CLEAR \ . , lENCE 



example, happens to a growing un- 

 stable wave in a thin stratum when it 

 reaches a dynamically stable layer in 

 which Ri is significantly greater than 

 Vi ? We do not know. This is one 

 of many important questions that 

 needs to be answered by further re- 

 search. 



Other aspects of the new radar ob- 

 servations that are relevant to flight 

 safety as well as to aircraft investiga- 

 tions of WIT and to its predictability, 

 are: (a) the sharp vertical gradations 

 in turbulence intensity (i.e., some- 

 times the turbulence is restricted to a 

 stratum no more than a few tens of 

 meters thick) and (b) the inter- 

 mittancy of K-H waves and turbu- 

 lence. 



It is not surprising that one air- 

 craft experiences significant turbu- 

 lence while the next one encounters 

 none in the same region. While the 

 radar observations demonstrate that 

 the base of the inversion and subsidi- 

 ary sheets within it are the seat of 

 K-H wave activity, their breaking is 

 self-destructive in that the shear and 

 stability to which they owed their 

 origin are decreased, and Ri thus in- 

 creased above its critical level. Ac- 

 cordingly, the breaking action acts as 

 an escape valve to release the pressure 

 for K-H activity, and turns the waves 

 and turbulence off. On the other 

 hand, the larger-scale atmospheric 

 processes work to restore the initial 

 conditions, and new K-H waves are 

 triggered. 



All this speaks to the difficult ques- 

 tions of aircraft experiments directed 

 to observing the initial conditions for 

 WIT, the energy budget involved, 

 and, indeed, its entire life cycle. Pre- 

 cisely where and when should the 

 measurements be made and how are 

 they to be interpreted in the light of 

 WIT's great spatial and temporal vari- 

 ability? Clearly, such experiments 

 should preferably be conducted si- 

 multaneously with a radar capable of 

 "seeing" the waves and turbulence di- 

 rectly. 



Prospects for Prediction — The prior 

 discussion raises serious doubts as to 

 the ultimate achievement of pinpoint 

 forecasts of WIT in either space or 

 time. While one may expect, eventu- 

 ally, to be able to predict the medium- 

 to large-scale processes that work to 

 develop and sharpen internal fronts 

 and shear, many presently unobserv- 

 able small-scale phenomena (gravity 

 waves, orographic lifting and tilting, 

 convective motions, and such) will 

 operate to reduce Ri to its critical 

 value locally and trigger wave activity 

 here and there. Accordingly, while 

 we may expect significant improve- 

 ments in the predictability of the 

 heights of internal surfaces, and thus 

 in the heights at which WIT is likely, 

 and probably in the predicted in- 

 tensity as well, the actual forecast will 

 probably remain a probabilistic one 

 for many years to come. We should 

 therefore direct a good share of our 

 attention to the remote-probing tools 

 that are capable of detecting both the 

 internal surfaces and the occurrence 

 of waves and turbulence. As in the 

 case of radar detection of thunder- 

 storms, such observations are likely 

 to provide the best short-term predic- 

 tions of WIT for the foreseeable fu- 

 ture. 



Instrumentation for Detecting WIT 



Although we have spoken exten- 

 sively of the capability of ultrasensi- 

 tive high-resolution radar techniques 

 in detecting WIT, a few additional re- 

 marks need to be made concerning 

 actual warning devices. 



Ground-Based Devices — High- 

 resolution FM-CW microwave radar 

 is an obvious candidate for this task. 

 At present, however, it is limited to a 

 detection range (based on over-all 

 sensitivity in detecting clear air in- 

 versions) of about 2 kilometers. An 

 increase of range to 15 kilometers is 

 attainable with available state-of-the- 

 art components. This would accom- 

 plish the detection of clear-air WIT 

 throughout the depth of the tropo- 



sphere. A network of such st, 

 across the nation, with fixed, ver- 

 tically pointing antennas, is econom- 

 ically feasible. Fortunately, the sig- 

 nificant internal fronts at which WIT 

 occurs are horizontally extensive, so 

 that detection of waves and turbu- 

 lence at one or more stations would 

 indicate the layers affected and the 

 likelihood of WIT at the same height 

 (or interpolated height for sloping 

 layers) in between stations. (Note that 

 we emphasize the need for observa- 

 tions with a high degree of vertical 

 resolution, capable of detecting the 

 suspect layers and measuring the 

 amplitude and intensity of breaking 

 waves.) 



Airborne Radar — With regard to 

 the use of high-resolution FM-CW 

 microwave radar on board aircraft for 

 purposes of detecting and avoiding 

 WIT along the flight path, the 15- 

 kilometer range capability would be 

 inadequate to provide sufficient warn- 

 ing even if a high-gain antenna of the 

 required dimensions (10' to 15' effec- 

 tive diameter) could be accommodated 

 in the aircraft. Moreover, since the 

 vertical resolution in such a use-mode 

 would correspond to that of the beam 

 dimension rather than the available 

 high-range resolution, the radar could 

 not discern wave amplitude and 

 heights with precision. However, the 

 use of such a radar in both down- 

 ward- and upward-looking directions 

 (from large antennas fitted within the 

 fuselage structure) does appear feas- 

 ible. Clear-air WIT could then be 

 avoided by detecting the heights of 

 internal surfaces and K-H wave ac- 

 tivity above and below flight level 

 and assuming continuity of layer 

 slope. Whether or not such a system 

 should be adopted depends on cost/ 

 benefit/risk ratios. The installation 

 of a $100,000 radar seems warranted 

 when aircraft carry more than 200 

 passengers. Certainly, it should be 

 adopted for experimental purposes in 

 connection with WIT research. The 

 potential benefits of airborne high- 

 resolution radar to both military and 

 commercial aviation could then be 

 better evaluated. 



Ill 



