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November 12, 2010  

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Turbulence above thunderstorms  Thunderstorm picture

     Most pilots reading this will never fly their personal aircraft above a thunderstorm.  However, if you get the opportunity to fly in a turboprop or turbocharged aircraft, you just may be able to climb over some lower-topped convection.  Because of the turbulence hazard in the clear air above and surrounding thunderstorms, the FAA suggests pilots "avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo," and "clear the top of a known or suspected severe thunderstorm by at least 1,000 feet altitude for each 10 knots of wind speed at the cloud top." (see the FAA Aeronautical Information Manual (AIM) Chapter 7, "Safety of Flight").

     However, scientists at the Research Applications Laboratory (RAL) have investigated aircraft encounters with turbulence above thunderstorms and find that the FAA guidance is a bit naive.  One of these incidents occurred on 10 July 1997 over Dickinson, North Dakota.  A commercial turbojet aircraft encountered severe turbulence as it was negotiating a path through a number of scattered thunderstorms.  At the time of the encounter it was passing directly over a developing deep convective cloud.  In this incident, 22 passengers sustained minor injuries and the aircraft sustained enough damage to cause it to make a precautionary landing at Denver, Colorado. This aircraft encountered what is referred to as convection induced turbulence (CIT)."  This type of turbulence is common enough that the CIT acronym is well known within the commercial aviation community; it is used to describe turbulence in the clear air either above the thunderstorm top, under the anvil, or near the lateral visible boundaries.  

     Researchers were able to model the turbulence encountered by this aircraft using high resolution two–and three–dimensional numerical simulations and compared the output to observations from ground–based radar and the aircraft flight data recorder.  The simulations showed that CIT simulation animationthe turbulence above the rapidly growing thunderstorm cloud was related to breaking gravity waves above the storm (click here to view the animation).  Pilots are familiar with the most common form of gravity wave, called mountain waves or lee waves.

     The blue areas in this diagram are cloud water content (above some threshold) and the red areas are model-produced turbulence (actually referred to as subgrid turbulence kinetic energy).   The waves were shown to propagate into the
clear air above the cloud top, become unstable and break due to nonlinear interactions with its critical level.  Comparisons to the NWS NEXRAD Doppler radar echoes showed excellent agreement to the simulated clouds, and the flight data recorder vertical velocity trace was well–reproduced by the simulated gravity wave induced velocity fields above the cloud.  Further, it was shown that the vertical extent of the turbulence was related to the strength of the wind shear above the cloud top, not the wind speed as suggested by the FAA guidelines.  Most importantly, the FAA guidelines outlined in the AIM were inadequate to avoid this turbulence encounter.  Keep all this in mind the next time you are tempted to fly over a thunderstorm!

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