Difference between revisions of "NOAAWinds"

Revision as of 05:43, 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

Each 6-hourly GFS cycle 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 generated by a weather simulation program, using laws of physics and inputs from instruments. 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:

  cycle      pub time
      
  t00z       03:30 - 04:30Z    <-- used in games

  t06z       09:30 - 10:30Z

  t12z       15:30 - 16:30Z    <-- used in games

  t18z       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 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 long-range forecasts

The NOAA GFS produces long-range Gribs out to 16 days in the future. VRTool can import these, but only during the following hours:

• 0100-0400 UTC
• 1300-1600 UTC

The reason for this is as follows: Each 6-hourly GFS run predicts wind at 3-hour forecast intervals up to 192h and then at 12-hour forecast intervals up to 384h. So each run produces a sequence of Grib files with the following time-stamps:

 f00, f03, f06, f09 ...... f189, f192, f204, f216 ...... f360, f372, f384
|<--------3-hour intervals----------->|<--------12-hour intervals------->|

When VRTool is set to import weather at 12-hour resolution it automatically chooses the appropriate Gribs from the current GFS run. If you look carefully at the table in the preceding section, in the third column you can see that the time-stamps that VRTool selects change depending on which GFS run is being used.

For example between 0400Z and 1000Z (the yellow part of the column) VRTool uses the t00z GFS run, and chooses the f06, f18, f30, f42 ..... gribs from that sequence. Beyond f192 VRTool will not find matching Gribs unless it is selecting time-stamps that are multiples of 12.

This means that forecasts beyond f192 can only be imported only when the t00z or t12z GFS run is current (during the green and blue parts of the table). A further restriction is that the f384 grib only becomes available once the GFS run is fully completed.