Figure 5-6 depicts a trapped lee wave.
Notice that this type of wave extends downwind from the mountain, does not develop to a high altitude, and has no upstream tilt, in contrast to the vertically propagating wave in Figure 5-1.
This class of wave presents less turbulence hazard at high altitude than do breaking vertically propagating waves, because the wave amplitude decreases with height within the "trapping layer," typically based within a few thousand feet of the ridge crest. As a result, these waves do not extend to as great an altitude. (An exception to this rule is when the atmospheric structure permits only partial trapping. This commonly occurs because the layer of wind shear that is instrumental in the trapping is weaker or shallower than necessary to do the job completely.)
However, at lower altitudes, trapped lee waves can create strong turbulence encounters for aircraft. Below lenticular clouds, the wind can be quite variable and gusty, although usually not extremely strong. The gusty winds can extend from the surface up to the base of the clouds, particularly during daylight hours of spring and summer when the sky is otherwise mostly cloud-free.
26 Cloud bases associated with trapped lee waves are typically one to several thousand feet above ridge level, and pilot reports in the vicinity frequently indicate.
moderate-to-severe turbulence beneath the clouds. The turbulence associated with trapped lee waves is related to the large horizontal and vertical wind shears below cloud level.
With this type of wave, there is frequently a strong shear layer near cloud base immediately to the lee of the mountain range. This separates a turbulent wake region below mountain-top level from the faster-moving, cloud-bearing air above. In the cloud layer itself, conditions typically range from turbulent near cloud base to smooth near cloud top. The clouds themselves give some indication of the degree of turbulence within them; smooth, laminar-looking edges and tops are associated with little or no turbulence, while a lumpy, non-uniform appearance and a visual impression of rolling motion about an axis parallel to the cloud is indicative of turbulence.
Superimposed on the smaller-scale turbulent motions that may be present are larger-scale up- and downdraft motions that are a part of the wave. Vertical shear of the horizontal wind is locally enhanced at the crests and troughs of the wave as a result of vertical transport (by the wave) of strong winds, leading to shear-induced turbulence.
Figure 5-7 shows lenticular clouds associated with a trapped lee wave. Note the laminar appearance of the flow within the cloud that has developed from expansional cooling and condensation of water vapor in the upward-moving portion of the wave.
8 6 Clouds Associated with Trapped Lee Waves I I I Figure 5-7. L..enticular clouds associated with a trapped lee wave (ajier Dar-ran and Klemp).
Figure 5-8 depicts a striking example of clouds associated with a trapped lee wave.
The rolling motions in these clouds are repetitive downstream, each cloud band corresponding to a wave crest. The bottom bright band of clouds is a Foehn wall on the mountains. Note the gaps in successive cloud bands. Although still pictures cannot show rotation within the clouds, time-lapse photographs of such cloud bands usually reveal a marked rolling motion, with descent on the downwind side of the band, ascent on the upwind side. Vertical motions measured by research aircraft in trapped lee wave clouds are typically The cloud gap below the uppermost wave cloud in Figure 5-8 reveals some cirrus clouds that may be associated with a vertically propagating wave, which coexists with this trapped lee wave (not an unusual occurrence). When the trapped lee waves are evident as several regular cloud bands beginning immediately downwind from the mountain ridge as in Figures 5-7 and 5-8;
any coexisting vertically propagating wave usually is not hazardous.
Figure 5-8. Clouds associated with a trapped lee wave (photograph ©, 1988, R. Holle).
5.1.2.1 Forecast and Observed Data Determination of the likelihood of trapped lee waves should be made using the same forecast and observed data as for vertically propagating waves. Both are favored by stability in the lowest several thousand feet above the level of the mountain top. If this condition is met, then the simplest procedure available is to examine the vertical wind shear. For a given strength of wind at 12,000 ft, a strong increase in wind speed between 12,000 and 18,000 ft (at least a doubling is typical for trapped waves) with no dramatic change in direction indicates the likelihood that trapped lee waves will be the principal source of turbulence. On the other hand, only a small increase in speed in the wind component across the mountain range between 12,000 and 18,000 ft should be cause for concern that breaking vertically propagating waves will occur, particularly for high mountain ranges and very strong (25 to 30 kt or more) ambient winds at ridge-top level.
From a practical standpoint, the forecast of wave activity and reports of lenticular and/or rotor clouds, along with pilot reports of turbulence in the vicinity of the ridges, are sufficient for the assumption that wave activity is occurring. However, when flying early in the day or late at night, the absence 30 of pilot reports may not be an assurance that no turbulence is present; similarly, the absence of clouds may mean only that the air is too dry for clouds to form. Therefore, in all cases, the forecast and/or measurement of ambient winds near ridge level of 20 kt or greater should cause pilots to assume that wave activity is present and make appropriate changes in takeoff/landing times, choice of departure or arrival airports, and en route course, unless reliable data indicate that there is no danger of a strong wave event. As is always the case in flight operations, the go/no-go decision should take into account the performance capabilities of the aircraft and the currency and experience level of the pilot.
5.1.2.2 Summary Comments on Trapped Lee Waves
• Trapped lee waves do not propagate vertically because of the capping effects of strong wind shear or low stabilty above.
• Aircraft turbulence encounters related to trapped lee waves are generally restricted to lower altitudes, whereas vertically propagating waves can affect all altitudes.
• Strong turbulence can develop near the bases of accompanying lenticular clouds, although such clouds may not be present.
• Forecasts of winds 20 kt or greater at ridge level should alert pilots to the likelihood of wave activity.
Figure 5-9. Conceptual view of a mountain lee wave rotor zone (1993, A.J. Bedara, Jr.).
Rotor Severe Downslope West Windstorm East Winds East Continental Boulder Strong West Winds Divide 40 km 10 km
5.1.3 Persistent Horizontal Roll Vortices (Rotors) When mountain waves are present, it is quite common for a rotor zone to develop near or below ridge level on the downwind side of the mountain, under a wave crest and associated lenticular cloud (if sufficient moisture is present). This is an area of potentially severe-to-extreme wind shear and turbulence.
Figure 5-9 shows a schematic of the wind flow associated with this feature. As case, the rotor is really part of the "jump" discussed in Section 5.1.1. Although strong rotation is typically present within the rotor zone and associated cloud, a pilot in a moving aircraft may not be able to detect such motion visually until the aircraft is quite close to the vortex. In fact, from a distance, a rotor cloud may look like a rather innocuous cumulus cloud; however, the downwind side of the rotor cloud will typically be rounded in the direction of rotation of the rotor, with cloud tags or streamers at the bottom of the cloud mass.
illustrated in this schematic, rotors typically The latter features appear to be rapidly mark the downwind terminus of a forming and dissipating, thereby giving downslope windstorm. When this is the some sense of rotation within the cloud.
Figure 5-10 is a photograph of a wave cloud and associated rotor cloud viewed from aloft. Noteworthy in this picture are the stacked lenticular clouds, which indicate that wave activity is occurring and point to the likelihood that the rather mnocent-looking, rounded cloud below is indeed a rotor cloud.
Because of their potential for causing turbulence and loss of aircraft control, rotor zones should be avoided. Doppler lidar measurements in rotors have documented rotational speeds greater than 20 kt. Rotor zones are of concern not only because of the likehhood of strong turbulence in their vicinity (particularly on the upwind side of the rotor), but also because of the potential for rolling moments that could exceed the roll authority of the aircraft or otherwise lead to loss of control. Rotors are especially dangerous at low altitudes, particularly during takeoff and landing.
when the aircratt is slowed and in a relatively high-drag configuration. As previously noted, there is evidence that translating rotors (moving vortices), with or without clouds, may be especially dangerous in causing aircraft upset and loss of control.
Since rotor zones can be a product of either type of mountain wave, the previously Figure 5-10. View from aloft of a discussed forecast and observed data and wave cloud and associated rotor the associated rules also can be applied in (photograph ©, NCAR) forecasting the likehhood of rotor zones.
When waves are present, one should assume that a rotor zone exists below ridge level and within about 20 nm of the ridge.
Pilots should be particularly cautious when they know or suspect that breaking 33
vertically propagating waves or a downslope windstorm are occurring. Under these conditions, the rotor or jump zone will very likely be a location of severe-to-extreme turbulence and, in addition to horizontal axis vortices, may also contain vertical axis vortices of great intensity.
5.1.3.1 Summary Comments on Horizontal Roll Vortices
• Rotor zones can develop below ridge level downwind of a mountain in association with waves.
• In flight, rotor clouds may appear to be "normal" cumulus clouds.
• Rotor clouds may have strong turbulence and can produce large aircraft rolling moments, which can lead to loss of aircraft control.
• Translating rotors can be especially hazardous, because the combination of their rolling moments and translation speed can exceed aircraft roll authority.
• When waves are present, assume rotors exist below ridge level within 20 nm of the ridges.
34 .......
5.1.4 Kelvin-Helmholtz Waves As noted in Section 4.3, another type of wave can be found in a stable atmosphere within a region that has very strong vertical shear of the (large-scale) horizontal wind through a concentrated layer. Figures 4-5, 5-11, and 5-12 show clouds associated with such Kelvin-Helmholtz (K-H) waves.
Figures 5-11 and 5-12 are more typical of cloud forms observed with K-H waves.
When K-H waves develop, eddies form within the shear zone and move with the background wind flow. In Figures 4-5, 5-11, and 5-12, the K-H waves cause localized changes in a larger cloud that is the visible manifestation of a mountain wave. The larger-size mountain waves (in the cases shown in these pictures, probably vertically propagating waves) contribute to locally increased vertical shear so that the wind above the cloud top is stronger than the wind below. The K-H waves are feeding on this difference in wind speed (wind shear). Thus, the existence of even small amplitude mountain waves increases the likelihood of encountcring shear-generated turbulence. Often with this type of disturbance, there may be insufficient moisture for clouds to form, making the turbulent layer invisible but no less bumpy.
Although gravity-shear waves are typically not associated with the magnitude of aircraft hazard represented by the mountain waves that we have previously discussed, they are a frequent contributor to turbulence at altitude and are occasionally associated with aircraft reports of moderate-to-severe turbulence (Bedard et al., 1986). For purposes of our discussion here, the presence of clouds like those in Figures 4-5, 5-11, and 5-12 should alert pilots to anticipate at least some degree of turbulence in their vicinity.
In general, the stronger the wind shear associated with K-H waves, the stronger the disturbance that develops at the shear interface. Since this effect usually occurs within a fairly shallow zone, a modest altitude change (normally several thousand fect) should allow aircraft to clear the turbulence.
Figure 5-11. Clouds associated with gravity-shear (Kelvin-Helmholtz) waves (photograph ©, 1985, P. Neuman).