CLEAR AIR TURBULENCE 



and other means (perhaps laser tech- 

 niques). Such a study has been 

 recommended by the U.S. Committee 

 for the Global Atmospheric Research 

 Program. 



Specific Questions About 

 Clear Air Turbulence 



The major questions concerning 

 clear air turbulence now requiring 

 answers fall into three groups: 



First, questions concerning the 

 origin or onset of clear air turbulence: 



1. Is clear air turbulence generally 

 the result of a particular fluid- 

 flow instability? If so, what are 

 the crucial parameters of the in- 

 stability? 



2. What are the typical atmos- 

 pheric features in which clear 

 air turbulence occurs and how 

 is their structure related to the 

 parameters of fluid instabilities? 

 (Of particular interest are the 

 relationships to vertical wind 

 shear, horizontal temperature 

 gradients, and the Richardson 

 number.) 



3. Are other small-scale proc- 

 esses, examples being gravity 

 waves or local heating, impor- 

 tant in the formation of clear 

 air turbulence? 



Second, questions concerning the 

 evolution of clear air turbulence: 



1. What is the precise evolution 

 of the atmospheric variables at 



various scales during an out- 

 break of clear air turbulence? 



2. How can this evolution be most 

 economically summarized or de- 

 picted? 



3. What are the temporal charac- 

 teristics of the transport of 

 momentum and heat, the flux 

 and dissipation of energy, and 

 the stress imposed on the 

 larger-scale flow during an 

 outbreak of clear air turbu- 

 lence? 



4. What are the relationships be- 

 tween processes occurring in 

 one part of a patch of clear 

 air turbulence and those of 

 another? Are there relation- 

 ships between apparently dis- 

 tinct patches of turbulence? 



5. What characterizes the termina- 

 tion of an outbreak of clear air 

 turbulence? What scars does 

 turbulence leave in its environ- 

 ment? 



Third, questions concerning the 

 implication of clear air turbulence: 



1. How often do patches of clear 

 air turbulence of various sizes 

 and intensity actually occur in 

 various regions of the atmos- 

 phere? 



2. What is the usual intensity of 

 turbulence in the free atmos- 

 phere in regions in which flight 

 is sensibly smooth? 



3. How important is cleai 

 bulence — quantitatively - 

 the atmosphere's energy cycle 

 compared to regions with the 

 usual intensity of turbulence 

 in air smooth for flight? 



4. How large are the terms ex- 

 pressing the effects of clear air 

 turbulence in the usual mete- 

 orological equations (used for 

 numerical prediction) compared 

 to other terms? 



The urgent needs of aviation and 

 aeronautical engineering for informa- 

 tion on clear air turbulence and for 

 reliable predictions of its occurrence 

 will be finally and completely satis- 

 fied only when a full scientific under- 

 standing of the phenomenon is ob- 

 tained. The same understanding will 

 permit accurate determination of 

 whether the effects of clear air tur- 

 bulence must be incorporated in 

 an attempt at extended numerical 

 weather prediction. If this is neces- 

 sary, and successful methods can be 

 found, it will mark the crossing of 

 a long plateau in attempts to under- 

 stand the interactions of large- and 

 small-scale motions. 



The economic and social benefits 

 that would accrue from a capability 

 for long-range weather prediction 

 and the needs of aviation make it 

 imperative that the importance and 

 characteristics of clear air turbulence 

 in the general circulation of the at- 

 mosphere be investigated and com- 

 prehended. 



Prediction and Detection of Wave-Induced Turbulence 



The phenomenon of "clear air 

 turbulence" is of particular impor- 

 tance to man's activities within the 

 atmosphere because: (a) it is both a 

 hindrance and hazard to aviation and 

 (b) it accounts for roughly 20 to 30 

 percent of the total dissipation of the 

 atmosphere's energy. The latter fact, 



only recently discovered, relates di- 

 rectly to our attempts to predict the 

 global circulation weeks in advance. 

 Without an adequate appraisal of this 

 significant portion of the total energy 

 budget, it will be impossible to model 

 and predict the future state of the 

 atmosphere. 



Dimensions of the Problem 



The name "clear air turbulence," 

 or "CAT," has conventionally been 

 used to refer to turbulence occurring 

 several kilometers above the earth's 

 surface and in air that is free of 

 clouds and strong convective cur- 



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