|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.