Module pygrib

Introduction

Python module for reading and writing GRIB (editions 1 and 2) files. GRIB is the World Meterological Organization standard for distributing gridded data. The module is a python interface to the GRIB API C library from the European Centre for Medium-Range Weather Forecasts (ECMWF).

Required

• Python 2.4 or higher.
• numpy N-dimensional array object for python. Version 1.2.1 or higher (version 1.5.1 required for Python 3).
• pyproj Python interface to PROJ.4 library for cartographic transformations or matplotlib and the basemap toolkit. Pyproj 1.8.9 is required for Python 3.
• GRIB API C library for encoding and decoding GRIB messages (edition 1 and edition 2). Version 1.8.0 or higher required. To be fully functional, the GRIB API library requires Jasper or OpenJPEG for JPEG200 encoding, and PNG for PNG encoding.

These dependencies are available via the package management system of most Linux distributions, and on MacOS X using macports. If you build grib_api yourself as a static library on a 64-bit system you may need to set CFLAGS to '-O2 -fPIC' before running the configure script. To use pygrib on Windows, you must use the Cygwin environment, since the grib_api library requires a posix environment. Cygwin installation instructions are available here.

Installation

• Download the source code.
• pygrib installation options can either be set with environment variables, or specified in a text file (setup.cfg). To use environment variables, set $GRIBAPI_DIR, $JASPER_DIR, $OPENJPEG_DIR, $PNG_DIR and $ZLIB_DIR so that the include files and libraries for GRIB API, Jasper, OpenJPEG, PNG and zlib will be found. For example, the include files for jasper should be found in $JASPER_DIR/include or $JASPER_DIR/include/jasper, and the jasper library should be found in $JASPER_DIR/lib or $JASPER_DIR/lib64. If any of those environment variables are not set, then it is assumed that GRIB API was not built with support for that library. If the libraries and include files are installed in separate locations, the environment variables $GRIBAPI_INCDIR and $GRIBAPI_LIBDIR can be used to define the locations separately (same goes for JASPER, OPENJPEG, PNG and ZLIB). To use setup.cfg, copy setup.cfg.template to setup.cfg, open setup.cfg in a text editor and follow the instructions in the comments for editing.
• Run 'python setup.py build' and then 'python setup.py install', as root if necessary. Note that if you are using environment variables to specify the build options, you cannot build and install in one step with 'sudo python setup.py install', since sudo does not pass environment variables. Instead, run 'python setup.py build' first as a regular user, then run 'sudo python setup.py install' if the install directory requires admin or root privileges.
• Run 'python test.py' to test your installation.
• Look at examples in test directory (most require matplotlib and basemap).

Example usage

• from the python interpreter prompt, import the package:

    >>> import pygrib
  

• open a GRIB file, create a grib message iterator:

    >>> grbs = pygrib.open('sampledata/flux.grb')  
  

• pygrib open instances behave like regular python file objects, with seek, tell, read, readline and close methods, except that offsets are measured in grib messages instead of bytes:

    >>> grbs.seek(2)
    >>> grbs.tell()
    2
    >>> grb = grbs.read(1)[0] # read returns a list with the next N (N=1 in this case) messages.
    >>> grb # printing a grib message object displays summary info
    3:Maximum temperature:K (instant):regular_gg:heightAboveGround:level 2 m:fcst time 108-120 hrs:from 200402291200
    >>> grbs.tell()
    3
  

• print an inventory of the file:

    >>> grbs.seek(0)
    >>> for grb in grbs:
    >>>     grb 
    1:Precipitation rate:kg m**-2 s**-1 (avg):regular_gg:surface:level 0:fcst time 108-120 hrs (avg):from 200402291200
    2:Surface pressure:Pa (instant):regular_gg:surface:level 0:fcst time 120 hrs:from 200402291200
    3:Maximum temperature:K (instant):regular_gg:heightAboveGround:level 2 m:fcst time 108-120 hrs:from 200402291200
    4:Minimum temperature:K (instant):regular_gg:heightAboveGround:level 2 m:fcst time 108-120 hrs:from 200402291200
  

• find the first grib message with a matching name:

    >>> grb = grbs.select(name='Maximum temperature')[0]
  

• extract the data values using the 'values' key (grb.keys() will return a list of the available keys):

    # The data is returned as a numpy array, or if missing values or a bitmap
    # are present, a numpy masked array.  Reduced lat/lon or gaussian grid
    # data is automatically expanded to a regular grid. Details of the internal
    # representation of the grib data (such as the scanning mode) are handled
    # automatically.
    >>> maxt = grb.values # same as grb['values']
    >>> maxt.shape, maxt.min(), maxt.max()
    (94, 192) 223.7 319.9
  

• get the latitudes and longitudes of the grid:

    >>> lats, lons = grb.latlons()
    >>> lats.shape, lats.min(), lats.max(), lons.shape, lons.min(), lons.max()
    (94, 192) -88.5419501373 88.5419501373  0.0 358.125
  

• get the second grib message:

    >>> grb = grbs.message(2) # same as grbs.seek(1); grb=grbs.readline()
    >>> grb
    2:Surface pressure:Pa (instant):regular_gg:surface:level 0:fcst time 120 hrs:from 200402291200
  

• modify the values associated with existing keys (either via attribute or dictionary access):

    >>> grb['forecastTime'] = 240
    >>> grb.dataDate = 20100101
  

• get the binary string associated with the coded message:

    >>> msg = grb.tostring()
    >>> grbs.close() # close the grib file.
  

• write the modified message to a new GRIB file:

    >>> grbout = open('test.grb','wb')
    >>> grbout.write(msg)
    >>> grbout.close()
    >>> pygrib.open('test.grb').readline() 
    1:Surface pressure:Pa (instant):regular_gg:surface:level 0:fcst time 240 hrs:from 201001011200
  

Documentation

• see below for the full python API documentation.

Changelog

• see Changelog file.