GRIB

GRIB data

Meteorological data from the WMO is supplied in GRIB format. See the GDAL GRIB format page.

• Some data available here: http://www.globalmarinenet.net/grib.htm

Newer versions of GDAL can read this data (i.e. newer than the 1.5 branch; SVN trunk is best), and it can be imported with r.in.gdal.

It often contains a small number of cells but many many bands (layers).

View band metadata with gdalinfo.

• see also the Meteorology wiki page

Processing

A mini-tutorial follows.

The data is in Lat/Lon on a Sphere. It is not quite correct to import into a Lat/Lon WGS84 location, but the resolution is so crude that you might be able to hold your nose and do so anyway.

• There may be a cell-registration bug: (grid vs cell-center convention) cells all offset to the east and south?
• Currently band metadata is not automatically copied with the import. Add it in manually with "r.support history=".

Convert latest u,v components of wind to GeoTiff (not really needed, but easier to debug) and import into GRASS:

No-data (NULL) values are flagged as "9999" in the file.

$ gdal_translate -b 1 -a_nodata 9999 TasmanSea.wind.grb  TasmanSea.wind.u_wind_latest.tif
$ gdal_translate -b 2 -a_nodata 9999 TasmanSea.wind.grb  TasmanSea.wind.v_wind_latest.tif

r.in.gdal in=TasmanSea.wind.u_wind_latest.tif \
  out=TasmanSea.wind.u_wind_latest -o
r.in.gdal in=TasmanSea.wind.v_wind_latest.tif \
  out=TasmanSea.wind.v_wind_latest -o

or import into GRASS directly:

r.in.gdal in=TasmanSea.wind.grb band=1 out=TasmanSea.wind.u_wind_latest
r.in.gdal in=TasmanSea.wind.grb band=2 out=TasmanSea.wind.v_wind_latest
r.null TasmanSea.wind.u_wind_latest setnull=9999
r.null TasmanSea.wind.v_wind_latest setnull=9999

Convert U,V velocity component maps into magnitide,direction maps for use with d.rast.arrow:

g.copy TasmanSea.wind.u_wind_latest,U_map
g.copy TasmanSea.wind.v_wind_latest,V_map

g.region rast=TasmanSea.wind.u_wind_latest -p
r.mapcalc 'magnitude = sqrt(U_map^2 + V_map^2)'
r.mapcalc 'direction = atan(U_map, V_map)'

Display:

d.mon x0
r.colors magnitude col=bcyr
d.rast magnitude
d.vect admin98 type=area fcol=225:225:255 color=none

d.rast.arrow map=direction type=grass  magnitude_map=magnitude \
  grid=none arrow=black

d.legend map="magnitude" at=90.3,94.7,1.8,26.5
echo "wind (m/s)" | d.text color=black at=13.8,80.6 size=3 align=lc

Export to a PNG image using the Cairo driver:

d.out.file -c TasmanSea_winds_latest

Resulting image:

Create 2 m/s contours:

r.contour in=magnitude out=current_wind_2ms step=2