For GrADS developers

[Ren Diandong]

Dear Sir/Madam,

I had sent over (three hours ago) the attached figure and the same two
subroutines as I sent previously to Nina and Javier for check.

If that is what is needed, would you please implement the attached *.f90
routine into GrADS as another option of the pdef so persons interested
in those kind of data can easily use grids to display and pinpoint the
data they need.

Suggestion for the options name as ROT.

Thanks,

Diandong Ren
OU/SoM

     subroutine RotFindIjForLatlon
(Nx,Ny,sgn,polelat,polelon,x1,y1,dx,dy, &
                      LOCATION,LOCATIONij)
      IMPLICIT NONE

      INTEGER            :: Nx,Ny,i,j,sgn
      real, dimension (1:nx,1:ny,1:2) :: LOCATION
      integer, dimension (1:nx,1:ny,1:2) :: LOCATIONij

      REAL :: polelat, polelon, x1, y1,dx,dy
      real, parameter :: pi=3.14159265,DR=pi/180.0,badvalue=-999.9
      REAL             :: sin_phi_pole, cos_phi_pole
      REAL             :: E_x,E_y
      REAL             :: arg,arg2,a_phi,term1,term2,a_lambda
      REAL, dimension (1:Nx,1:Ny) :: wrot

      IF (polelat.ge.0) THEN
         sin_phi_pole = sin(DR*polelat)
         cos_phi_pole = cos(DR*polelat)
      ELSE
         sin_phi_pole = -sin(DR*polelat)
         cos_phi_pole = cos(DR*polelat)
      ENDIF

    DO j=1,Ny
      DO i=1,Nx

            a_lambda=180.0+LOCATION(i,j,1)-polelon
               a_phi=LOCATION(i,j,2)*DR
               arg=sin(a_phi)
                term2=COS(a_phi)
                 IF (ABS(term2).lt.1e-5) THEN
                    a_lambda=0.0
                 ELSE
                    a_lambda=a_lambda*sgn
                    arg=cos(DR*a_lambda)
                    term1 =term2*arg
                 ENDIF
            E_y=asin(sin_phi_pole*arg-cos_phi_pole*term1)
            E_x=acos((arg*cos_phi_pole+term1*sin_phi_pole)/cos(E_y))


              if(sgn>0.0)then
              E_x=E_x-2.0*pi
              E_y=pi-E_y
                         endif
 LOCATIONij (i,j,1)=(1+int((E_x/DR-x1)/dx))
            LOCATIONij (i,j,2)=(1+int((E_y/DR-y1)/dy))
      ENDDO
      ENDDO

      end subroutine RotFindIjForLatlon


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