PART II. ATLAS OF VISUAL INDICATORS
6.0 VISUAL INDICATORS OF
OROGRAPHIC WIND FIELDS Our purpose in this section is to provide an atlas of photographs that show striking examples of the strong wind flow patterns described in Part I. The goal is to assist aircrews in diagnosing the presence and qualitative strength of these features so that umely operational decisions can be made, either prior to departure or while airborne and approaching the area of strong and turbulent wind flow.
6.1 LARGER-SCALE FEATURES Lee waves that result from fairly uniform (in direction) winds over a broad area of terrain can be considered larger-scale features. Figures 6-1 through 6-4 depict wave clouds that show the local air motion through the large-scale wave events.
Figure 6-1 is a photograph of an isolated lenticular eloud near Pikes Peak, Colorado. Layered Cloud The air is moving through the cloud from right to left in the picture. The limited vertical extent of this feature is shown by Figure 6-la. An isolated the lack of involvement of the overlying Pikes Peak, Colorado lenticular cloud near cirrus clouds. Figure 6-1b is a schematic of the lenticular with streamlies of the wind * (photograph O, 1990, R. Holle).
flow superimposed.
Obstacle Figure 6-1b.
Figure 6-2 shows another wave cloud whose layered structure makes the general patter of the wind flow (right to left) in the wave quite evident. Note that the largest amplitudes are at lower levels in this cloud feature, implying that the most disturbed air also is at lower levels of the wave.
The wave cloud in Figure 6-3a clearly shows the air motions that have created it.
The streamlines of the wind motion are added to Figure 6-3b. The wave cloud in Figure 6-4 is almost parallel to the upper level flow; this orientation may be responsible for its corkscrew appearance.
Although these photographs show outstanding examples of the way in which air motion associated with lee waves can be "mapped" by accompanying cloud features.
some lee waves will not generate lenticular clouds because of a lack of water vapor in the moving air mass.
Figure 6-2. A wave cloud over Laramie, Wyoming (photograph ©, B. Marmer).
Figure 6-3a. A wave cloud over Nederland, Colorado (photograph Q, 1993, P. Neiman).
Figure 6-3b.
Figure 6-4. A wave cloud oriented parallel to the upper-level flow looking west from Dillon, Colorado (photograph ©, R. Reinking).
Mountain Peak Figure 6-5a. Cloud Circular lenticular Locations clouds produced by a pair of eddies in the tee of an isolated mountain peak near Nederland, Colorado (photograph 0, 1990, P. Neiman). Figure 0-5b.
6.2 SMALLER-SCALE FLOWS In the previous section, we reviewed the wave clouds and the flow patterns associated with waves that result from the large-scale movement of an air mass over a mountain range. Now we present photographs of clouds resulting from air motion over and around individual (or isolated) mountain peaks.
Figure 6-5a shows the results of flow over and around a set of higher peaks (to the left in the photograph). Various interpretations of these spectacular cloud forms are possible. One of these is shown in Figure 6-5b in which the pair of circular lenticular clouds represent a pair of eddies initiated by flow around these peaks.
Another interpretation is that these lenticulars result from greater amplitude of the larger-scale mountain wave directly downwind of the peaks. Note that there are indications of disturbances in the lower portions of this cloud field, whereas the cloud appears smooth at higher levels.
Significantly, the clear air below the central region of these clouds may contain intense small-scale vortices that could be hazardous for aircraft penetrating the area.
Figure 6-6 is a stunning photograph of a lenticular cloud that resulted from the strong flow around Mt. McKinley, Alaska.
Figure 6-6. A three-dimensional In this case, the wind flowing around the lenticular cloud that has developed mountain developed a stationary eddy in the in the strong flow around lee of the peak. Although this cloud Mt. McKinley, Alaska appears quite smooth and laminar, the shear (photograph ©, 1981, B. Marmer).
zones created by such flow perturbations have the potential to produce destructive turbulence.
Figure 6-7. A field of circular lenticular clouds that have developed in the complex flow around a number of mountain peaks in Mt. McKinley National Park, Alaska (photograph ©, 1979, B. Marmer).
56 If such turbulence were occurring at the time of this photograph, it likely occurred in clear air away from the cloud.
An instance of flow around a number of isolated mountain peaks (also occurring at Mt. McKinley National Park) is shown in Figure 6-7. This field of circular, three-dimensional lenticulars probably has associated with it centers of strong turbulence, interspersed with areas of relative calm. Of concern to flightcrews who might attempt flight through this type of disturbed airflow is the fact that regions of strongly circulating wind flow can produce localized areas of very strong and complex air motion patterns.
Figures 6-8 and 6-9 are photographs of lenticular clouds taken from an aircraft showing the effects of flow around complicated, heterogencous topography.
Figure 6-10 concludes this section with a photograph of a lenticular cloud at sunset.
Once again, the concer with this type of smaller-scale wave activity lies in its potential for producing localized areas of severe-to-extreme turbulence in small-scale eddies. These extremely strong vorces may have no associated cloud features to make them visible as the aircraft approaches, perhaps with catastrophic
Figure 6-8. A view of three-dimensional lenticular clouds taken from an aircraft (photograph Q, NCAR).
Figure 6-9. A view of three-dimensional lenticular clouds north of Boulder, Colorado, taken from an aircraft (photograph Q, 1988, S. Holle).
6.3 ROTORS AND OTHER TURBULENT ZONES We have previously discussed the formation of rotor zones in association with lee waves, along with their potential for subjecting aircraft to strong turbulence and rolling moments. This section provides a sample of well-developed rotor clouds associated with lee-wave events.
Figure 6-11a depicts an example of clouds associated with vertically suppressed, trapped lee waves. These waves are occurring downstream (east, in this case) of the Continental Divide. In Figure 6-11b, the air motion is superimposed on the cloud fields. Because these clouds are flat and not very ragged, it is unlikely that turbulence within them is more than light-to-moderate.
In contrast to the vertically suppressed clouds of Figure 6-11 are the vertically enhanced clouds, viewed from the same location and also associated with trapped lee waves, shown in Figure 6-12a. The air Figure 6-10. A lenticular cloud over Boulder, Colorado, at sunset motion in these rotors is depicted in Figure 6-12b.
(photograph ©. B. Martner).
Figure 6-1 la. Clouds associated with vertically suppressed, trapped lee waves &t Boulder, Colorado (photograph ©, 1987, P. Neiman).
Pigure 6-1 1b.
Figure 6-13a. Rotor cloud near State College, Pennsylvania (photograph ©, 1985, P. Neiman).
Figure 6-13h Foehn cloud Rotor FlOW - Cloud Figure 6-14. Schematic of the flow associated with a rotor zone in the lee of a mountain.
Figures 6-13a and 6-13b depict a rotor cloud over much lower terrain near State College, Pennsylvania. A Foehn cloud can be seen in the background of this picture.
The flow associated with this feature is shown in the Figure 6-14 schematic. Note the well-defined wave that has resulted from the air motion over the ridge. The event shown in these photographs is graphic evidence that extremely high or rugged terrain is not a requirement for the development of significant wave activity.
The turbulence in this situation is likely to be considerably greater than in the instance depicted in Figure 6-1 1a.
Rotor clouds can develop complex structures, as shown in Figure 6-15.
Figure 6-15a shows a group of rotor clouds, one of which has apparently developed a helical orientation. This structure is shown in Figure 6-15b, along Note Helical Structure with the orientation of the rotary motion of the airflow within the cloud layer. The Figure 6-15a. A group of rotor clouds over Boulder, Colorado, helical structure may indicate much more one of which has developed a complex, turbulence-producing air motion helical structure (photograph 0). in this area.
1992, F.M. Ralph). Figure 6-16 depicts a small vortex that has formed near the turbulent mixing region at the leading edge of cold, moist air at low levels, interacting with the foothills and westerly downslope flow just above.
Figure 6-15b.