angles.f

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!                   *****************
                    SUBROUTINE angles
!                   *****************
!
     &( xlamd , dtplus, dtmoin)
!
!***********************************************************************
! TOMAWAC   V6P1                                   08/06/2011
!***********************************************************************
!
!brief    COMPUTES THE ANGLES OF RESONANT VECTORS IN THE CASE
!+                OF THE STANDARD INTERACTION CONFIGURATION - DIA METHOD
!+                PROPOSED BY HASSELMANN AND HASSELMANN (1985).
!+
!+
!+            PROCEDURE SPECIFIC TO THE CASE WHERE THE DIRECTIONS
!+                ARE EVENLY DISTRIBUTED OVER [0;2.PI].
!
!note     THE ANGLES DTPLUS AND DTMOIN ARE IN DEGREES AND ARE BOTH
!+          POSITIVE BY CONVENTION.
!
!reference  HASSELMANN S., HASSELMANN K. ET AL.(1985) :
!+                     "COMPUTATIONS AND PARAMETERIZATIONS OF THE NONLINEAR
!+                      ENERGY TRANSFER IN GRAVITY-WAVE SPECTRUM. PART2 :
!+                      PARAMETERIZATIONS OF THE NONLINEAR ENERGY TRANSFER
!+                      FOR APPLICATION IN WAVE MODELS". JPO, VOL 15, PP 1378-1391.
!
!history  M. BENOIT
!+        26/06/96
!+        V1P2
!+
!
!history  N.DURAND (HRW), S.E.BOURBAN (HRW)
!+        13/07/2010
!+        V6P0
!+   Translation of French comments within the FORTRAN sources into
!+   English comments
!
!history  N.DURAND (HRW), S.E.BOURBAN (HRW)
!+        21/08/2010
!+        V6P0
!+   Creation of DOXYGEN tags for automated documentation and
!+   cross-referencing of the FORTRAN sources
!
!history  G.MATTAROLO (EDF - LNHE)
!+        08/06/2011
!+        V6P1
!+   Translation of French names of the variables in argument
!
!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!| DTMOIN         |<--| ANGLE ASSOCIATED TO THE FREQUENCY (1-XLAMD).FREQ
!| DTPLUS         |<--| ANGLE ASSOCIATED TO THE FREQUENCY (1+XLAMD).FREQ
!| XLAMD          |-->| DIA STANDARD CONFIGURATION LAMBDA COEFFICIENT
!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
!
      USE declarations_tomawac, ONLY : gradeg
!
      USE declarations_special
      USE interface_tomawac, ex_angles => angles
      IMPLICIT NONE
!
!
!.....VARIABLES IN ARGUMENT
!     """""""""""""""""""""
      DOUBLE PRECISION, INTENT(IN)    :: XLAMD
      DOUBLE PRECISION, INTENT(INOUT) :: DTPLUS, DTMOIN
!
!.....LOCAL VARIABLES
!     """"""""""""""""""
      DOUBLE PRECISION AUX
!
!
!.....1. CHECKS THAT LAMBDA RANGES BETWEEN 0 AND 0.5.
!     """"""""""""""""""""""""""""""""""""""""""""""""""""""
!
      IF(xlamd.LT.0.d0.OR.xlamd.GT.0.5d0) THEN
        WRITE(lu,1002) xlamd
        CALL plante(1)
        stop
      ENDIF
!
!.....2. COMPUTES TETA_PLUS (DTPLUS) AND TETA_MOINS (DTMOIN).
!     """""""""""""""""""""""""""""""""""""""""""""""""""""""
      aux=2.d0*xlamd*(1.d0+xlamd*xlamd)
      dtplus=acos( (1.d0+aux)/(1.d0+xlamd)**2 )*gradeg
      dtmoin=acos( (1.d0-aux)/(1.d0-xlamd)**2 )*gradeg
!
!
 1002 FORMAT('/!/-----------------------------------------------/!/'/
     &       '/!/        PROGRAM STOP IN SUBROUTINE ANGLES      /!/'/
     &       '/!/-----------------------------------------------/!/'/
     &       '/!/       THE VALUE OF THE LAMBDA PARAMETER       /!/'/
     &       '/!/             USED IN THE DIA METHOD            /!/'/
     &       '/!/  MUST BE INCLUDED IN THE INTERVAL [ 0. ; 0.5 ]/!/'/
     &       '/!/  THE VALUE HERE IMPOSED IS : ', g10.4,'       /!/'/
     &       '/!/-----------------------------------------------/!/')
!
      RETURN
      END