Grib news vol. 3 (June 30 2003)
This is an irregular newsletter about grib weather data from Saildocs. If you wish, you can enter a subscription and a copy will be sent whenever there is an update, see below for info. Previous copies are available via Saildocs by requesting "gribnews-1" for vol-1 (Dec-2002), "gribnews-2" for vol-2 (Mar 2003). The most-recent version will always be available as "gribnews".
In March we started downloading each of the GFS/AVN model runs, every 6 hours. Other than very occasional problems with the NOAA/NCEP server that has gone smoothly. As discussed in Gribnews vol-2, the requested times are referenced back to 00:00 UTC, this "valid-time fudging" ensures consistency irrespective of when the grib is requested but can add confusion. See the last newsletter for complete details ("gribnews-2").
Recent developments include adding some additional forecast-times to each GFS download, creation of an alternative grib-prefix to avoid the fudging valid-times where it is not desired, and availability of data from the COAMPS regional model. (Some of this terminology is explained below)
Data availability: The primary grib data from Saildocs continues to be from the GFS model. We've been able to add more forecast-times (valid-times), and data is currently available at 6-hour increments out to 96 hours, and at 12-hour increments out to 180 hours. Additionally, for the 00z and 12z model-run (available at 06:00 and 18:00z) data is available out to 360 hours (15 days) on a 2.5x2.5 grid. (Remember that forecasting beyond 4 or 5 days becomes increasingly speculative). The primmary supported dataset continues to be the original 12-hour increment out to 96 and 24 out to 240. What that means is that those times have priority and get downloaded first, and then (assuming that things are working OK) the "odd" times (6,18,30 etc) are downloaded.
Valid-times: The "fudging" of valid-times to reference everything to 00:00z is intended to provide consistency for folks who do not necessarily request a grib file at the same time each day. But for those who want the most-recent data and don't care about consistency from their previous request, an alternative "gfs:" prefix can be used instead of "grib:". The syntax is identical, the only differences are that the valid-times will not be altered from the request, and data is available out to 360 hours for each model-run.
Note that Airmail's grib-request window does not yet support these new codes, but the request can be generated and then modified before sending.
Regional models: Current global models do a very good job over open water, but suffer from insufficient resolution near shore and over land. As computer technology develops the models are run on finer grids, but there are practical limits. An alternative is to take the data from the global model, and use it as input to a regional (mesoscale) model on a finer mesh with more detailed modeling of land effects. One such effort is the US NAvy's COAMPS ("Coupled Ocean/Atmospheric Mesoscale Prediction System") model, which is available for the eastern Pacific, Central Americal and western Atlantic with a resolution of 0.2 x 0.2 degree. This data has recently become available on the internet, and is available from Saildocs on an experimental basis. Currently, only the eastern Pacific region is available (29N-60N, 160W-114W), but we expect to add the others in the next few weeks. The syntax is the same as for a basic grib, except the prefix "coamps:" is used in place of "grib:", and the grid can be specified as small as "0.2,0.2". For best results, stick with 0.2 or multiples of that, to avoid interpolation.
The availability of regional models is an exciting area particularly for coastal sailors, as it has the promise of providing better predictions of near-coastal weather. (The definition of "near-coastal" varies, but generally the effects of a large landmass will be felt out to around 200 miles). Many weather services run global or regional models but unfortunately most governments have not followed the US lead in making data produced with public funds available to the public. We are fortunate and grateful, and hope that citizens of other countries will take the message home that making this information available benefits everyone.
Other news: An update for Airmail's Viewfax grib/fax viewer will be available shortly, adding support for printing gribs as well as some annotation capability.
To subscribe to this: send an email to: query@saildocs.com with the following message:
sub gribnews days=0
That will enter an indefinite subscription to the "gribnews" newsletter. General information about grib files is available by requesting "gribinfo" (or mail to: gribinfo@saildocs.com), and general info about Saildocs from info@saildocs.com.
Good sailing, and thanks for your support.
Jim Corenman
saildocs@siriuscyber.net