Weather analysis

How can we calculate the ETA in les Sables d'Olonne?

Ensemble routings January 6th 2017
© Great Circle

© Great CircleWith the first competitors about to return in the Northern hemisphere, many raise the question of the date of arrival. What is the ETA (Estimated Time Arrival) for the boats in Les Sables d'Olonne?  Marie-Martine who is a medium in Les Sables d'Olonne said it would be January 16th.

In  practice, the weather models diverge beyond 7 or 8 days. It is therefore not easy to establish an ETA for an arrival planned in 10 to 13 days. To rationalize things, we use the "combination models". For an available model, the Americans calculate 20 variations of the model and the European Meteorological Center 50 variations of the model. They modify the initial conditions and look at what happens. It is what we also call the butterfly effect. The legend says that the beating of a butterfly's wing can generate a depression 10 or 15 days later. It is probably a little exaggerated, but it is true that a higher temperature of the Gulf Stream or a little stronger cold spell in Canada is going to have consequences on the depressions which are going to form in the North Atlantic. By changing these initial conditions for the whole world, we obtain different models for the long term.

In theory, every model has the same probability as the others of occuring. By routing the competitors with all the available models, we obtain a set of routes. We can then establish statistics with a distribution, as mathematicians do, around the date of arrival. It turns out generally to be rather accurate.

There are two limits in the exercise. The first one is that only the skippers have the exact polars of their boats, even if after 60 days we begin to have a good idea of the performances of each of the boats. The second is that we do not know the breakage that they have on the boats and how it can affect the performance.

With the models of January 6th, 2017, the probability is high that the winner will finish between January 16th and 18th.

Christian Dumard and Bernard Sacré / Great Circle

© Great Circle© Great Circle

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