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Board » Technical Discussion » Gribs

Posted by
Jan van der Puil

Users avatar

Posts: 58
Registered at: 2013-08-06 21:35:15

2019-03-06 07:45:49 - Gribs

When the Sardinia Cup opened yesterday, some racers noted that the wrf gribs outside the racing area and out to the weather system looked odd, and they do, and they always have done. As we do not sail there and the software that modulates the synoptic forecasts to generate local so-called 'meso' effects has to do a lot of 'crunching', the wrf gribs that are fed into SOL only cover the racing area. The rest of the weather that you see on SOL or that you download from SOL via brainaid or AGL is generated by the SOL bilinear interpolator. Since between the race area boundary and the weather boundary there is only one piece of data available for interpolation, that being the data at the race area boundary, the result of this interpolation is constant wind out to the weather system boundary and this looks odd especially if the weather system boundary is a long way 'out'. Those of you mathematically-minded may well wonder why does the weather not reduce linearly to 0 at the edge, then. Well, because that's not how the algorithm is written.
Some of you may also wonder why we show weather out beyond the race area at all. The reason for that is that the predictor line(s) pointing out from your bow will go haywire if it/they run(s) out of wind in less than six hours ahead. So we have to have wind six hours ahead at your boat's Vmax at right angles out from the boundary.
As said, it has always been thus, from the very first time we used wrf for the Boston to Newport and then for such iconic SOL perennials like Pacific North West and The Vineyard and all our Sprints including those in 60kn DN's requiring gribs extending across vast areas of frozen forests, prairies, tundras and the like.

Clipping along

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