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Gravity wave Monday morning

| April 14, 2009 @ 2:39 pm | 52 Replies

(Raw Doppler velocity image at 104 am CDT from NEXRAD. Areas of green scattered in the bright red indicate “folding”, or velocities greater than the radar can interpret (>57 knots, 65 mph).

The gravity wave/wake low that moved across much of Alabama Sunday night and Monday morning was one of the most significant, widespread weather events so far this year. I heard that over 150,000 Alabama Power customers were without power yesterday morning, and this event, unlike tornadoes or even floods, affected almost everyone.

We at UAH have been studying atmospheric gravity waves and their impacts on the weather, and carrying this information into NWS offices, since 2005. Gravity waves may cause tornadoes News story from AIP, Science at NASA story, produce fascinating cloud movies Watch this., initiate thunderstorms, or, as we saw the other night, cause damaging winds.

The gravity wave that came through Alabama a little over 24 hours ago was a “wake low” type event, where a gravity wave forms at the back edge of a rain mass associated with thunderstorms. There are still several theories out there to explain how this happens, but many of them are consistent, and here’s my best explanation of what happened.

At the back edge of the large rain mass, there was descending air. This could have been caused by dry air coming in aloft, getting cooled by rain, and then subsiding because it was cool and heavy. The UAH MIPS wind profiler showed downward motion then upward motion.

Downward motion causes warming in the atmosphere. But, this warm air is buoyant, so it rises back up pretty soon. This down and up motion is the wave. Sometimes air can oscillate several times up and down before it comes to rest, like waves on top of water if you disturb the surface.

The charts below (time-height sections of potential temperature and specific humidity, quantities that are conserved as air moves up and down) show how the atmosphere responded to the downward and upward motion.



Notice how, around 0440 UTC (1140 pm CDT), the lines move down, then around 0455 UTC (1155 pm CDT), they move back up? That’s the wavelike motion. Since the downward-moving air warmed up, it became lighter, lowering the air pressure below it. This caused the sudden drop in pressure seen at stations across the area. Below is a 24-hour chart of the wind speed (top) and pressure (bottom) at Tuscaloosa.


Notice the rapid drop in pressure from 30.00″ to 29.65″ in less than 2 hours. This is a larger pressure change than we often get with large cold fronts over a whole day! Normally, pressure changes occur slowly enough that the atmosphere can stay in fairly good balance. But, when the pressure changes this quickly, the air just rushes into the low.


In the above pressure chart, notice how close together the isobars are over west Alabama. The atmosphere can’t possibly come to equilibrium with this, so the air just flows toward the low pressure, or toward the WNW. That’s why, even though the disturbance came in from the NW, winds blew from the SE. And, this one moved slowly, allowing more time for the air to accelerate, causing higher wind speeds.

We continue to research waves at UAH, and Kevin Knupp and I currently have a paper under peer review that would help forecasters identify the factors leading up to a wind damage event from a gravity wave or wake low. Two of the main factors are amplitude (this one was very large) and speed (it moved slowly).

Category: Met 101/Weather History

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