Early this afternoon, an outflow boundary, or gust front, that was produced by storms very early this morning over southeast Missouri and northeast Arkansas, is moving through the metro area. Trailing this outflow is a set of gravity waves. I posted a pair of radar images (reproduced below) on Facebook and Twitter that clearly show the waves.
So what is a gravity wave? A gravity wave is really nothing more than a wave moving through a stable layer of the atmosphere. If you think of a rock being thrown into a pond, you know ripples migrate from the point the rock hits the water outward. An up and down motion is created as these "waves" spread out. The strongest waves are near where the rock hit the water and they get smaller as they spread out.
In atmospheric terms, something causes the air to lift - in this case, thunderstorm updrafts. In unstable air, that air continues to lift. However, if the lift occurs into an area of stable air, the tendency will be for the air to sink back downward. The first wave is created. As the now-downward moving air sinks past it's point of equilibrium (where the air will neither rise nor fall), it begins to rise again. The air is attempting to achieve a balance in the vertical. Thus, gravity waves are born. The video below is an excellent example of how these waves "look" when the stable layer includes enough moisture to produce a cloud layer. Watch the clouds undulate as they roll past the fixed camera. Truly amazing!
The converging and diverging air associated with gravity waves is easily picked up by Doppler radar if they are strong enough and close enough to the surface. Based on radar interpretation of this morning's waves, they existed between about 1,000-5,000 feet above the ground. The base of the clouds created by these waves was about 4,000 feet, according to observations from Memphis International Airport. The rising air that created the clouds also initiated thunderstorms.
So, this is exactly what happened this morning. Thunderstorms to our north sent out gravity waves in a stable layer that propagated into the metro region. Here, they encountered a more potent atmosphere - an unstable, moist, and warm one. The waves produce areas of converging air, which many times in a summertime atmosphere is the trigger needed to produce thunderstorms. Thunderstorms producing additional thunderstorms - hundreds of miles downstream and hours later!
For more information on gravity waves, check out these resources:
TheWeatherPrediction.com by Meteorologist Jeff Haby of Mississippi State Univ.
Gravity waves on Wikipedia
How gravity waves can influence tornado storms, from NASA
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