Gravity Wave| weather phenomena explained

Eric Frei · 22nd April 2009, 11:20 PM

In a news story many trees and power lines down by what is termed a "gravity wave".

The news story is here: Gravity wave leaves fallen trees, power lines - WAFF.com: North Alabama News, Radar, Weather, Sports and Jobs-

Quote:

GUNTERSVILLE, AL (WAFF) - Four counties in north Alabama are reporting damage after an overnight severe weather event called a gravity wave.

Morgan and Madison counties had trees uprooted and power lines down. The most significant damage was in Jackson and Marshall counties, where power has been out for most of the day.
Trees are blocking major roads, and 90-percent of one campground has been destroyed.

A hard hit area in Jackson County left dozens without homes and damaged campers and other property. People there have described the area as a war zone. Crews and homeowners have worked feverishly since 4 a.m. Monday morning to get it cleaned up.

As straight-line winds whipped through Jackson County Park, 92 of the 162 campers split in two when nearly 100 100-foot trees came crashing down.

So what is a gravity wave?

Source: GRAVITY WAVE DEFINED

Quote:

METEOROLOGIST JEFF HABY

The word "gravity" in the word gravity wave can make the term more confusing than it really is. It has little to do with having a special relationship with gravity. ALL air motions are influences by gravity. Once the word gravity is eliminated, all that is left is the word wave. Air can have one of two motions, which are either STRAIGHT or WAVE. These waves can be vertical or horizontal. When you look at a 500-millibar chart with the troughs and ridges you are looking at horizontal waves (waves on a more or less horizontal plane).

A gravity wave is a vertical wave. The best example I can think of in describing what a gravity wave looks like is to think of a rock being thrown into a pond. Ripples or circles migrate from the point the rock hits the water. An up and down motion is created. With increasing distance from the point where the rock hit the water, the waves becomes less defined (the waves are dampening).

Now let's look at what a gravity wave is in the atmosphere. To start a gravity wave, a TRIGGER mechanism must cause the air to be displaced in the vertical. Examples of trigger mechanisms that produce gravity waves are mountains and thunderstorm updrafts. To generate a gravity wave, the air must be forced to rise in STABLE air. Why? Because if air rises in unstable air it will continue to rise and will NOT create a wave pattern. If air is forced to rise up in stable air, the natural tendency will be for the air to sink back down over time (usually because the parcel forced to rise is colder than the environment). The momentum of the air imparted by the trigger mechanism will force the parcel to rise and the stability of the atmosphere will force the parcel of air to sink after it rises (you have now undergone the first steps into creating a wave).

It is important to understand the concept of momentum. A rising or sinking air parcel will "overshoot" its equilibrium point. In a gravity wave, the parcel of air will try to remain at a location in the atmosphere where there are no forces causing it to rise or sink. Once a force moves the parcel from its natural state of equilibrium, the parcel will try to regain its equilibrium. But in the process, it will overshoot and undershoot that natural position each time it is rising or sinking because of its own momentum. At a sufficient distance from where the trigger mechanism caused the parcel to rise, the intensity of the gravity wave will decrease. At increasing distance, the parcel of air becomes closer to remaining at its natural state of equilibrium.

In a gravity wave, the upward moving region is the most favorable region for cloud development and the sinking region favorable for clear skies. That is why you may see rows of clouds and clear areas between the rows of clouds. A gravity wave is nothing more than a wave moving through a stable layer of the atmosphere. Thunderstorm updrafts will produce gravity waves as they try to punch into the tropopause. The tropopause represents a region of very stable air. This stable air combined with the upward momentum of a thunderstorm updraft (trigger mechanism) will generate gravity waves within the clouds trying to push into the tropopause.

And a Youtube video:

You Tube

Jeff Darby · 23rd April 2009, 11:59 AM

Very interesting, you would not notice this with out the video footage. I wonder if the air waves match the swell out to sea?

Eden Keeper · 5th December 2010, 06:14 AM

This extreme weather event was later said to be a wake low rather than a gravity wave, but few people heard the correction. Most of the damage was concentrated in areas where the trees had insufficient roots due to either overcrowding (mostly pines) or a shallow water table such as in the flood zone near the lake. Miraculously there were direct no injuries much less deaths; on a whim at 8:00 pm the evening before my uncle decided drive an hour+ to see his grandsons rather than stay in the campground where his camper was destroyed during the pre-dawn storm where one of the trees that hit it fell across his bed.

Since then it seems like everybody in this area that owns both a chainsaw and a bucket truck thinks they are qualified arborists. I describe the work done by most of them (specifically the illegitimate and obscene practice of topping trees advertised by 67-80% of them) as tornado chic since the results are aesthetically indistinguishable from the damage (to the trees left standing) that it is intended to prevent. (A 6 year old's reaction to a topped shade tree at his grandparents, "Why did you do that for it looks awful... if it blows down now it will punch a bunch of holes in the roof.") The low bid contractor to remove storm debris in the County Park got it because no license, etc. was required if the bid was less than $50,000 (USD), but to keep it legal a judge effectively waived $80k-100k in County Landfill dump fees for the stumps (rather than using them as fish reefs), but hopefully not the limbs (since the city sells the chips), thus the contractor was paid to salvage timber including nearly all of the pines that were still standing (about half total the forested area of the park) that any lumberjack would have paid to get to harvest. As a result these rip-off artists have wiped out virtually all private and public budgets for landscaping.

I've considered starting a thread about an indirectly related near miss accident, but decided to describe it here instead. A recent widow afraid of another storm was having all the pine trees in her yard cut. She picked a tree service based on seeing one "good" job they did, but for her they cut one and left the mess. After she complained for a couple months (no explanation for not returning her calls) they finally came back to finish the job. This time one of the trees fell on the AC unit, but unbelievably missing both the corner eave of the house (perhaps there was a kickback after it got below the eave) and a worker by inches. The root causes of the tree pivoting on the stump include a narrow (and downward angled) notch, little to negative hinge step in the (slightly angled) backcut, and cutting through the hinge, while the worker nearly hit was probably pulling harder on one of the ropes being used to initiate, but obviously failing to control the direction of fall after it went slack. Whoops, they also lied about being insured. Her resulting out of pocket cost to replace the AC (during one of the hottest weeks this year) resulted in my job of replanting safer ornamental trees being indefinitely postponed or canceled. (Other landscape jobs I've lost due exclusively to the HVAC company is a story for another forum/thread, but the short version is leaky exhaust pipes terminated with a funnel that looks like a rain cover.)

Eric Frei · 5th December 2010, 09:10 AM

Interesting

YouTube - Dr. Greg Forbes Describes Wake Lows and Jumps Rope

23rd April 2009, 11:59 AM	#2
Jeff Darby Moderator - Previously known as JayD Join Date: Feb 2007 Location: TreeWorld, Sydney Australia Posts: 2,030	Re: Gravity Wave\| weather phenomena explained Very interesting, you would not notice this with out the video footage. I wonder if the air waves match the swell out to sea? __________________ Member: Australian Tree Association Join the Australian Tree Association...Have your voice heard ! Arboriculture, A life long study for some, a passing phase for others © Jeffrey J Darby 2011

5th December 2010, 06:14 AM	#3
Eden Keeper Semi-mature vigorous tree Join Date: Dec 2010 Location: Alabama Posts: 116	Re: Gravity Wave\| weather phenomena explained This extreme weather event was later said to be a wake low rather than a gravity wave, but few people heard the correction. Most of the damage was concentrated in areas where the trees had insufficient roots due to either overcrowding (mostly pines) or a shallow water table such as in the flood zone near the lake. Miraculously there were direct no injuries much less deaths; on a whim at 8:00 pm the evening before my uncle decided drive an hour+ to see his grandsons rather than stay in the campground where his camper was destroyed during the pre-dawn storm where one of the trees that hit it fell across his bed. Since then it seems like everybody in this area that owns both a chainsaw and a bucket truck thinks they are qualified arborists. I describe the work done by most of them (specifically the illegitimate and obscene practice of topping trees advertised by 67-80% of them) as tornado chic since the results are aesthetically indistinguishable from the damage (to the trees left standing) that it is intended to prevent. (A 6 year old's reaction to a topped shade tree at his grandparents, "Why did you do that for it looks awful... if it blows down now it will punch a bunch of holes in the roof.") The low bid contractor to remove storm debris in the County Park got it because no license, etc. was required if the bid was less than $50,000 (USD), but to keep it legal a judge effectively waived $80k-100k in County Landfill dump fees for the stumps (rather than using them as fish reefs), but hopefully not the limbs (since the city sells the chips), thus the contractor was paid to salvage timber including nearly all of the pines that were still standing (about half total the forested area of the park) that any lumberjack would have paid to get to harvest. As a result these rip-off artists have wiped out virtually all private and public budgets for landscaping. I've considered starting a thread about an indirectly related near miss accident, but decided to describe it here instead. A recent widow afraid of another storm was having all the pine trees in her yard cut. She picked a tree service based on seeing one "good" job they did, but for her they cut one and left the mess. After she complained for a couple months (no explanation for not returning her calls) they finally came back to finish the job. This time one of the trees fell on the AC unit, but unbelievably missing both the corner eave of the house (perhaps there was a kickback after it got below the eave) and a worker by inches. The root causes of the tree pivoting on the stump include a narrow (and downward angled) notch, little to negative hinge step in the (slightly angled) backcut, and cutting through the hinge, while the worker nearly hit was probably pulling harder on one of the ropes being used to initiate, but obviously failing to control the direction of fall after it went slack. Whoops, they also lied about being insured. Her resulting out of pocket cost to replace the AC (during one of the hottest weeks this year) resulted in my job of replanting safer ornamental trees being indefinitely postponed or canceled. (Other landscape jobs I've lost due exclusively to the HVAC company is a story for another forum/thread, but the short version is leaky exhaust pipes terminated with a funnel that looks like a rain cover.)

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