Figure 6-16. A small vortex structure formed near a Figure 6-17. Turbulent cloud structures near the tropopause turbulent mixing zone (photograph O, A.J. Bedard, Jr.). over Boulder, Colorado (photograph ©, 1993, P. Neiman).
Figure 6-17 exhibits turbulent structures in a portion of wave cloud near the tropopause. The turbulence in this cloud, together with its great altitude, is strong evidence that the cloud is produced by a breaking vertically propagating wave. The fragmented and ragged edges of the clouds in this picture are clear indicators of turbulence. Such cloud forms have been observed in coincidence with severe or greater turbulence reported by turbine aircraft at altitude Figure 6-18. A linear contrail showing relatively smooth air (photograph ©. 1991, A.J. Bedard, Jr.)
Figure 6-19. A contrail in the vicinity of mountain wave activity near Boulder, Colorado, showing areas of turbulence (photograph 021988, R. Holle).
Figure 6-20. A contrail showing a significant 6.4 INTERPRETING CONTRAILS area of turbulence aloft One aid to interpreting air motion (photograph Q, characteristics and turbulence potential at K Langford). jet cruise altitudes is to study the contrails left by other aircraft. Contrails can provide a direct indication of atmospheric conditions when no clouds are present.
They can yield information on atmospheric stability and wind shear and may allow the selection of a routing or cruise altitude that will remain clear of turbulence.
In relatively undisturbed air, contrails will usually remain linear and smooth in appearance (Figure 6-18). They may spread out because of the natural diffusion of the aircraft exhaust, or they may occasionally exhibit small-scale oscillations caused by the interplay of wingtip vortices in air that is only slightly disturbed. Flight through the air mass depicted in Figure 6-18 will most likely be turbulence-free.
A more typical contrail found over mountainous terrain is shown in Figure 6-19. Although only a few lenticular clouds suggest the presence of a mountain wave, the contrail reveals a detailed history of the rather turbulent passage of this aircraft through the wave area.
The aircraft was flying from left to right in the picture and, because the contrail does not exhibit violent mixing or dissipation over most of the area shown, strong and persistent turbulence was probably not encountered. However, it appears that the aircraft had encounters with large-scale (on the order of 10 km) waves, with superimposed strong chop at several locations.
Another significant event is located at the point in Figure 6-19 where the contrail has nearly vanished because of the turbulent mixing that is occurring. The amplitude of the wave is largest here, while the wavelength is shorter than is found in the other portions of the visible contrail. This is an indication of a major vertically propagating gravity wave. Supporting evidence of this is the lenticular cloud that appears above the exiting contrail on the right side of the picture. This latter area should be avoided, or if that is not possible, the aircraft should be configured for turbulent air penetration prior to the turbulence encounter. The contrail appears to straighten out as it moves east of the mountain range, implying that wave energy was being transported vertically, rather than downstream at the same altitude.
Other turbulent histories are evident from the contrails in Figures 6-20 through 6-23.
In Figure 6-20, the aircraft flew beneath an area of standing lenticular clouds, encountering a significant area of turbulence that peaked at the upstream edge of the cloud. Here the atmosphere has strong rising motion and the contrail is highly contorted and twisted, in contrast to that seen in the rest of the photograph.
Once the aircraft cleared the leading edge of the cloud area, the contrail became smooth and laminar, indicating that the turbulence had probably ceased.
Figure 6-22. Contrail associated with lenticular and rotor clouds, showing very turbulent conditions aloft (photograph ©, K. Langford).
Figure 6-21 shows yet another encounter with a turbulent zone near the upwind edge of a lenticular cloud. The highly turbulent mixing of the contrail at this point leaves no question about the strong shear present, a characteristic of this upstream region.
Figures 6-20 and 6-21 point to the fact that the upwind edge of an area of lenticular clouds is frequently quite turbulent. As the aircraft in Figure 6-21 continued on course, it encountered a longer-wavelength oscillation and then a smooth flight path. It should be noted that similar concern should be given to the downwind edge of lenticular clouds.
Figure 6-23. Contrail located above rotor and lenticular clouds, indicating smooth conditions aloft (photograph 82, 1988, R. Holle).
A striking photograph of very turbulent conditions is shown in Figure 6-22. This contrail, located below lenticular clouds and above an area of rotor clouds, shows evidence of wave disturbances and very turbulent conditions that were likely manifested as severe turbulence. In this instance, disturbed parcels are violently displaced on very small horizontal scales.
The longer-wavelength disturbance in Figure 6-22 is likely contributing to passenger and crew discomfort, but the more serious problem is defined by the pulty nature of the contrail.
The final contrail photograph (Figure 6-23) shows that (as most pilots know) there are always exceptions to the rules applied to descriptions of the atmosphere. Even though lenticular clouds and rotor clouds indicate the potential for turbulence, a contrail often can provide some indication Warmer air of the most turbulence-free altitude in such above inversion Weak flow aloft an area. In Figure 6-23, the cloud signature Figure 6-24a. Smokestack is indicating a rotor cloud topped by a series plumes with an inversion of lenticular clouds that are related to laver in Boulder, Colorado vertical gravity wave propagation.
(photograph ©,1992, However, the contrail history indicates that P. Neiman). the conditions encountered by the aircraft Plume A Warmest air were relatively benign. Thus, the presence of laminar contrails in areas that contam | Cold air mountain waves can aid pilots in selecting turbulence-free altitudes.
Figure 6-24b.
6.5 OTHER VISUAL INDICATIONS OF AIR
MOTION NEAR COMPLEX TERRAIN The atmosphere presents a number of opportunities for an observer with a trained and discriminating eye to diagnose the processes that are occurring in the nearby motion field. We have already reviewed a number of these atmospheric signposts. In this final section of our atlas of visual indicators, we present a potpourri of photographs that expand upon the issue of interpreting the type and level of activity of air motion near mountainous terrain.
Figure 6-24a depicts the plumes from several smokestacks located in Boulder, Colorado.
Perhaps you have encountered similar patterns on your way to the airport before a trip. In this example, the light wind at lower levels is blowing from right to left, and the plumes do not significantly expand with height. This indicates that the steam is moving upward rapidly and that the plume is much warmer than the low-level air. As the plumes penetrate higher, the largest plume breaks through the stable (inversion) layer into warmer air and expands more rapidly. This process is shown schematically in Figure 6-24b. Although not depicted in Figure 6-24, had the wind flow above the inversion been stronger, the plume would have been sharply bent downwind as it penetrated the stable layer and encountered the Figure 6-25. Blowing snow near mountain peaks, indicating likely wave activity (photograph ©, R. Reinking).
Figure 6-26. Low-level wind indicators on a lake surface Figure 6-27. Wave cloud occurring above a layer of weak (photograph ©, 1988, A.J. Bedard, Jr.). instability (photograph ©, NCAR).