Gravity waves from Missouri storms result in Labor Day showers in the metro

Pop-up showers around mid-day today were a bit of a surprise, as they were not indicated by computer models early this morning and, according to our forecasts over the weekend, Labor Day appeared to be a dry day. However, an event currently not easily "forecast-able" by the models was the cause for these showers - a set of gravity waves!

Sometimes these atmospheric waves can be detected in radar imagery, but if conditions are just right, they are more easily detected using visible satellite imagery, as was the case this morning. The image below, taken at 8:30am, shows an arc of clouds from OK through AR, into western TN and KY. This arc is the outflow boundary from the MO storms. However, the outflow was accompanied by a series of gravity waves as well, most clearly visible in OK in the circled region (the technical term for this phenomena is an undular bore). Click here for an animation of this satellite imagery clearly showing the waves propagating away from the storms between 7-10am CDT (animation download is large and recommended for high-speed internet users). Also, the waves can be faintly seen in this (large) radar loop from about 6am-10am this morning around Oklahoma City, Little Rock, and over east AR from the Memphis radar.

An outflow boundary from decaying thunderstorms in KS/MO moved across OK, AR, TN, and KY this morning as shown in the satellite image taken at 8:30am. Gravity waves were also present behind this outflow, best seen in the circled area in OK. Click here for a satellite loop clearly showing the waves propagating away from the storm complex (Warning: VERY large animation - best with high-speed connections).

So what conditions were responsible for the waves that occurred this morning and why did showers result in the Mid-South?

Just as when a pebble is thrown into a calm body of water, waves in the atmosphere spread out from a large disturbance such as a thunderstorm complex. In meteorology, these waves are called "gravity waves" and they form when the wave(s) are trapped in a stable layer of the atmosphere, rising like the crest of a wave on the water, then being pulled back towards earth by gravity. As long as the stable layer of air exists, the waves continue to propagate.

The waves are visible on satellite imagery when they move through an area of saturated air that exists in or just below the stable layer. Rising air from the wave causes clouds to form in the saturated air and sinking air promotes drying or clear skies. The alternating rising and sinking air result in cloud bands that form perpendicular to the direction the wind is blowing at that level (transverse bands). Satellite imagery is the best way to see these bands of clouds that are the result of the gravity waves!

The rising air from the waves can also be strong enough to promote precipitation from the resultant clouds, especially when they encounter less stable air, as was in place over the Mid-South at mid-day today. The showers were the result of the rising air in the gravity waves. Since we didn't know these waves would occur, nor that they would arrive as the atmosphere was destabilizing, we didn't have a good handle on the formation of showers either. As the waves dissipated, the showers ended by early afternoon.

Erik Proseus, MWN Meteorologist

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Gravity waves move across the Memphis metro

Gravity waves are captured on radar as they move through the metro on 2/21/13 around 7:30pm

Earlier this evening, NEXRAD radar picked up a set of low level gravity waves as they moved through the metro head of a cold front.  A side-by-side radar screenshot is shown below with the waves marked in the velocity (wind) data.  The waves existed in the lowest ~1,000' of the atmosphere.

Gravity wave are basically vertical motion, or "waves," in a stable layer of air. Think of ripples created in a still pond when you throw a pebble in. Water is displaced by the pebble and mini waves are created.  As teh ripples, or waves, propagate out, they become smaller as equilibrium is reached. 

Atmospheric waves are vertical displacement of air up and down caused by something that forced the air to rise in the first place (typically thunderstorms). Even after the source of the waves (i.e., the thunderstorms) dissipate, the waves can propagate downstream. Just like ripples in a pond that get smaller as they propagate out, gravity waves eventually get weaker and dissipate altogether.  

The waves are identifiable on radar because they cause the wind to shift direction at the level where the  waves exist.  In tonight's case, the waves existed from very low levels up to about 1,000' and, in fact, the wind direction switched from east to west and back to east again at MemphisWeather.net as they passed over (over many minutes).  These rising portion of these waves also can cause clouds and precipitation, while the sinking air promotes clear (or less cloudy) skies, just like low pressure and high pressure, respectively.  Looking at the precipitation panel in the image above (left side), you can faintly make out "waves" of enhanced reflectivity as blues and greens.  These enhancements are clouds and light rain caused by the rising air associated with the waves!

So, next time "gravity waves" are mentioned in weather discussion, you'll know what they are! Here's another post that explains how gravity waves caused thunderstorms to erupt over the metro on July 4, 2011, as well as a cool video of waves in action in a cloud timelapse.

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Well-documented pair of gravity waves move over the Memphis metro

A set of gravity waves (click link for a description of gravity waves) trained over the Memphis area between 7:00-8:00 this morning. The waves were visible on radar imagery as well as visually - as a pair of linear cloud features that moved quickly by from north to south at about 1,000 feet.

Below is a short video I took of the waves passing over Bartlett (with narration), as well as several photos submitted by Twitter and Facebook followers.  Thanks to all those who submitted their pics!





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Gravity waves: an explanation and what happens when they interact with an unstable atmosphere

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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