Received: from hplms26.hpl.hp.com by opus.hpl.hp.com with ESMTP (1.37.109.18/15.5+ECS 3.3+HPL1.1) id AA128136701; Sat, 5 Apr 1997 10:58:21 -0800 Return-Path: <MilesLoyd@aol.com-DeleteThis> Received: from emout15.mail.aol.com (emout15.mx.aol.com) by hplms26.hpl.hp.com with ESMTP (1.37.109.16/15.5+ECS 3.3+HPL1.1S) id AA065666699; Sat, 5 Apr 1997 10:58:19 -0800 Received: (from root@localhost) by emout15.mail.aol.com (8.7.6/8.7.3/AOL-2.0.0) id NAA09733 for wind_talk@opus.hpl.hp.com-DeleteThis; Sat, 5 Apr 1997 13:54:37 -0500 (EST) Date: Sat, 5 Apr 1997 13:54:37 -0500 (EST) From: MilesLoyd@aol.com-DeleteThis Message-Id: <970405135436_-1905162269@emout15.mail.aol.com-DeleteThis> To: wind_talk@opus.hpl.hp.com-DeleteThis Subject: wind prediction Content-Type: text/plain; charset=unknown-8bit Content-Transfer-Encoding: 8bit
Ken Poulton describes some of his attempts to make sense of NWS data to
forecast winds:
>I did a study of the SFO afternoon forecasts several years ago and got
>about a 5 knot standard deviation of prediction minus measurement.
>In lay terms: they were wrong by more than a sail size (+-3 knots) half
>the time! Today they were wrong by 5 knots (from 3 to 5 PM).
>
>Sounds pretty bad, huh? The NWS folks say that forecasting wind in this
>area is hard because of the terrain and the strong interaction with the
>marine layer ("fog"). I suspect that a 5 knot std dev is just fine
>for most purposes and its just us windsurfers who are hyper-sensitive to
>the difference between 21 knots (big fun on my 5.4!) and 14 knots (forget
>it).
I have looked at this a bit over the past few years, too. For the most part,
I have not come up with predictions that make sense to talk about. Ken’s 5
knot std. dev. result is essentially worthless, and that is as good as I have
done. However, I have a few ideas about the subject.
I have looked mostly at the west bay from Candlestick to Palo Alto. For
windsurfing, there is an enormous difference in the sites along there. At
any time, one may be good and others poor. In fact, at any one site as one
stands on shore and looks out at the various wind lines and gusts rippling
the water, it is obvious that the variation at a single site is also an issue
to be taken into account. This is further reinforced when you sail out
through the conditions. The sort of computer codes the NWS uses for
prediction would require computers that are many times larger than anything
that exists in order to resolve the necessary detail to do good forecasts
using their methods.
The better approach to the problem is to correlate the data and forecasts
with the actual situation on the water, like Ken has tried for SFO. (Other
data sources, such as windtalkers may also be useful.} With the differences
in conditions at various sites along the west bay, good correlation of NWS
data to SFO results would be only slightly better than the present situation.
However, this correlation between NWS data and SFO results is a necessary
capability if one is to do the real problem. If one can not do that, it is
doubtful that predictions can be made at the windsurf sites.
If correlation to windsurf site conditions are ever to be made, data are
needed on the windsurf site conditions. We need a data base of actual
conditions at the sites over a period of time to try to correlate with. I
see two ways that these data might be collected. A grand array of
instrumentation would be very expensive, and I do not see any likely source
of funding. However, others might have thoughts along this line.
The other way to make such a data base is for each of us to fill out a form
and e-mail it to a central location. This would also provide data to answer
the question: “How was it yesterday?”, so there is an short term value to the
people who fill out the forms. If you also save your forms on your computer,
you get a log of your sailing.
This approach depends on enough people filling out the forms for enough sites
to make a good data base, so the present question is would YOU fill out a
form after most windsurf sessions and e-mail it out that evening or the next
morning? If you would do that, send me an e-mail saying so and telling me
what places you usually sail: not just the west bay. I will summarize the
responses and post the results. If it looks like we can make a good data
base, we will then need to work on the details.
Miles Loyd MilesLoyd@AOL.com-DeleteThis
This archive was generated by hypermail 2b29 : Mon Dec 10 2001 - 02:31:32 PST