Clear-Air Turbulence (CAT) is at least annoying and at worst hazardous to aircraft, yet at this time very little guidance is available to aviators. In heavily traveled corridors, e.g. between Europe and North America, pilots encountering CAT inform trailing aircraft. An airborne instrument has been devised for detecting CAT several kilometres ahead (1), but there are no plans yet for commercial implementation. The instrument uses an infra-red laser beam to illuminate naturally occurring aerosols, and their turbulent motions modulate the signal they reflect to the instrument's receiver.
Model initial fields are too coarse and not enough is known about CAT to forecast its occurrence accurately (2). The US Aviation Weather Center produces experimental mountain wave turbulence forecasts and the National Center for Atmospheric Research publishes equally experimental (general) turbulence advisories. These CAT forecasts are based on two resources:
The instability in question is the Kelvin-Helmholtz instability, which occurs when the kinetic energy in a sheared flow exceeds the potential energy stored in a stably stratified layer of atmosphere. In particular, observations suggest that a stable layer of air is unstable when the Richardson's number (the ratio of the stability over the wind shear) is less than 0.25, i.e. the mechanical wave production exceeds the buoyancy damping by a factor of 4. CAT is often observed in the vicinity of jet streaks, where the vertical wind shear is large.
Observations show that CAT is common in anticyclonic flow (2). This may be because flow around an upper-level ridge is super-geostrophic (Note 12.D), therefore enhancing the vertical wind shear. Another explanation is that anticyclonic vorticity is inertially unstable. This instability occurs when the centrifugal and pressure gradient forces exceed the Coriolis force. It leads to secondary circulations which offset the geostrophic balance. In an attempt to restore geostrophic balance ('geostrophic adjustment'), the layer of air may produce gravity waves.
(1) Anon 1998. Sensor reveals Clear-Air Turbulence in time to issue warnings. Bull. Amer. Meteor. Soc. 79, 1126-7.
(2) Knox, J.A. 1997. Possible mechanisms of Clear-Air Turbulence in strongly anticyclonic flows. Mon. Weather Rev. 125, 1251-9.