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!

10 comments:

Wondering K said...

oh that's gorgeous! I'm from the midwest and used to go chasing after the darn things when I worked at a tv station....I miss the big scary summer storms!

Justin said...

It really looked like one of them whirled quite low on the west side of Tiger Mountain. Either that or a mini-microburst?

JewelyaZ said...

Sweet! Sure gets the scared-of-tornadoes-after-living-in-North-Carolina heart to beat a little bit faster!

I also really enjoyed watching the people climb up and down the side of the building next door, and all the motion on the roof. :-)

I like the idea of a video channel. Thanks

The General said...

I was looking out of the 15th story window of my wife's office in the Metropolitan Towers (the "keyboard buildings") at 4:45, and saw two or three similar rotating cloud formations around Queen Anne. It's neat seeing this post, because I even commented to my wife "looks like little tornadoes trying to form."

Gary said...

Cliff says "In a convergence zone there is a lot of horizontal wind shear . . . "

Why is it called "horizontal" shear? Aren't the two airstreams flowing past each other in the vertical plane and not one over another in a horizontal plane? The rotation develops around a vertical axis, too. Could someone please explain?

Kevin Purcell said...

Gary asks "Why is it called "horizontal" shear?"

Because there is a wind velocity gradient (i.e. a change in speed and/or direction) in a horizontal plane.

It's easier to think of gradients of wind velocity than two slabs of air moving pas each other.

Gary asks "Aren't the two airstreams flowing past each other in the vertical plane and not one over another in a horizontal plane?"

The two flows are in the same horizontal plane "beside each other".

Clearly there has to be a gradient (shear) where they "meet". And this gradient is in that horizotnal plane: hence the name.

How large the gradient is influences how much rotation is seen.

It doesn't have to be two flows against each other it just has to be a gradient in the wind field so at a given location one side has a higher velocity (and a given direction) than the other.

Think about a clockwise rotating mass of air: it has horizontal wind shear (a wind gradient) because the air flow on the north edge is to the right and the air flow on the south edge is to the left. Even if the speed of the wind is the same the directions are different (so there is shear/grsdient). As you move to the center of the rotation the wind velocity will drop (that's shear too because there's a gradient in speed).

Now you can see why it generates rotation about a vertical axis. For a one side being pushed more in one direction than the other side so the air rotates.

Winds gradients make rotations; rotations make wind gradients.

Vertical wind shear: is a gradient in flow (speed and/or direction) in the vertical direction.

That can give rise to hoirzontal rotation e.g. rotors in mountain waves.

deanchurchill said...

Am looking at GFS forecast for PacNw beginning around Feb 16. See large, cold upper low, moisture, a train of shortwaves coming down the Alaska coast, lasting for days. What's the common word for solid precipitation?

Bham_Guy said...

Above 1000'? Snow

Gary said...

Response to Kevin, just above:
"It's easier to think of gradients of wind velocity than two slabs of air moving pas each other."

That's really the crux of it--thank you. And "slabs" is the word I was looking for. I was thinking of plates or slabs moving with respect to each other (geology).

[shear : to subject to a shear force b : to cause (as a rock mass) to move along the plane of contact]

So . . . the watchword is "gradient"!

snapdragon said...

deanchurchill,
Don't snowtease me, man. I don't think I can handle my hopes being dashed yet again this "winter".