Snow and horseshoe vortices

With snow blanketing much of Western Europe at the moment, here's a chance to do some aerodynamic investigation in the field.

Somewhat counter-intuitvely, it turns out that under certain conditions snow fails to accumulate on the windward side of a tree. The reason for this is that the snow at the base of the tree is scooped out by a horseshoe vortex.

A high-pressure stagnation point forms on the windward side of the tree, and this creates an adverse pressure gradient for the flow approaching the tree. The adverse pressure gradient causes the boundary layer of the flow across the ground to detach and roll-up into a vortex. (The photo and first diagram here are taken from Internal flow: concepts and applications, p117, Greitzer, Tan, and Graf, 2004). The vortex is then bent down the sides of the tree, where it also scoops out some snow.

Similar vortices can be found on cars or aircraft at the junctions between a wing-section and the endplate or fuselage.