Zezo forum:

Post by milonga2 | 2020-11-17 | 18:21:11

Hello, so maybe it's not the place to ask this, but I still don't understand why should we care about the ITCZ for our routing.

If we sail with a simulated weather based on the 1� GFS, how can we be impacted by local weather such as storms and calms that occurs in real life in the doldrums.

Should I go straight to Fernando island as zezo tells me or do I have to look for storms etc. on windy.com as the chief meteorologist of the Vendee Globe suggests... ?

6 Comments

Post by zezo | 2020-11-17 | 18:26:53

A good question.

Problem with ICTZ is that it's unpredictable.

What you see as suggestion is based on static wind field. The router can find a route through it, but in reality it is not static. It consist of storms and local wind holes.

So the router finds a route between the holes, always seeking and finding 10 kt wind from good direction.

In (virtual) reality you set your course, go to sleep, and in the morning find yourself in the middle of a wind hole brought by the new GRIB. So you are stuck with 2 knots wind for 6 hours, and are not even sure in which direction to steer because by the time you reach the supposed wind, it will change.

That's what makes the direct route appear better than it will be. Often seen as possible route close to the African coast.

Post by milonga2 | 2020-11-17 | 20:16:33

Wow, thanks... So now I know I am not going to sleep well ! :)

Post by zezo | 2020-11-17 | 20:39:16

We've now passed the Northern ITCZ. The Southern one is different beast. Like there are no trade wind. Well, they are there, but against us and prohibit approach along the African coast.

The choice then depends on the position of the St. Helena high and the timing of the low pressure systems running around the globe. Timing is less than ideal so far, and the chances of it changing decrease as we go South and the forecast time becomes shorter term, so the chief meteorologist has a valid point ;-)

Post by zezo | 2020-11-17 | 20:20:25

That said, a route via st. Helena high can work, BUT that's a choice that was made few days ago when deciding which side to pass the Theta storm.

Changing your mind now may lose you you more positions than it could possibly gain.

On the positive side, these boats are not fast enough to outrun a Southern ocean weather system, so there may be some compression of the fleet and second chances on the way back North.

Post by milonga2 | 2020-11-17 | 20:41:58

Actually my concern right now is just not to get stuck in some calm area in the ITCZ. As suggested by the Voiles&voiliers article, I looked at the windy forecast with ECMWF model. There you can see some kind of wave of lower and higher winds. But to what extend these variations affect the VR game weather ? I don't see any holes in the particle movement in the game. I suppose we are just lucky to go through it tonight...

I am not changing my mind, I am at the eastern side of the western group, so simply heading to Fdo de Noronha I guess !

Post by zezo | 2020-11-17 | 20:51:24

There seems to be a nice flow indeed. Main problem of the shortcut is passing straight via the st. Helena High which will happen in about 10 days. Haven't used ECMWF myself, not sure how far in the future in can be trusted in those places.