Sunday, November 23, 2008
Twin Horseshoe Vortices! (6-17-08 in Nebraska)
Looking back at some photos my wife Shawna took on 6-17-08 while we were in northwest Nebraska waiting for storms to develop, I ran into these shots above that we had forgotten about. We've both seen plenty of "horseshoe vortices", but never a twin pair like this, south of Alliance. Shawna has a good eye for picking them out (I would have missed these).
There are probably several ways such vortices can form, but the most common idea is shown in the black and white explanation figure above. As a cumulus updraft builds, it may encounter rapid and strong vertical wind shear (differing wind speeds and/or directions within a short distance of height). This may cause the updraft to develop horizontal spin that, when stretched further by the small updraft, spins faster and is deformed into a "horseshoe" shape. Condensation within the cloud allows us to visually see the vortex. As dry air mixes into the cloud, it evaporates, except for the spinning horseshoe vortex, which resists dry air entrainment for a little while longer. In reality, horseshoe vortex formation is complex (otherwise they would happen all the time), but the diagram above summarizes some basics.
An estimated wind profile for Alliance is also shown above. Note the rapid and sharp wind shift from southeast to northwest at about 3000 ft above ground. This may help explain the horizontal spin that developed and stretched into "horseshoe" shapes with condensation as these cumulus updrafts on 6-17-08 encountered this sudden wind shear with height.
I don't think I've ever seen a photo of twin horseshoe vortices before. Thanks, Shawna, for catching these!
- Jon Davies 11/23/08