Recipe: Fortran program to read data from downloaded file

Problem

We want to read a downloaded data file using Fortran.

Solution

For the purpose of this example, we assume that we have already downloaded the file c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4 from the ftp server. For more information about file naming conventions, and how to download a file from the ftp server, please follow the links in the #See Also section.

Here we use the NetCDF-4 library to read the downloaded file (since NetCDF-4 uses HDF-5 as the underlying data format, HDF-5 library can also be used directly). First, we ensure that out NetCDF-4 library has been built with Fortran and HDF5 support. If both the queries

> nc-config --has-f90
> nc-config --has-hdf5

return "yes", we have a compatible NetCDF-4 library. Here nc-config is a utility bundled with NetCDF-4 package.

Below are two programs to read

1. global temperature data
2. temperature data over mainland US

from the downloaded file.

Read global temperature data

Code

program g5nr_reader_global

  use netcdf           ! for reading the NR files

  implicit none

  ! File name
  ! ---------
  character(len=256) :: T_file

  ! 4D array: (lon,lat,lev,time)
  ! ----------------------------
  real, allocatable :: T(:,:,:,:)    ! global data

  ! Miscellaneous
  ! -------------
  integer :: ierr
  integer :: im, jm, lm
  integer :: ncid, varid
  integer :: start(4), count(4)

  ! file name and dimensions
  ! ------------------------
  im = 720
  jm = 361
  lm = 72
  T_file = "c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4"

  ! Open file and get var id
  ! ------------------------
  call check(nf90_open(T_file,NF90_NOWRITE,ncid), "opening T file")
  call check(nf90_inq_varid(ncid,"T",varid), "getting T varid")

  ! Read global temperature data
  ! ----------------------------
  allocate(T(im,jm,lm,1))            ! global 4D array with 1 time level
  start = [1, 1, 1, 1]
  count = [im, jm, lm, 1]            ! 1 time level, 3D (lon,lat,lev) array
  write(*,*) 'Reading global T...'
  call check(nf90_get_var(ncid,varid,T,start=start,count=count), "reading T")
  write(*,*)'T: ', maxval(T),minval(T)

  ! close file, release memory
  call check(nf90_close(ncid), "closing T file")
  deallocate(T)

  ! All done
  ! --------

contains

  subroutine check(status, loc)

    integer, intent(in) :: status
    character(len=*), intent(in) :: loc

    if(status /= NF90_NOERR) then
       write (*,*) "Error at ", loc
       write (*,*) NF90_STRERROR(status)
    end if

  end subroutine check

end program g5nr_reader_global

See Also

1. Recipe: File naming conventions
2. Recipe: Retrieve (global) data from FTP server