NCL: Reading Supported Formats

There are several supported formats in NCL. Support for GRIB2 files was added in version 4.3.0.

Pointers to supported format files

The NCL function addfile is used to access supported format files. It takes two arguments, the filename, and an action flag (e.g. "r" for read).

The filename must have a suffix that identifies the file format: '.nc' for netCDF, '.grb' and '.grib' for GRIB, '.grb2' and '.grib2' for GRIB2, '.hdf' for HDF Scientific Data Set, '.hdfeos', '.he2', and '.he5' for HDF-EOS and '.ccm' for CCM History Tape.

addfile is a function, so it must be set equal to some variable. This variable is the pointer to the file, and it will be used extensively.

Actual Script

    begin
    ;************************************
    ; create pointer to several types of
    ; files. Note the syntax is the same
    ;************************************
       in = addfile("./uv300.nc","r")                    ; netcdf
       h  = addfile("./S2000759.hdf","r")                ; hdf
       g1  = addfile("./monthly_1979_1151.tar.grb","r")  ; grib1
       g2  = addfile("./monthly_1979_1151.tar.grb2","r") ; grib2
    ;************************************
    ; read variable from the file
    ; no space between in and the variable name
    ;************************************
       u = in->U    ; on RHS, case sensitive
    end

Many netCDF and HDF files store data originally of type "float" or "double" as numeric type "short" or "byte". The purpose of the file creator is to reduce the size of the file. To accomplish this the original data are scaled and offset. The function pack_values located in contributed.ncl describes one approach. To unpack these data use the short2flt or the byte2flt function.

Actual Script

    load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
    begin
       in = addfile("./h500.nc","r")              ; netcdf [,hdf]
       h  = short2flt( in->HGT  )                 ; HGT [short] , h [float]
       printVarSummary( h )
    end