Assuming it's very impractical to get down to reach the tree line; and that the wind is changing direction very often - so a setup with several sides closed would be preferred - what would be a good tarp setup for very strong, direction changing winds?
The kind of tarp I am interested into is the square one (that's what I have), 3x3 mt or 2.5x2.5 mt. Possibly, it should provide room for two persons, eventually leaving the gear out.
Thank you
If the wind is blowing from all directions, then you need to get as low as possible to the ground. Do your best to find a spot that is somewhat sheltered from the wind. The lee of a crest usually works, but if you have wind blowing from all directions then try to find a recess in the ground - a low spot where the ground that slopes up in all directions away from your bed, just make sure the ground you're setting up on won't pool water. If you can, build up a wind screen around your tarp using rocks, even if the best you can manage is a short wind screen of only an unstacked ring of rocks around your tarp.
Your priority is to seek shelter from the wind. It would be unwise to try and use your tarp to do this in such an exposed environment (not to mention deafeningly noisy). I've tried setting up a tent in the Nevada Desert with 90km+ wind gusts, personally, I think the easiest thing to do is simply lay your tarp flat on the ground, pile rocks around the edges then slip in underneath it. It's not worth bothering with poles and tie lines, you don't want any wind to get underneath your tarp.
If no rocks are available, then your best bet would probably be to use the tarp as an emergency blanket, and wrap yourself up in it, then lay on top of the edges. If you're serious about trying to set up your tarp, the configuration I'd try first would be the Mushroom fly from the illustration below, which isn't much different from my suggestion to pile rocks on the edges of your tarp and slide in under it, except that you've got a small pole in the middle.
