Unusual rotation in supercell?

[dmckemy]

I was out chasing yesterday and caught a long lived supercell that traversed across much of Western and Central South Dakota. While this supercell didn't produce any tornadoes, it had some pretty strong winds with it (see the SPC reports page). Toward the end of its life, I set up my video camera to do a time lapse and caught something that I thought was interesting... Here's the video below:

Watch video >

When I sped up the video, it seems like the lower levels of the storm are rotating clockwise, while the mid to upper levels of the updraft are rotating counter clockwise (it's easier to see it when viewing in HD on youtube). Maybe it's the angle that I'm looking at it from.... Has anyone else encountered anything like this while chasing? The shear profiles that the supercell formed in yesterday were interesting to say the least...backing flow near the surface, but veering flow aloft.

 

[Skip Talbot]

Very interesting, and nice video. Perhaps the updraft base is bowing out with bookend vortices and you're watching the anticyclonic southern end?

 

[Zachary Biggs]

That is more or less what I was thinking.
Nearly every supercell I've seen has had similar motion to its base when viewed from the south. It seems to occur when a large RFD cuts into the backside of the meso causing an anti-cyclonic rotation area on the south side and a very pronounced cyclonic rotation on the north. Or sometimes the meso simply bows out into more of a gust front.

 

[Jeff Duda]

The above hypotheses certainly seem valid. However, even when I watched the HD resolution version I didn't see any sense of clockwise rotation above the very bottom of the base. I think it could be an optical illusion and what you are seeing is simply infow coming into the storm moving from right to left while the cyclonic rotation dominates above. Since the lowest layer of the base was smooth, it was very difficult for me to discern any rotation in it at all. However, I did see some sinking motion along the tags at the bottom of the cloud. That certainly makes the RFD theory seem like a good candidate.

 

[dmckemy]

Thanks for the input! I do agree with what Skip has posted above...the storm did seem to be in an outflow stage (although to be honest, it seemed to have been in that stage for quite some time...I'm not too sure what was keeping it going!). And Jeff, it is definitely plausible that the angle I was viewing the storm from could cause that illusion to where the lower levels look like they are spinning clockwise...the video I have on my computer is a little better quality than what is on youtube, so it's a little easier to see the cloud motions near the surface, though they do seem to be moving in some sort of clockwise fashion at the surface.