Replace values in ECMWF initial condition in GRIB format

grd2grb.f90

program grd2grb
use grib_api
implicit none

  character(len=*), parameter :: &
    ifname = "ICMSHepc8INIT", gfname = "out.grd", ofname = "out.grb"

  integer, parameter :: &
    ug = 41, ntrunc = 21, nv = (ntrunc + 2) * (ntrunc + 1)
  real*4, dimension(nv) :: sh
  integer :: ifile, ofile
  integer :: igrib, iret, n = 0 , i

  open(unit =  ug, file = gfname, access = "direct", &
       status = "old", action = "read", recl = nv * 4)
  call grib_open_file(ifile, ifname, 'R')
  call grib_open_file(ofile, ofname, 'W')
  do
    call grib_new_from_file(ifile, igrib, iret)
    if (iret == GRIB_END_OF_FILE) exit
    n = n + 1 
    read(unit = ug, rec = n) sh
    print *, "message #", n, " # of values = ", nv
    call grib_set(igrib, "values", sh, iret)
    call grib_write(igrib, ofile)
    call grib_release(igrib)
  end do

  call grib_close_file(ifile)
  call grib_close_file(ofile)
  close(unit = ug)

end program