Gravity waves are global events. Much like the ripples on a massive pond, these large-scale waves can propagate from an atmospheric disturbance over thousands of miles. These waves are maintained by the gravitational force of Earth pulling down and the buoyancy of the atmosphere pushing up. Until now it has been hard to link atmospheric gravity waves with other atmospheric phenomena, but new research suggests that gravity waves passing over storms can spin up highly dangerous and damaging tornados… Suddenly gravity waves become very important and may help to forecast where and when tornados may strike…
In a nutshell, meteorologist Tim Coleman of the National Space Science and Technology Center in Huntsville (Alabama) sums up what gravity waves are:
“They are similar to waves on the surface of the ocean, but they roll through the air instead of the water. Gravity is what keeps them going. If you push water up and then it plops back down, it creates waves. It’s the same with air.” – Coleman
A large number of things may cause gravity waves (not to be confused with gravitational waves, the ripples in space-time), including intense disturbances caused by storm systems, a sudden change in jet stream location or wind shear. The strong oscillation will then travel for hundreds or even thousands of miles.
See a gravity wave in action over Iowa…
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Far from gravity waves being a mild curiosity, it seems that they have a large part to play with other atmospheric dynamics down here on the ground.
Tim Coleman and colleagues have found that the passage of gravity waves over the top of storms could intensify or even create tornados. It is all down to the angular momentum of the spinning storm. When storms are large, they slowly rotate. If for some reason they shrink in scale, the spin will increase (imagine an ice-skater spinning on the ice with her arms outstretched, as she brings her arms in, she spins faster). This is the fundamental rule of angular momentum conservation, as the size of a storm contracts, the faster it spins. Ultimately, if the conditions are right, intense tornados can be generated, a huge amount of angular momentum in a tiny volume.
Now gravity waves are believed to have a part to play. As they pass over a storm, the pressure of the overlying gravity wave propagation will compress the storm. As this occurs, a vast amount of angular momentum is forced into a smaller volume. The seeding of baby tornados is therefore possible. Gravity waves also come in sets; one wave will follow another, each periodically compressing the storm, intensifying tornado generation.
So keep your eyes peeled for incoming gravity waves during a storm… tornados may spin to life…