Sunday, December 29, 2013

Breaking Waves In the Atmosphere: The Mystery

On Christmas Day around 4 PM, a number of you in Seattle reported seeing a strange cloud pattern in the sky.  One so remarkable that the emails and smartphone images flooded in to your poor weather reporter.   The clouds looked like breaking waves on a beach...let me show you a few.



Credits, top to bottom:  Tom Ackerman, Michael Cameron, Dan Pichard

Others blogs have also noted this clouds, including Scott Sistek KOMO Weather Blog.  On that blog, he had a very nice picture taken by Dan Magden.


And perhaps most extraordinary of all, the cam on the top of the atmospheric science building caught the brief generation and dissipation of this breaking wave clouds (click on picture to view):


Now the origin of such beautiful, wave-like clouds is no stranger to meteorologists.  Known an Kelvin-Helmholtz Instability (KHI) clouds, they occur when the horizontal winds changes relatively quickly in the vertical, something we call vertical wind shear.  An objective criterion to tell us when such instability formsis dependent on both vertical wind shear and vertical stability (the tendency for air parcels to be displaced in the vertical).   Too much vertical stability and the breaking wave motions don't occur.  Too little stability, and not enough shear can build up to produce these circulations.

And, of course, you need the atmosphere to be close enough to saturation to get such clouds.

Meteorologist can use our numerical models to simulate such KHI.  You can view one by clicking on the picture below:

If you look at the photos above you will notice, that many of the waves are breaking to the left, but some...on the right hand side...seem to be breaking to the right.  This seemed really strange to me and to a number of you.  I had never seen this before.  Perhaps it is just an illusion.

So let's play detective a bit.

How high were the clouds?  Looking at probably the closest airport  to the action (Boeing Field), the reported cloud base was 6000 ft, which seems reasonable from the pictures.

What were the winds and temperatures aloft at that time?  Well, we have some good luck...an aircraft landed at Seattle-Tacoma Airport during the previous hour (3:20 PM) and took atmospheric measurements.

Here is the sounding from the plane. Heights are given in terms of pressure:  keep in mind that 850 represents about 5000 ft and 700 about 10,000 ft. Blue is wind speed and white is temperature.  In the area of the wave clouds the winds were from the west, with little change in direction.  There is modest wind shear in that layer (increasing from roughly 7 meters per second at 5000 ft to 12 meters per second at 10000 ft).  Roughly double winds in meters per second to convert to knots.


So in the layer of the clouds the wind was increasingly westerly with height, which should tend to shear the waves to the east as we see for most of the clouds.   But why did some of the clouds on the western side of the line appear to shear in the opposite direction?

Perhaps the winds over the western part of the Sound were different aloft.

But we don't have any observations there.  But wait, we do have our very high resolution WRF model.  Here are the winds forecast for 4 PM at 850 hPa (again around 5000 ft) for the central Puget Sound from that model.  The winds do appear to shift across the Sound to an easterly direction there..you can see the winds splaying out over Seattle.  The winds above the airport are much more westerly.    So one speculation is that there was a layer of opposite shear west of the city and that produced the opposite cant of the waves there.  But if there was a reversal of the shear, I can not see how there would be a continuous line of the wave clouds. So that supports the illusion theory.

   I am open for suggestions on this one....


And if you still hunger for more clouds, click on this wonderful cloud video produced by Greg Johnson.  It even has music.

http://vimeo.com/82890411

Fracking and Ozone

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5 comments:

Bob said...

Looking at the timelapse videos from the UW with finer time resolution -

http://www.atmos.washington.edu/~ovens/loops/flash2.cgi?/images/webcam2/movies/20131225.swf

- it appears to me that the "west-breaking waves" are more likely easterly moving swells that haven't "broken" yet, along with some optical illusion caused by the thinness of the cloud layer; the whole train still appears to move to the east during the video.

Very cool, our liquid atmosphere!

Trevor said...

Interesting temps in Shelton today. If the obs station is to be believed, it hit 67. Anyone live there able to confirm?

JewelyaZ said...

I noticed these clouds on Christmas day and said something to my daughter about the "Kelvin-somebody wave clouds." She told me that I'd been reading your blog too long, because they were WAY more exciting-looking clouds than my blase explanation. :-)

Happy New Year, and thanks for the mild holiday break and safe travels.

garyinseattle said...

I am skeptical of the optical illusion explanation for the point at which the waves appear to be breaking in opposite directions. I was at a position head-on to the demarcation line. At 4PM on Northlake Way near Dunn Lumber it was perfectly symmetrical like the part down the middle on a full head of hair. I was very unhappy I didn't have my camera. I'm surprised somebody in Wallingford didn't submit a great picture.

Anne Cisney said...

I have just such a photo in my own phone, which I took while uttering the words, "I wonder whether I could send this to Cliff Mass?" Lo and behold, no need. You were ahead of me. Thank you!