Chapter 16, Mountain Weather 16-7
(Source: Durran and Klemp, 1983)5
Figure 16-6. Schematic Showing Locations of Jump and Wave Breaking Region of a Vertically Propagating Mountain
Wave
5 Durran, D. R., & Klemp, J. B. (1983). A compressible model for the simulation of moist mountain waves. Monthly
Weather. Review, 111, 2341-2361.
Chapter 16, Mountain Weather 16-8
Figure 16-7 shows a schematic of the jump feature, with a pronounced wave and associated strong shear
layer. The shear layer (shown in the inset) is a source of the turbulence found with the jump.
Figure 16-7. Schematic of the Strong Shear Zone Associated with a Hydraulic Jump in a Mountain Wave
16.2.4 Trapped Lee Waves
In the preceding section, an important type of mountain wave that propagates (i.e., transports its energy)
vertically was discussed. Next , consider a second type of mountain wave, often manifested by a train of
Altocumulus Standing Lenticular (ACSL) clouds extending far downwind of the mountain (although
trapped lee waves frequently occur without clouds). These waves are of concern for takeoff and landing
operations and en route flight below FL250. The associated lenticular (lens - or airfoil-shaped) clouds may
appear turbulent or smooth and, depending on the moisture stratification upwind of the mountain,
multilayered. They are evident as relatively straight lines or bands of cloud s (with clear spaces between),
parallel to the mountain range, but downstream from it.
The waves that produce these cloud features often are referred to as “trapped lee waves,” because the wave
energy is confined below a certain altitude. The mechanism confining this energy is strong wind shear
above ridge level. Trapped lee waves are most likely to occur when the wind crosses a narrow mountain
range, with a layer close to ridge level and upstream of the mountain that has strongly increasing wind
speed with height and high stability, capped by a layer of strong flow and low stability.
Chapter 16, Mountain Weather 16-9
Figure 16-8 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 16-5.
(Source: Durran and Klemp, 1983)6
Figure 16-8. Computer Simulation of Trapped Lee Waves Behind a 300-m-High Mountain
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 laye r of wind shear that is instrumental in the trapping is weaker o r 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.
Cloud bases associated with trapped lee waves are typically one to several thousand feet above ridge level,
and PIREPs 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 mountaintop level from the
6 Durran, D. R., & Klemp, J. B. (1983). A compressible model for the simulation of moist mountain waves. Monthly
Weather Review, 111, 2341-2361.
Chapter 16, Mountain Weather 16-10
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 draft 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 16-9 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.
The rolling motions in these clouds associated with a trapped lee wave are repetitive downstream, each
cloud band corresponding with a wave crest.
(Source: Durran and Klemp, 1983)7
Figure 16-9. Lenticular Clouds Associated with a Trapped Lee Wave
16.2.5 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.
7 Durran, D. R., & Klemp J. B. (1983). A compressible model for the simulation of moist mountain waves. Monthly
Weather Review, 111, 2341-2361.
Chapter 16, Mountain Weather 16-11
Figure 16-10 shows a schematic of the wind flow associated with this feature. As illustrated in this figure,
rotors typically mark the downwind terminus of a downslope windstorm. When this is the case, the rotor is
really part of the “jump” discussed earlier. 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 dista nce, 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.
The latter features appear to be rapidly forming and dissipating, thereby giving some sense of rotation
within the cloud.
(Source: Bedard, 1993)8
Figure 16-10. Conceptual View of a Mountain Lee Wave Rotor Zone
Because of their potential for causing turbulence and loss of aircraft control, rotor zones should be avoided.
Rotor zones are of concern not only because of the likelihood 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 as the aircraft is slowed and in a relatively high-drag
configuration.
16.2.6 Smaller-Scale Hazards
The following smaller -scale phenomen a represent specific weather hazards for aircraft operating near
mountains.
16.2.6.1 Lee-Side Inversion with Shear Flow (Mountain-Induced Shear with No Wave
Development)
Occasionally, an extremely strong low -level temperature inversion can occur in mountainous areas, with
the inversion top below ridge level (perhaps 900 to 1,000 ft AGL) and a pool of very cold air at the surface.
If this phenomenon occurs with strong wind flow above the inversion layer, there will be a concentrated
8 Bedard, A. J., Jr. (1993). Atmospheric Turbulence Aloft: A review of possible methods for detection, warning, and
validation of prediction models. Washington, DC: American Institute of Aeronautics and Astronautics.
Chapter 16, Mountain Weather 16-12
shear zone near the inversion, which can lead to both significant turbulence encounters and abrupt airspeed
changes for aircraft that penetrate the inversion on climbout or during descent. This situation is true
particularly when significant mountain wave activity is present above the inversion in the strong flow aloft.
In this case, the surface-based pool of cold air and the inversion above it shelter the surface from what might
otherwise be a damaging windstorm.
16.2.6.2 Non-Steady Horizontal Roll Vortices (Moving Horizontal Vortices)
The surge of wind across a ridge can initiate a vortex downwind of the ridge ( Figure 16-11). The vortex
rolls up to maximum strength of rotation as it continues to move downwind away from the ridge and slowly
dissipates. In its wake, with a return to steady flow, K-H waves develop at the top of the shear layer.
Extreme gustiness is a characteristic of the surface winds during severe downslope windstorms. The
interaction of these gusts with strong large-scale winds moving perpendicular to a ridge may produce strong
horizontal vortices of small scale.
(Source: Bedard, 1993)9
Figure 16-11. Development of a Strong Roll Vortex Associated with a Wind Surge Down the Lee Slope of a Mountain
9 Bedard, A. J., Jr. (1993). Atmospheric Turbulence Aloft: A review of possible methods for detection, warning, and
validation of prediction models. Washington, DC: American Institute of Aeronautics and Astronautics.