Recipe: Fortran program as OPeNDAP client

Back to G5NR Data Access Guide.

Problem

By accessing the collection inst01hr_3d_T_Cv via the OPeNDAP server

http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km

we want to read the surface temperature data inside the box bound by latitudes 25^oN, 50^oN and longitudes -130^oW, -65^oW for 2006/Sep/18, 9z and compute its min/max.

Solution

First, we ensure that our NetCDF-4 library has been built with Fortran and OPeNDAP support. If both the queries

> nc-config --has-f90
> nc-config --has-dap

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

The metadata for the collection inst01hr_3d_T_Cv is available at

http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv.info

Code

This code accesses the collection inst01hr_3d_T_Cv from the OPeNDAP server and reads a subset of the temperature data.

NOTE:

1. For hourly data, starting at 2005/May/15, 22z, the 11772^th timestep corresponds to 2006/Sep/18, 9z.

program g5nr_reader_dap

  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(:,:,:,:)

  ! Miscellaneous
  ! -------------
  integer :: ierr
  integer :: im, jm, lm
  integer :: ncid, varid
  integer :: start(4), count(4)
  integer :: imin, imax, jmin, jmax
  real :: minlat, minlon, maxlat, maxlon

  ! We replace filename by opendal url
  ! ----------------------------------
  T_file = "http://opendap.nccs.nasa.gov:9090/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv"

  ! file name and (subset) dimensions
  ! bounding box:
  !   lons = -130:0.5:-65
  !   lats =   25:0.5:50
  ! ---------------------------------
  ! array indices from the
  ! bounding lat/lon values
  imin = nint((-130. + 180.)/0.5)
  imax = nint(( -65. + 180.)/0.5)
  jmin = nint((  25 +  90.)/0.5)
  jmax = nint((  50 +  90.)/0.5)
  ! array sizes
  im = imax-imin+1
  jm = jmax-jmin+1
  lm = 72

  ! Open url 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 temperature data
  ! ---------------------
  allocate(T(im,jm,lm,1))            ! global 4D array with 1 time level
  start = [imin, jmin, 1, 11772]     ! time level 11772 corresponds to 2006/09/18, 9z
  count = [im, jm, lm, 1]            ! 1 time level, 3D (lon,lat,lev) array
  write(*,*) 'Reading 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_dap

Compile and link

We use the utility nf-config (included in the NetCDF-4 installation) to identify the linking rules

For a typical NetCDF-4 installation, the above code, g5nr_reader_dap.f90 can be compiled and linked to the NetCDF-4 library via

gfortran -o g5nr_reader_dap.x `nf-config --fflags` g5nr_reader_dap.f90 `nf-config --flibs`

creating the executable g5nr_reader_dap.x.

NOTE:

1. You can use your favorite Fortran compiler instead of gfortran.
2. If the NetCDF-4 library was built with parallel I/O support, you will need to use mpif90 to link, even if your code does not use the MPI library.

Run

Running the executable,

./g5nr_reader_dap.x

produces the output

Reading T (subset)...
T (subset):    305.6048       191.6956

Access data via OPeNDAP server

Discussion

The four (4) differences from the case where an individual downloaded file was being read are

1. Line 31: Instead of reading a downloaded NetCDF-4 file, we read an OPeNDAP URL.
2. Line 36: While in the downloaded file, the temperature variable appears in the uppercase (T), on the OPeNDAP server, this variable is in lowercase (see http://opendap.nccs.nasa.gov:9090/dods/OSSE/GEOS-5.12/BETA9/0.5000_deg/inst/inst01hr_3d_T_Cv). This is reflected in the call to nf90_inq_varid.
3. Line 41: Via the OPeNDAP URL, we now have access to all times for which data exists. The hourly inst files are available starting at 2005/15/15 2200z. Our desired time, 2006/09/18 0900z is then the 11772^th file.
4. Line 63: Same as the difference above, but for retrieving a subset of data.