr(50)-2.166
C
C
C*********************************************************************

      !  Statement functions

      dxt(i) = SNGL (xU(i)   - xU(i-1)) * Longitude_to_cm
      dxu(i) = SNGL (xT(i+1) - xT(i))   * Longitude_to_cm
      dyt(j) = SNGL (yU(j)   - yU(j-1)) * Latitude_to_cm
      csu(i) = cos(SNGL(yU(i)) * Degrees_to_radians) 
      xjt(i) = 3.166*dyt(i)/dyt_ref-2.166
      diff_fe(i,j,k,l,iLat,iLon) = am * xjt(iLat)/csu(iLat)/dxt(iLon)
     .                  *(arg_1(i+1,j,k,l) - arg_1(i,j,k,l))





      ! Get axis data

      CALL ef_get_res_subscripts (id, res_lo_ss, res_hi_ss, res_incr)
      CALL ef_get_arg_subscripts (id, arg_lo_ss, arg_hi_ss, arg_incr)
      CALL ef_get_bad_flags      (id, bad_flag,  bad_flag_result)
      CALL ef_get_coordinates    (id, ARG1, X_AXIS, 
     .                            arg_lo_ss(X_AXIS,ARG1),
     .                            arg_hi_ss(X_AXIS,ARG1), xU)
      CALL ef_get_coordinates    (id, ARG1, Y_AXIS, 
     .                            arg_lo_ss(Y_AXIS,ARG1),
     .                            arg_hi_ss(Y_AXIS,ARG1), yU)
      CALL ef_get_coordinates    (id, ARG2, X_AXIS, 
     .                            arg_lo_ss(X_AXIS,ARG2),
     .                            arg_hi_ss(X_AXIS,ARG2), xT)
      CALL ef_get_coordinates    (id, ARG2, Y_AXIS, 
     .                            arg_lo_ss(Y_AXIS,ARG2),
     .                            arg_hi_ss(Y_AXIS,ARG2), yT)

      ! Check axis units:  bail out if not lat and lon.

      CALL ef_get_axis_info      (id, ARG1, ax_name, ax_units, 
     .                                backward, modulo, regular) 
      IF     ( ax_units(1) .NE. 'deg'       .AND.
     .         ax_units(1) .NE. 'degrees_E' .AND.
     .         ax_units(1) .NE. 'longitude' .AND.
     .         ax_units(1) .NE. 'Longitude' .AND. 
     .         ax_units(1) .NE. 'LONGITUDE' ) THEN
        WRITE (6,*)'Longitude axis units =', ax_units(1) 
        !CALL ef_bail_out(id,'Longitude axis must be in degrees')
      ENDIF

      IF     ( ax_units(2) .NE. 'deg'       .AND.
     .         ax_units(2) .NE. 'degrees_N' .AND.
     .         ax_units(2) .NE. 'latitude'  .AND.
     .         ax_units(2) .NE. 'Latitude'  .AND. 
     .         ax_units(2) .NE. 'LATITUDE' ) THEN
        WRITE (6,*)'Latitude axis units =', ax_units(2) 
        !CALL ef_bail_out(id,'Latitude axis must be in degrees')
      ENDIF

      l1 = arg_lo_ss(T_AXIS,ARG1) 
      DO l = res_lo_ss(T_AXIS), res_hi_ss(T_AXIS)
         
         k1 = arg_lo_ss(Z_AXIS,ARG1)
         DO k = res_lo_ss(Z_AXIS), res_hi_ss(Z_AXIS)

            ilat = 2  
            j1 = arg_lo_ss(Y_AXIS,ARG1) + 1
            DO j = res_lo_ss(Y_AXIS), res_hi_ss(Y_AXIS)

               iLon = 2
               i1 = arg_lo_ss(X_AXIS,ARG1) + 1
               DO i = res_lo_ss(X_AXIS), res_hi_ss(X_AXIS)

                  IF (arg_1(i1,  j1,  k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1+1,j1,  k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1-1,j1,  k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1,  j1+1,k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1+1,j1+1,k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1-1,j1+1,k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1,  j1-1,k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1+1,j1-1,k1,l1) .EQ. bad_flag(ARG1) .OR.
     .                arg_1(i1-1,j1-1,k1,l1) .EQ. bad_flag(ARG1) )THEN
                     result(i,j,k,l) = bad_flag_result
                  ELSE
                     result(i,j,k,l) =
     .                   (diff_fe(i1,j1,k1,l1,iLat,iLon) - 
     .                    diff_fe(i1-1,j1,k1,l1,iLat,iLon-1) ) 
     .                       /dxu(iLon)/csu(iLat)*sec_per_month
                  ENDIF
                  iLon = iLon + 1
                  i1 = i1 + arg_incr(X_AXIS,ARG1)
               ENDDO

               iLat = iLat + 1
               j1 = j1 + arg_incr(Y_AXIS,ARG1)
            ENDDO               

            k1 = k1 + arg_incr(Z_AXIS,ARG1)
         ENDDO

         l1 = l1 + arg_incr(T_AXIS,ARG1)
      ENDDO

      END











