-wind_dir¶

Introduction¶

The -wind_dir option takes the zonal and meridional winds, computes the wind direction and writes it out in grib format. The results is in degrees where 0 degrees is a wind from the north and 90 degrees is a wind from the east. For this option to work, (1) the zonal and meridional winds must be earth relative (not grid relative winds) and (2) the meridional wind must follow the corresponding zonal wind for that level, time and forecast hour. (Condition 2 is the same restriction as for the -wind_speed option. If the U and V in your grib file do not have this order, the file must be sorted. However, many NCEP forecast files already have this order.

The winds need to be earth relative. The -new_grid option can convert between earth and grid relative winds.

You can save computer time by using the -match option to restricting the decoding of records to U and V.

The precision of the wind direction is set to the nearest degree (wgrib2 v3.0.0). Earlier versions used the precision entering the routine. That would be precision of the V field unless the precision was changed by a scaling option like -set_scaling.

Calm Winds¶

The calculation of the wind direction breaks down when the zonal and meridional winds are zero. The wind direction for calm winds depends on the implementation. (C's atan2 is well defined but some machines do not implement negative zero.)

Usage¶

-wind_dir output_grib_file

   Comment: the -wind_speed option can write to the same file as -wind_dir

Example¶

$ wgrib2 gep19.t00z.pgrb2af180 -wind\_dir wind.grb -wind\_speed wind.grb -match "(UGRD|VGRD)"
4.1:86046:d=2009060500:UGRD:200 mb:180 hour fcst:ENS=+19
4.2:86046:d=2009060500:VGRD:200 mb:180 hour fcst:ENS=+19
8.1:226831:d=2009060500:UGRD:250 mb:180 hour fcst:ENS=+19
8.2:226831:d=2009060500:VGRD:250 mb:180 hour fcst:ENS=+19

$ wgrib2 wind.grb
1:0:d=2009060500:WDIR:200 mb:180 hour fcst:ENS=+19
2:97922:d=2009060500:WIND:200 mb:180 hour fcst:ENS=+19
3:179554:d=2009060500:WDIR:250 mb:180 hour fcst:ENS=+19
4:277476:d=2009060500:WIND:250 mb:180 hour fcst:ENS=+19