Monday, February 7, 2011
Rotating Clouds in a Strong Convergence Zone
There was lots of excitement in my department around 4:45 PM today!
A strong convergence zone had set up this afternoon in cool, unstable air--in fact, there were multiple convergence zones evident in the radar at 4:47 PM as shown here:
One convergence zone extended from the mountains of Vancouver Island and a double feature was downstream of the Olympics. The precipitation was highly convective with ice pellets observed at some locations and heavy rain at others.
I was looking out the window and saw complex tendrils extending down out of the convective cloud and a lowering and thickening of the lower portion of the cloud base reminiscent of a wall cloud. (see image at top). I rushed to the corner office of Professor Robert Houze, a national expert on convection. We watched the clouds hanging down form the cloud base and they were CLEARLY rotating. (The action was happening south of the UW towards Capitol Hill.) I could not see a distinct funnel. We did notice a few other similar features evident nearby.
Here is a video (click on the image, and ignore the request for registration if you get it--hit the x, trying a new utility that saves all my videos for you to see later)
You can see the feature I am talking about, but it is not as good as being there. Look carefully around 4:48 PM (16:48) and for the next minute.
Where does the rotation come from? In a convergence zone there is a lot of horizontal wind shear between southwesterlies on the south side and the northerlies or northeasterlies on the north side. Here is a Doppler image for the same time as the radar image shown above...look at the sharp color change between the University district and downtown--that is indicating shear.
Image putting a pinwheel into that area of shear, with southwest flow on one side and NE flow on the other...IT WOULD ROTATE. Now the heavy rain and hanging cloud filaments were associated with the updraft of the convective cell, an updraft that increases with height. This tend to stretch the nascent rotation from the shear, amplifying it into a more intense vortex. This process, described in great detail in my book, is similar to the speed up of an ice skater as he/she brings in their arms starting with a weak rotation...the skater goes faster and faster!
Convergence zone rotation and funnels are not rare around here...happens pretty much every year...and is rarely dangerous. One time it even picked up some kids at a preschool in West Seattle. They were fine for their airborne adventure.
UPDATE: Here is a picture from Jessica Clawson, an attorney located in the Columbia Tower, of what looks like a descending features from the cloud base. She thought it was rotating too.Bottom Line: You don't have to go to Kansas or Oklahoma to see rotating clouds!
Posted by Cliff Mass at 6:27 PM