Wednesday, November 16, 2011

Altostratus Undulatus (wavy clouds!) visit the Mid-South

This afternoon as the weather system that has been plaguing the Mid-South for a couple of days was departing, atmospheric conditions came together to produce some stunning clouds.  After researching the conditions, the pictures received, and observing the clouds firsthand, I came to the conclusion that these were undulatus clouds, or more specifically Altostratus Undulatus.  You'll notice the word "undulating" in the name and that is exactly what they looked like.  As my 6-year-old astutely described them, they looked like the ocean, only up in the sky!  Below is a short video clip and a couple of pictures I took of the "wavy clouds" near Memphis International Airport this afternoon.

Pics and video above of the undulatus clouds as seen at Memphis International Airport. All  were taken by  MWN meteorologist Erik Proseus.
Being able to see undulatus clouds is fairly rare, even though the conditions that cause them are not. There are several factors that seem to come into play to produce visible undulatus.  The presence of abundant moisture at the right level in the atmosphere to create clouds in the first place is obviously important. Without moisture, there are no clouds and any atmospheric waves slip by unnoticed. Second, a trigger for the rising air is needed - in this case a weak upper level disturbance or set of gravity waves moving by could very well have been that trigger. Finally, to get the wave motion, the rising air needs to be forced back down and the cycle repeated (upward motion, then downward motion of the air). This is accomplished with a "stable layer" in the atmosphere.

Think of it in this manner - if a pond is undisturbed by wind, animals, bugs, or any other external force, it is perfectly flat (stable). Drop a pebble in the pond and ripples move out from the point where the pebble was dropped in. The water rises and falls creating miniature waves.  It doesn't keep rising; it doesn't keep falling. The waves move out from the center and try to regain equilibrium, or stability, which is the flat and still pond. In the atmosphere, rising air reaches a stable point, gets forced back down once it rises above that stable point, then starts rising again, over and over as it attempts to regain equilibrium.

The atmospheric cross-section (or profile) from 3pm this afternoon is shown below.  The red line is temperature, the green line is dewpoint (or moisture), the ground is at the bottom of the picture, and as you go up in the image, you are going up into the atmosphere. Wind is shown on the far right.  This cross-section shows a stable layer (or inversion base) at about 3,000'.  It also happens to be where the red and green line meet, representing 100% humidity and the presence of clouds.  In today's case, the upper level disturbance or gravity waves moving through triggered rising air, which got to about 3,000 feet, reached the stable layer, and then rose and fell around that inversion base or stable layer - thus "ripples" or waves in the atmosphere. We wouldn't have "seen" those ripples except that that level was also where the moisture existed (the clouds).  The stable layer separates wind above it that was blowing from the west and wind below it that was blowing from the north to northeast.

Atmospheric sounding from the NAM forecast model for 3pm CST. Details described in text above.
Here's one more interesting piece of evidence regarding the waves.  Check out the radar loop below during the same time frame.  There are a series of bands of light rain moving from west to east and oriented northeast to southwest. This could very well be the "triggering mechanism," i.e., the upper-level trough or gravity waves, reflected in some bands of light precipitation near the surface.  A very interesting weather phenomena today no doubt!

If you snapped any pics or video of the clouds, feel free to send them to our Twitter account (@memphisweather1), put them on our Facebook wall (link below), or send them to photos MemphisWeather net.

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