University of Alberta

The Atmosphere, EAS 372

Jan.-Apr., 2012

Introduction to using GrADS to display GEM forecast

GrADS Help

About CMC's grib2 files

The GEM forecasts are archived, with public access for a limited time. In EAS 372 we will learn how to obtain these files and display fields in GrADS.

The model outputs are stored in a format called ".grib2," and the file naming convention (for the high resolution run, i.e. 15km mesh) is:

CMC_reg_Variable_LevelType_level_ps15km_YYYYMMDDHH_Phhh.grib2

where the most frequently used cases (for us) will entail

To download .grib2 files manually, or learn about what is available, start at www.weatheroffice.gc.ca/grib/index_e.html.

To this PATH, one tacks on init_time/fcstrange/filename, where init_time has two digits and fcstrange has three. The convention for filename is given above. So one might issue

http://dd.weatheroffice.gc.ca/model_gem_regional/high_resolution/grib2/12/036/CMC_reg_HGT_ISBL_850_ps15km_2012040212_P036.grib2

which will grab the 36 hr prog for the 850 hPa height field, initialized at 12Z on 2 April 2012. (One could, alternatively, download this file automatically using the wget instruction in a terminal -- as below).

For the GEM global runs, grib2 files have the naming convention:

CMC_glb_Variable_LevelType_Level_projection_YYYYMMDDHH_Phhh.grib2

These files can be sourced on the PATH http://dd.weather.gc.ca/model_gem_global/25km/grib2/lat_lon/, to which one appends init_time/fcstrange/filename, where init_time has two digits and fcstrange has three. So (for example) to obtain the 120-h forecast for the 700 hPa height field from a run initialized at 00Z on 13 March 2013 one would issue

http://dd.weather.gc.ca/model_gem_global/25km/grib2/lat_lon/00/120/CMC_glb_DEPR_ISBL_700_latlon.225x.225_2013031300_P120.grib2

Running GrADS in EAS 1-39

  1. We will manipulate the .grib2 files and run GrADS from a terminal: Start Menu => Cygwin => XWin Server (nb! do not choose the "Cygwin terminal"). The terminal opens, and you will be in directory c:\cygwin\home\yourccid. Useful commands in this terminal:

    pwd (confirms what directory you are in)

    ls -a (lists all your files)

  2. In the windows file manager (browser), copy the file "g2ctl" from C:\cygwin\home\administrator to C:\cygwin\bin

  3. Where are your .grib2 files? This depends on the setting in the browser (Firefox), which may be adjusted to save you the need to cut and paste files around: it will be most convenient to download your .grib2 files directly to c:\cygwin\home\your_ccid by making the needed adjustment in Firefox => Tools => Options. Otherwise, or if you had already downloaded the files to some place else, now move or copy them, using a file browser, to the following (temporary) destination:

    c:\cygwin\home\your_ccid

    (substitute your own ccid for "your_ccid").

  4. If "name.grib2" represents a .grib2 file, then the final two preparatory steps (to be done in your XWin Server) for each grib2 file you wish to display are:
    • g2ctl name.grib2 > easiername.ctl
    • gribmap -i easiername.ctl
    for example,
    • g2ctl CMC_reg_HGT_ISBL_850_ps15km_2012040212_P036.grib2 > height850.ctl
    • gribmap -i height850.ctl

  5. However, if you want to plot more than one field on a single chart, you need to combine your .grib2 files, e.g. (in your XWin Server terminal)

    cat file1.grib2 file2.grib2 > both.grib2

    g2ctl both.grib2 > both.ctl

    gribmap -i both.ctl

    grads

    The file names here are arbitrary; don't forget the "pipe" (>) in the cat and g2ctl commands. Now you are ready to run GrADS.

  6. In the XWin Server terminal, start grads (i.e. type: grads)

  7. Within grads, a typical sequence of instructions might be the following (this example produces a map of 850 hPa height and temperature covering western Canada, and saves it as a .png image):

    • q ctlinfo (returns the variable names accessible to be plotted, e.g. hgt850mb)
    • open hgt_tmp.ctl
    • set lat 35 70
    • set lon 230E 290E
    • set cint 5
    • set cstyle 2
    • display tmp850mb
    • set cint 60
    • set cstyle 1
    • display hgt850mb
    • printim hgt_tmp_850.png png white
    Another example, producing shaded contours of vertical velocity:
    • open hgt_vvel.ctl
    • set lat 35 70
    • set lon 230E 290E
    • set gxout shaded
    • display vvel850mb
    • set gxout contour
    • set cint 60
    • set cstyle 1 (1=solid, 2=long dash)
    • set ccolor 0 (0=black, 1=white, 2=red)
    • display hgt850mb
    • draw title GEMreg 12h vld 00Z Tues 12 Mar. 2013\850 height & v.veloc.
    • printim hgt_vvel_850.png png white (ensures white background)
    To get a map displaying solid contours of height and dashed contours of temperature, both black on white (note: here the lat/lon are set to see a region around New Zealand):
    1. open hgt_tmp.ctl
    2. set lat -60 -30
    3. set lon 150E 180E
    4. set ccolor 0 (0=black, 1=white, 2=red)
    5. set cstyle 1 (1=solid, 2=long dash)
    6. set cint 60
    7. d hgt850mb
    8. set cstyle 2
    9. set cint 5
    10. d tmp850mb
    11. draw title ...
    12. printim hgt_tmp...png png white (ensures white background)

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Last Modified: 20 Mar., 2013