Gravity Waves

[Alex Whittingham]

From New Scientist, 11 Oct:

The existing method of warning pilots of CAT, called Graphical Turbulence Guidance, relies on pilot reports and observations of the atmosphere, including lightning data, but is not particularly accurate. To improve on these predictions Williams and his colleagues decided to focus on the cause of the turbulence: the gravity waves generated at the boundary of fast-moving high-altidude jet streams with slower moving air. Their model uses wind speed measurements to predict where these boundaries lie, and so where the gravity waves are likely to be strongest.

After initially thinking they were talking about fluctuations in the space-time continuum I realised the reference was to fluid dynamics. Done a bit of googling and I can understand the idea of gravity waves initiated by terrain but I'm still not getting the idea of gravity waves induced by jetstreams. Anyone come across this?

[Fitter2]

It's complex enough to need computer modelling, but essentially if you get an unstable layer below a stable one, any random vertical movement is amplified by energy fed between the layers.

The disturbance propogates downwind as a wave which can acheive vertical velocities of several thousand feet/min.

[criticalmass]

The term was also used in an ABC documentary on research into massive thunderstorms that develop around Darwin, Australia. A "gravity wave" was set up when the core of a large thunderstorm collapsed under its own weight, placing in an undulating motion millions of tonnes of the surrounding air and giving rise to extreme clear air turbulence.

[FullWings]

There's also the "Morning Glory" - a special sort of gravity wave called a "soliton": morninggloryaustralia... (http://www.morninggloryaustralia.com/)

With regard to jetstream waves, you get "Kelvin-Helmholtz Instability" developing sometimes in areas of strong shear, producing billows and breaking ripples which can cause severe CAT: K-H Instability... (http://en.wikipedia.org/wiki/Kelvin–Helmholtz_instability)