PART IV — DYNAMICS OF THE ATMOSPHERE-OCEAN SYSTEM 



rents. But our understanding of 

 CAT processes, which reached a new 

 climax only in 1969-70, strongly im- 

 plies that identical mechanisms also 

 occur within clouds and storm sys- 

 tems. 



Cumulative evidence is sufficiently 

 persuasive to conclude that CAT 

 occurs in internal fronts or layers in 

 which the air is statically stable and 

 across which there is strong shear of 

 the wind either in speed or direction. 

 Such conditions commonly prevail at 

 both warm and cold fronts marked 

 by clouds and precipitation. Increas- 

 ingly abundant aircraft incidents, 

 some of them fatal, also suggest that 

 the CAT mechanism occurs at such 

 frontal boundaries. The fact that 

 the process has not been clearly 

 identified as such is due to the general 

 assumption by pilots and meteorolo- 

 gists that turbulence within clouds 

 or storms is more commonly due to 

 convective- or thunderstorm-like ac- 

 tivity. But this assumption is un- 

 tenable when no direct meteorologi- 

 cal evidence of convective activity 

 exists and when aircraft undergo 

 forced maneuvers that can only be 

 associated with waves and breaking 

 waves; the latter are now recognized 

 to be the primary, if not the sole, 

 origin of what we have previously 

 called "CAT." 



Newly Recognized Features — Be- 

 cause it now seems clear that severe 

 turbulence of the nonconvective va- 

 riety also occurs within clouds and 

 storms, it is fallacious to continue the 

 usage "clear air turbulence." And 

 since turbulence in both clear air and 

 clouds and storms owe their origin 

 to breaking waves, it has been pro- 

 posed that such turbulence be re- 

 named "wave-induced turbulence," or 

 "WIT." Unless we recognize these 

 important facts, we shall fail to ap- 

 preciate the full dimensions of the 

 problem. For example, while CAT 

 generally occurs at relatively high 

 altitudes, thus allowing the pilot time 

 to recover from a turbulence-caused 

 upset, severe WIT within frontal 



storms may occur at very low alti- 

 tudes without the possibility of safe 

 recovery. Indeed, it is now reason- 

 able to suppose that many previously 

 unexplained fatal and near-fatal air- 

 craft accidents owe their origin to 

 WIT. Until this phenomenon is fully 

 appreciated both by pilots and me- 

 teorologists, aircraft will continue to 

 encounter potentially fatal hazards 

 without warning. 



Another deceptive aspect of the 

 acronym "CAT" is its exclusion of 

 wave-induced turbulence near cloud 

 boundaries. This may have unfortu- 

 nate consequences, since it is well 

 known that cloud tops commonly oc- 

 cur at the base of temperature inver- 

 sions, and the latter, when marked by 

 sufficiently strong wind shear, are the 

 seat of wave-induced turbulence. In- 

 deed, there is reason to believe that 

 the presence of clouds below the in- 

 version will enhance the chance of 

 WIT above. This is because radiative 

 and evaporative cooling from cloud 

 tops induces convective overturning 

 and this decreases the wind shear be- 

 low the inversion while enhancing the 

 shear in and above the inversion. 

 Similar arguments suggest that cloud 

 bases may also be preferred regions 

 of WIT. 



Finally, recent radar observations 

 (both ultra-high-resolution acoustic 

 and microwave) of stable and break- 

 ing waves indicate that WIT is an 

 almost ubiquitous feature at the low- 

 level nocturnal inversion and the ma- 

 rine inversion. On occasion, there- 

 fore, especially in association with the 

 low-level nocturnal jet, we may ex- 

 pect moderate to severe turbulence at 

 low levels. These situations would be 

 excluded from the present definition 

 of CAT, which is restricted to turbu- 

 lence at heights above the middle 

 troposphere. Equally important, how- 

 ever, is the recognition that WIT 

 plays a role in the mixing processes 

 at the top of the boundary layer. This 

 may have significant consequences for 

 the metamorphosis of the boundary 

 layer, and thus upon air-pollution 

 meteorology. 



Hazards and Cost Implications — 

 All this is by way of indicating that 

 WIT is far more widespread than is 

 presently recognized. The associated 

 hazards are also greater; and the con- 

 sequences, both in terms of basic 

 atmospheric processes and of ultimate 

 operational predictability, are more 

 far-reaching. This is so simply be- 

 cause our present classification of 

 CAT excludes the many occurrences 

 at low levels that might be confused 

 with ordinary boundary-layer turbu- 

 lence, and those within and near 

 clouds and precipitation that are often 

 misinterpreted as convectively pro- 

 duced turbulence. 



Statistics on CAT occurrence, and 

 its associated hazards and cost to 

 aviation, must therefore be viewed as 

 gross underestimates of the broader, 

 but identical, phenomenon of WIT. 

 Even so, the statistics compiled by 

 the National Committee for Clear Air 

 Turbulence indicate that damage to 

 aircraft may have cost the Depart- 

 ment of Defense $30 million from 

 1963 to 1965, to say nothing of crew 

 injuries or the effect of turbulence in 

 reducing combat effectiveness. The 

 committee reported a study that 

 showed the cost to commercial avia- 

 tion in 1964 to have exceeded $18 

 million, of which a major portion was 

 the increased expense caused by di- 

 versions around areas in which turbu- 

 lence was forecast or had occurred. 



The WIT Mechanism and Its 

 Predictability 



Our knowledge of the WIT mecha- 

 nism is substantial — at least com- 

 pared to the state of knowledge be- 

 fore 1968. A great deal more needs 

 to be learned, however. Newly de- 

 veloped observational tools promise 

 major advances in understanding the 

 WIT mechanism which should open 

 the way to a more realistic appraisal 

 of the climatology of WIT and the 

 physical conditions under which it oc- 

 curs. Together, the instruments and 

 the increased understanding should 

 lead to improved predictability, al- 



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