Difference between revisions of "NOAAWinds"

Revision as of 02:37, 25 October 2014

NOAA GFS winds in Virtual Regatta games

Winds used in the Virtual Regatta games are from NOAA's Global Forecast System. The GFS makes the forecasts available at this location:
http://nomads.ncep.noaa.gov/cgi-bin/filter_gfs.pl

This forecast predicts wind at 1° latitude/longitude resolution, at 3 hour intervals up to 192h, then at 12 hour intervals up to 384h. This data is fabricated by a of weather simulation program, using laws of physics and inputs from instruments [which?]. Server responses are in grib 2 format. Variable "wind at 10m" is used. Winds are given by its two U and V components, in latitude/longitude directions.

Game winds for 06Z are extracted from folder YYYYDDMM00. Time 00 comes from file gfs.t00z.pgrbf06.grib2, time 12 from gfs.t00z.pgrbf18.grib2 and so on. Winds for 18Z are extracted from folder YYYYDDMM12. Time 0 comes from file gfs.t12z.pgrbf06.grib2, time 12 from gfs.t12z.pgrbf18.grib2.

Note that a new folder is created by NOAA every 6 hours. Only two of the four daily simulations are used in the game. GFS files are added gradually to latest folder during publication. Publication schedule is:

 forecast      pub time
      
  00         03:30 - 04:30Z    <-- used in games

  06         09:30 - 10:30Z

  12         15:30 - 16:30Z    <-- used in games

  18         21:30 - 22:30Z

This means forecast 0Z is already history when published at 3:30Z, and is meant to be used as an interpolation point (in time). Game wind updates are at 6Z and 18Z, two hours after GFS release.

Game wind downloads

VRTool game wind forecasts are gathered from different files from GFS forecasts. Current wind (forecast 00) is the same as the game's ( file 06 from NOAA folders 00Z and 12Z ). Future winds (12,24,36 etc) are taken from the latest files available at the time of the download.

Note that VRTool future winds (12,24,etc) are different from game's most of the time, because they are taken from newer predictions (probably better).

How VRTool downloads NOAA winds

If you are using weather routing or the track designer, you should update NOAA winds right after the following times: 4Z, 6Z, 10Z, 16Z, 18Z, 22Z (Z=GMT) and recalculate your route. Winds for 4Z and 16Z are particularly important, because the 12h forecast is an accurate preview of winds that will be used in the game two hours later. So you get 14 hours of 100% sure winds (instead of the 12 in the game app)

Getting a long-range forecast

As explained above the GFS predicts wind at 3-hour intervals up to 192h and then at 12 hour intervals up to 384h. Each GFS sequence produces Grib files with the following time-stamps:

f00, f03, f06, f09 ......f186, f189, f192, f204, f216 ...... f360, f372, f384

If you have set VRTool to import weather at 12 hour resolution it automatically extracts the appropriate Gribs from the most recent GFS sequence. If you look carefully at the table in the preceding section you can see in the third column that the time-stamp that VRTool selects changes depending on when the current GFS sequence was generated.

For example between 0400Z and 1000Z (the yellow part of the column) VRTool get forecasts from the t00z GFS run, and selects the f06, f18, f30, f42 ..... gribs from that sequence.

A consequence of this is that VRTool will not find a matching Grib beyond f192 unless it is selecting time-stamps that are multiples of 12.