4. CLEAR AIR TURBULENCE 



Clear Air Turbulence and Atmospheric Processes 



Understanding of atmospheric 

 processes appears to decrease rapidly 

 with decreasing scale or typical size 

 of the phenomena considered. Thus, 

 it has only recently been recognized 

 that turbulence in clear air in the 

 upper troposphere and lower strato- 

 sphere is an important part of the 

 energy cycle of the atmosphere. 



Although motions in the atmo- 

 sphere at scales less than a kilometer 

 are often turbulent to some degree, 

 the occasional outbreaks of moderate 

 or severe turbulence that have 

 plagued aviation for the past decade 

 or more have important implications 

 for the study and prediction of large- 

 scale atmospheric motion. 



These motions are a result of dif- 

 ferential heating. In the process of 

 attempting to restore a uniform dis- 

 tribution of heat in the atmosphere, 

 the motions and processes of the 

 atmosphere create narrow layers in 

 which both wind and temperature 

 variations are concentrated. The 

 sharpest of these occur in the boun- 

 dary layer, in the fronts associated 

 with weather systems, and in the 

 vicinity of the jet stream near the 

 tropopause. 



In each of these regions of strong 

 gradients, turbulence typically occurs 

 when the gradients become strong 

 enough. The turbulent motions cause 

 mixing and tend to smooth the varia- 

 tions of wind and temperature. In 

 the process, a considerable amount of 

 heat and momentum may be trans- 

 ported from one region to another, 

 and with all turbulence there is a 

 conversion of kinetic energy to ther- 

 mal energy. 



The basic cycle of events in the 

 atmosphere may thus be viewed as 

 a sequence in which: 



1. Large-scale gradients created by 

 differential heating result in 

 large-scale motions. 



2. The large-scale motions con- 

 centrate the variations caused 

 by this differential heating into 

 narrow zones which now con- 

 tain a significant fraction of the 

 total variation. 



3. As the degree of concentration 

 increases, turbulence arises in 

 these zones, destroying the 

 strong variations and thus mod- 

 ifying the larger-scale structure 

 of the atmosphere. 



In this sense, turbulence in the zones 

 of concentrated variation is an essen- 

 tial part of the thermodynamic proc- 

 esses of the atmosphere. 



Atmospheric scientists have long 

 known that both the transport of 

 heat and momentum and the dissipa- 

 tion of kinetic energy were strong 

 in the boundary layer and in frontal 

 regions. The importance of these 

 same processes in clear air turbulence 

 near the jet stream is a recent dis- 

 covery. 



Perhaps the most important prac- 

 tical implication of this development 

 concerns the feasibility of long-range 

 numerical weather prediction. Such 

 predictions cannot be reliable for ex- 

 tended periods unless the computer 

 models correctly simulate the energy 

 budget or energy cycle of the atmo- 

 sphere. It now appears likely that this 

 cannot be done without taking ac- 

 count of the role of clear air turbu- 

 lence — a phenomenon of too small 

 a scale to be revealed by present 

 standard sounding techniques or to 

 be represented directly with the data 

 fields used in the computer models. 



The importance of clear air tur- 

 bulence in the energy budget is illus- 

 trated by its contribution to the rate 

 of dissipation of kinetic energy in the 

 atmosphere. Although the exact 

 value is subject to some controversy, 

 the total dissipation rate probably 

 will be somewhere in the range 5 to 

 8 watts per square meter, of which 

 2 to 3 watts per square meter prob- 

 ably occurs in the boundary layer. 

 Studies of dissipation by Kung, using 

 standard meteorological data, and by 

 Trout and Panofsky, using aircraft 

 data, both arrive at an estimate of 

 1.3 watts per square meter for the 

 dissipation in the altitude range 

 25,000 to 40,000 feet near the tropo- 

 pause. 



Thus, despite the present uncer- 

 tainty of these estimates, it appears 

 that the region near the tropopause 

 contributes on the order of 20 per- 

 cent of the total dissipation of the 

 atmosphere. The rate of dissipation 

 in severe turbulence is about 400 

 times as large as that in air reported 

 smooth by pilots and about 20 times 

 as large as that in light turbulence. 

 The estimates of Trout and Panofsky 

 show that the light and moderate 

 turbulence contributes the major 

 fraction of the total dissipation in 

 the layer near the tropopause; fur- 

 thermore, their estimates show that 

 about equal fractions of the dissipa- 

 tion in this layer are probably due to 

 the severe clear air turbulence and 

 to the smooth air. (It should be 

 noted that the estimate of the con- 

 tribution of severe turbulence is un- 

 doubtedly too low, because pilots 

 attempt to avoid it if at all possible.) 



Available Observational Data 



Most of what is known about the 

 structure of clear air turbulence and 



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