PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM 



the regions in which it occurs results 

 from investigations motivated by its 

 impact on aviation. Clear air tur- 

 bulence has caused injuries to crew 

 members and passengers on commer- 

 cial airlines, loss of control, and 

 damage to aircraft structures. For 

 these reasons, two main types of 

 investigations have been carried out. 



Pilot Reports — In the first type, 

 reports of civil and military pilots are 

 used in conjunction with standard 

 weather data in an attempt to derive 

 a gross climatology of both the fre- 

 quency of occurrence of clear air 

 turbulence and its association with 

 wind and temperature fields. These 

 data are biased because pilots try to 

 avoid clear air turbulence; further- 

 more, the pilot reports are subjective 

 and not uniform, due both to varying 

 pilot temperament and to varying 

 aircraft response to characteristics. 



Instrumented Aircraft — In the sec- 

 ond, aircraft specially instrumented to 

 measure the gust velocities compris- 

 ing the clear air turbulence are flown 

 into such regions. The resulting data 

 have been analyzed in a variety of 

 ways. These programs have con- 

 tributed significant and valuable in- 

 formation about the internal physics 

 of the turbulent motion and about 

 certain aspects of its statistical char- 

 acteristics. The data are biased, how- 

 ever, by the fact that turbulence was 

 being sought by pilots; thus, they 

 cannot be used directly to establish 

 the frequency of occurrence of clear 

 air turbulence. 



A more serious defect, from the 

 scientific standpoint, is that these 

 programs were conceived on the basis 

 of the needs of aviation and aero- 

 nautical engineering; they were not 

 designed to reveal information about 

 the physics of turbulence or its de- 

 tails or interactions with larger-scale 

 flows. Nevertheless, the available 

 data could be used for scientific pur- 

 poses more extensively than they 

 have been. 



In the past few years attempts to 

 conduct scientific studies of the phys- 

 ics of clear air turbulence with spe- 

 cially instrumented aircraft (in some 

 cases with simultaneous use of 

 ground-based radars) have been 

 started in the United States, Canada, 

 England, and the Soviet Union. Al- 

 though the preliminary results from 

 these programs appear both promis- 

 ing and encouraging, no definitive 

 body of knowledge has yet emerged. 

 The problem is that the accuracy of 

 data required for scientific study of 

 the physics of clear air turbulence 

 and its interactions with the envi- 

 ronment leads to requirements for 

 basic sensors that severely test, or 

 even exceed, current instrumenta- 

 tion capabilities. 



Theoretical Knowledge 



Despite these deficiencies in the 

 collection of empirical data about 

 clear air turbulence, there does appear 

 to have been recent theoretical prog- 

 ress. Atmospheric scientists have 

 long suspected that clear air tur- 

 bulence is primarily a result of a 

 particular mode of fluid-flow in- 

 stability that occurs when there is 

 weak density stratification relative to 

 rapid vertical variation in the flow 

 velocity. This phenomenon has been 

 modeled in the laboratory by Thorpe, 

 seen under water in the Mediterra- 

 nean by Woods, and the character- 

 istic shape has appeared on the scopes 

 of radars used in turbulence studies 

 by Hardy, Glover, and Ottersten as 

 well as in a few photographs taken 

 when the process was made visible 

 by clouds. The hypothesis that clear 

 air turbulence is indeed a mani- 

 festation of this particular fluid-flow 

 instability provides an important con- 

 ceptual basis for planning the struc- 

 ture of empirical investigations. 



Recent work has also suggested 

 that internal gravity waves in the at- 

 mosphere may be linked with the 

 formation of clear air turbulence. An 

 interesting possibility is that the 



waves may be absorbed in shear 

 layers, and thus may act as a trigger 

 for the outbreak of turbulence. The 

 fact that gravity waves are often 

 generated by flow over mountains 

 may explain why clear air turbulence 

 occurs more frequently in mountain- 

 ous regions. 



Basic Equations — Although the 

 basic laws that govern clear air tur- 

 bulence are the same mechanical 

 and thermodynamic ones that apply 

 to all fluid motion and can be ex- 

 pressed mathematically, there has 

 been little success in applying the 

 equations to the problem. The main 

 reason is that mathematical theories 

 that provide solutions to these equa- 

 tions do not seem to exist. The es- 

 sential difficulty is that turbulence is 

 a distinctly nonlinear process, and 

 interactions on different scales are 

 a crucial part of the physical phe- 

 nomenon. 



It is precisely this that makes clear 

 air turbulence important to the en- 

 ergy cycle of the atmosphere. The 

 kinetic energy destroyed by the tur- 

 bulence comes from the kinetic en- 

 ergy of much larger-scale flows — 

 those that we attempt to predict with 

 numerical methods. The equations 

 used in the computer models apply 

 to averages of the variables over 

 quite a large region, and should in- 

 clude terms that express the effect 

 of smaller-scale motions within the 

 region of averaging upon the aver- 

 aged variables. Here again, the 

 mathematical form of the correct 

 equations is known, but some prac- 

 tical method must be found for rep- 

 resenting in the models the contribu- 

 tions to these terms from the intense 

 processes occurring in both clouds 

 and clear air. 



This probably can be accomplished 

 for clear air turbulence only when a 

 great deal more is known about its 

 characteristics and its interactions 

 with the large-scale processes. The 

 most pressing need is for a thorough 

 empirical study with aircraft, radar, 



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