subroutine Hx &
               (H_obs,x,y)
! **********************************************************************
! *              .      .    .
! * PROGRAM: observation operator
! *
! * PRGMMR:    D. ZUPANSKI                  ORG: CIRA/CSU   DATE: 2003-10-01
! *
! * ABSTRACT:  Calculate obs operator: y=H(x) 
!------------------------------------------------------
!    H(x)=x
!    H(x)=x**2
!    H(x)=x**3
!    H(x)=x**4
!    H(x)=exp(x)
!    H(x)=10*log(138.*x**1.75) !! rain radar reflectivity
!----------------------
! *
! * PROGRAM LOG:
! *
! * 10/01/2003    ..... D. ZUPANSKI
! *
! **********************************************************************

      real x
      real y
      real zz
      character*3 H_obs

!==============================================================
!------------------------------------------------------
!--- observation operator (transformation) ------------
!-----------------------------------------------------

      if(H_obs.eq.'pw1') then
      y=x
        elseif(H_obs.eq.'pw2') then
      y=x**2
          elseif(H_obs.eq.'pw3') then
      y=x**3
            elseif(H_obs.eq.'pw4') then
      y=x**4
              elseif(H_obs.eq.'exp') then
      y=exp(x)
                elseif(H_obs.eq.'log') then
              if (x.gt.0.) then
             zz=10.*log(138.0*x**1.75)
             y=max(0.,zz)
              else
             y=0.0
              end if
        else
       write(*,*) "out of options for H_obs !!!"
      end if

!---------------------------------------------------
       end subroutine Hx