Sunday, May 7, 2017

Wind Shear: When the Atmospheric Seems to be Tearing Itself Apart

Sometimes the atmosphere seems to be acting strangely, with the clouds at one level moving very differently from neighbors above or below.  As described below, the change of horizontal wind with height, known as wind shear (or vertical wind shear), can produce spectacular contrasts in cloud behavior, is sometimes associated with severe thunderstorms, and can endanger aircraft.

There is no better way to be impressed with wind shear than through videos of cloud fields, and who better to provide the imagery than weather cam maestro, Greg Johnson of Skunk Bay Weather (who was spotlighted by Eric Lacitus of the Seattle Times here).

Check out this video, which shows sequences in which clouds, only a few thousand feet apart in the vertical, are moving in nearly opposite directions!

OK, now you are ready for a bit of wind shear 101.    Vertical wind shear can occur in three ways.  One is pure speed shear, where the wind speed changes with height  (see illustration below).  The other is pure directional shear, where the wind direction changes with height.  Finally, there is a combination of both, which is the most frequent situation in the atmosphere.

Now wind shear is very typical near the surface, since the rough surface (with trees, hills, building, and even ocean waves) slows down the wind.  Thus, wind speed generally increases with height above the surface.  The drag from the rough surface also tends to cause a deviation in wind direction, with a component towards lower pressure near the surface.   This variation of wind with height due to surface drag is visualized as the Ekman spiral (see below).

If you ever want to REALLY be impressed with wind shear, go out on a field on a windy day and lie down on the ground.... MUCH less wind near the surface.  You will be amazed.

Terrain can produce lots of wind shear, particularly in the lower atmosphere.  Here in western Washington, our mountains greatly influences the wind field-- blocking incoming flow, causing large whirls and eddies in the lee of barriers such as the Olympics, producing gap flows in narrow troughs in the mountains, and much more.

For example, on Friday evening at 8 PM, there was west-northwesterly flow on the coast, but northerly flow over northern Puget Sound and southwesterly flow over the south Sound--the result was one of our favorite local weather features:  the Puget Sound Convergence Zone.   But aloft the winds approaching our region from off the Pacific Ocean were westerly near the surface, turning
to southwesterly and then southerly aloft--something we know because of the radiosonde (balloon launched weather observations) observation at 5 PM at Forks, on the WA coast (see below)

Wind shear can have another origin:  from horizontal temperature gradients.  When temperature changes horizontally, it causes a tilt of pressure surfaces with height, resulting a changes in winds with height.   For example, because temperature cools to the north in the midlatitudes, there are increasing westerly winds with height.   There is an equation that describes this effect:  the thermal wind equation.

Let me demonstrate this to you.  Here is the forecast wind speed in the upper troposphere (300 hPa, around 30,000 ft above sea level) for 8 PM on April 17th.  The yellow color  shows strong winds speed heading into northern CA.
 And here is an analysis of lower atmosphere temperatures, with warm being red/orange and cool being greens and blues.)  The strong winds (the jet stream) are associated with large horizontal temperature changes (or gradients).

Finally, wind shear can be produced by the outflows of thunderstorms, as cool, moist flow rushed out from the thunderstorm cell below warm air coming from a different direction

So wind changes with height have a variety of causes, but they can produce extraordinarily interesting cloud images and videos, with perhaps the most dramatic being the effects of wind shear on falling precipitation from high cirrus clouds.  Such fallstreaks are often curved and delicate.

Finally, wind shear is a big issue for aviation.  When wind shear is large aloft, light to strong turbulence can occur.   And wind shear near the surface, usually produced by thunderstorms, can be a severe threat to planes taking off and landing.


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Eric Blair said...

you should try taking one of the short hop flights from Aspen to Denver, particularly when it's not ideal weather conditions. Like being in a paint can mixer for an hour.

John Marshall said...

The Skunk Bay time-lapse is amazing. I spend a lot of time watching clouds from my hilltop deck, but it's rare to get a true sense of the extreme variability in cloud movements and layers by watching real time.

I remain completely in awe that computer models can predict many of these movements and the associated weather. The atmosphere is so complex and chaotic and continually changing. Atmospheric science is totally awesome.

DEORTMAN said...

Yes, very interesting video of cloud layers moving at different speeds and different directions, something very commonly observed here in Seattle (although nearly as interesting as the ad that proceeds the video).

Ground wind sheer is the bane of athletic departments trying to block wind at track facilities or at baseball/football fields where wind direction can cost a team a game. Lucky that all basketball has to worry about is indoor rain.