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code/obs_operators/interp.f

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+      SUBROUTINE INTERP_AP( ya, na, xa, yb, nb, xb )
+
+      ! Linear interpolation xa (ya, na) >> xb (yb, nb)
+      ! ya, yb == pressure levels
+      ! NOTE: yb:bottom->top ; ya: top->bottom
+
+      integer :: na, nb
+      integer :: i, j
+      real*4, dimension(na) :: xa, ya
+      real*4, dimension(nb) :: xb, yb
+      real*4 :: slope, biais
+
+      do j = 1, nb
+         do i = 1, na-1
+            if (( yb(j) .ge. ya(i)) .and. ( yb(j) .lt. ya(i+1))) then
+               if ( (xa(i) -xa(i+1)) .ne. 0.) then
+                  slope = ( ya(i) - ya (i+1) ) / (xa(i) -xa(i+1))
+                  biais = ya(i) - slope * xa(i)
+                  xb(j) = ( yb(j) - biais) / slope
+               else
+                  xb(j) = xa(i)
+               endif
+            endif
+         enddo
+      enddo
+
+
+      !Return to calling program
+      END SUBROUTINE INTERP_AP