K-H waves are quite common in the atmosphere; they can form in the vicinity of thunderstorms, in shear layers near the jet stream, and in association with stable regions of the atmosphere that are topped by a strong wind shear layer (such as the top of a pool of cold air on the lee side of a mountain). In fact, K-H instability induced by the wind shear associated with strong winds aloft is likely the chief source of high-level turbulence away from mountain ranges (clear air turbulence, or CAT). The mechanism that causes this type of disturbance can be compared to that of a flag flapping in a breeze. The flapping is a result of instabilities created by the wind shear along the flexible surface of the flag, analogous to the wind shear through a very stable (but shallow) layer of the atmosphere.
4.3.1 Summary Comments on Gravity Waves and Shear-Induced Waves
• A parcel of air within a stable air mass moving over a mountain will undergo wave motion.
• The resulting wave is a gravity wave with up-and-down motions.
• Gravity waves can grow in amplitude until they "break" into turbulence.
• If the magnitude of wind shear exceeds a critical value, turbulence will occur.
4.4 BREAKING WAVES AND TURBULENCE As indicated in the previous section, waves frequently develop in areas of the atmosphere that are characterized by stable air that is in motion over terrain, and in areas where the direction and/or speed of the horizontal wind changes rapidly with increasing altitude (that is, locations with strong vertical shear of the horizontal wind). It is important to understand that these waves can be quite powerful, in terms of the vertical motions within the wave, while being relatively turbulence-free. In this case, updrafts and downdrafts can be strong enough to produce significant altitude excursions or, if altitude is maintained, large changes in indicated airspeed (at fixed power settings). In fact, for an aircraft at cruise, indications that a wave is being encountered may include pitch and trim changes (manual or autopilot) necessary to maintain altitude with corresponding changes in airspeed, even in the absence of accompanying turbulence. However, the air may be extremely rough, perhaps destructively so in zones of shear and rotation under the waves, or when shear-induced waves roll up and then break down into small-scale turbulence (Figure 4-5a-b).
The concerns for pilots operating in mountainous areas when winds are strong include the potential for loss of aircraft control and possible turbulence-induced structural damage to the aircraft.
Our purpose here is to provide information about the meteorological events that can occur, along with suggestions for determining the likelihood of their presence and relative strength by using commonly available reports and forecasts during pre-flight planning and visual indicators prior to takeoff and while airborne. We have identified two important atmospheric characteristics related to wave motion: air stability, and wind direction and speed First, we need to know something about the overall stability of the air moving over the mountains. Second, we need a measure of the wind direction and speed at various altitudes near ridge level and above.
5.0 ATMOSPHERIC DISTURBANCES
IN MOUNTAINOUS AREAS
5.1 LARGER-SCALE HAZARDS This section describes some of the most important types of waves that can develop in mountainous areas, the atmospheric environments in which they develop, and strategies for discerning their presence and avoiding them.
When the atmosphere encounters a mountainous barrier, a number of responses are possible. If the wind is weak or the moving air mass exceptionally dense, the mountains may act as a dam, preventing the motion of air over the barrier. More frequently, strong winds flow over or around mountains or ridges. If the surrounding atmosphere is unstable, the vertical displacement of the air will (if sufficient moisture is present) lead to thunderstorm formation or at least the development of deep convective clouds.
However, if the wind is sufficiently strong and the surrounding atmosphere is stable, a wave will develop, as previously described.
The wave that results from vertical displacement of a stable air mass over a mountain or ridge can generally take one of two forms: vertically propagating mountain waves and trapped lee waves. Both types of waves can be hazardous to aviation operations. The particular type of wave or combination of waves that forms depends on the nature of the mountain range and on atmospheric properties upwind of the mountain. It is possible for both types of waves to exist at the same time, as will be seen in some of the pictures included in this AC. It also is possible to have hybrid or intermediate forms, that is, waves that are only partially trapped.
Vertically Propagating Mountain Wave Figure 5-1. Schematic of a vertically propagating mountain wave (after Durran and Klemp, 1983).
Height (km) Air motion Aircraft hazards associated with these assess its likely strength and location, and features can range between extremes of no prepare for an encounter (reduce airspeed to effect at all (for weak, laminar-flow waves) turbulent air penetration speed, secure loose to potentially destructive turbulence (for objects, etc.) or plan an appropriate large-amplitude breaking waves with strong diversion to avoid the area containing the rotor zones). The pilot's task is to be aware disturbance.
of the potential for wave development, 16
5.1.1 Vertically Propagating Mountain Waves Figure 5-1 shows a schematic of a vertically propagating mountain wave. This feature is essentially a standing gravity wave whose energy propagates vertically. For this class of wave, nothing is preventing vertical propagation, such as strong wind shear or neutrally stable atmospheric layers. Once again, the mere fact that a wave has developed in air moving over a mountain (or other barrier) does not in itself indicate problems for an aircraft operating in the vicinity. The potential for hazard is a function of the strength of the wave and whether or not an area of the wave "breaks" into turbulent motions that, in the extreme, can lead to structural damage or failure of an aircraft component.
With this type of wave feature, air that is moving nearly perpendicular to the barrier is deflected upward and accelerated as it passes over the crests and down the lee slopes of the terrain. Notice in Figure 5-1 that the standing wave has developed vertically above the mountain crest and that the resulting wave tilts upwind with height.
This vertical propagation of the wave means that the effects of the mountain range can be felt at heights significantly above the actual altitude of the peaks (at times reaching in excess of 60,000 ft). As a result, aircraft flying at virtually any altitude may have to deal with significant turbulence and wave-induced altitude excursions. In fact, the amplitude of this type of wave actually increases with height above the mountain (in the absence of atmospheric features, such as strong inversions or shear layers, that would tend to partially reflect or absorb the upward-moving wave energy). This amplification is a consequence of the normal decrease in air density with altitude.
The amplitude of the wave will be larger, for the same upstream conditions, the higher the mountain range above the surrounding terrain. Although, even very modest terrain relief can cause appreciable wave activity under the proper conditions.
Wave amplitude also will tend to be larger for stronger cross-mountain wind components at mountain-top level.
However, the actual amplitude depends on complex relationships between upstream atmospheric wind and temperature profiles and the height and shape of the particular mountain range. As you might imagine, stronger flow across the mountain leads to a deeper wave, given the same atmospheric stability. However, the greater the background stability, the shallower the resulting wave, at fixed-wind speed.
Breaking Vertically Propagating Wave As we have noted, the primary concern for pilots with this type of feature is that the vertical motions of the air moving through the wave may become strong enough to "break" into turbulence. Such wave breaking is believed to have been the source of the turbulence encountered in the 9 December 1992 accident at FL310 west of Denver, Colorado (Table 2-1, and Ralph et al., 1994).
What do we mean by "wave breaking"?
Looking again at the streamlines that show the airflow in Figure 5-1, we can see that high above the ridge there is a region of updraft. With a wave of modest amplitude (in which the vertical displacement of air moving through the wave is relatively limited), an aircraft flying through this region would likely experience appreciable "wave action," with altitude and/or airspeed fluctuations, but little turbulence. However, with sufficient amplitude, the wave breaks Air motion and localized updrafts and downdrafts occur. The consequences for a pilot flying through this region include airspeed and altitude deviations and the possible sudden onset of severe or extreme turbulence. This -60 -30 30 4 60 20,000 ft and 39,000 ft msl and is therefore type of turbulence occurs typically between → Windstorm Downslope primarily of importance to turboprop and jet aircraft at cruise as they approach and Figure 5-2. Schematic showing locations of jump and overfly the mountain range.
wave breaking region (after Durran and Klemp, 1983). Jump 18
Often accompanying these high altitude effects is the occurrence of very strong surface winds that result from the wave breaking aloft. In this case, strong downslope winds on the lee slopes can reach 100 kt in gusts, creating a low-level turbulence hazard for all aircraft. Further, these extremely strong low-level winds often abruptly terminate in a "jump" located some distance down the lee slope or well to the lee of the mountains themselves. These features are indicated schematically in Figure 5-2. The jump region is frequently an area of extreme turbulence extending to 10,000 ft or more above the surface. The area of the jump is sometimes marked by a line of ragged rotor clouds exhibiting very turbulent motion. Downwind of the jump, turbulence decreases in intensity but still may be quite strong.
Figure 5-3 depicts an intense mountain wave event that was investigated with aircraft, in which severe turbulence was encountered. The severe turbulence associated with this wave was widespread, occurring at high levels in and near the wave-breaking region and closer to the surface in the near vicinity of the jump.
Figure 5-4 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 5-5 is a photograph of the jump at the downstream edge of a region of strong downslope winds near Boulder, Colorado.
In this picture, one can see the smooth, relatively laminar Foehn (wall) cloud below the much more ragged rotor cloud extending horizontally across the scene.
The Foehn cloud is obscuring the mountains along the Continental Divide and is probably 3,000 to 6,000 ft thick. This feature is formed by condensation in the stable air that is being forced upward over the mountains. The area of the gap between this cloud and the overlying rotor cloud is a region of strong surface winds. The rotor cloud marks the downstream extent of this high wind, and probably the downstream limit of the strongest turbulence as well.
We will have more to say later about rotor zones. For now, we can stress that the airspace near and below a rotor cloud frequently contains severe-to-extreme turbulence and definitely should be 19
SABRELINER - QUEEN AIR 8 (K) 11 JANUARY 1972 *** Flight Track ¡(k)ft 200 mb ++++ Turbulence Encountered Potential Temperature (K) 325- 327.5. •K - 327.5 325 - 40 320- 322.5~ 322.5 - 35 250 320 300 317.5 30
• 317.5 350 315 25 400
312.5 315 20 500 307.5,.. 310- 310 312.5 305 • 310
302.5 307.5 600 305 700
- 302.5 ContIneni 800 / Divide / 900 WEST: • Distance - ‹ (Denver) 'EAST 1000
• 110 -100 -90 -80 - 70 - 20 30 40 50 Figure 5-3. Aircraft flight tracks and turbulence encounters associated with a wave-induced high-wind event (taken from Lilly, 1978).
5.1.1.1 Forecast and Observed Data Hydraulic Jump What types of forecast and observed data are available for detecting the likelihood of vertically propagating waves? For forecast wind direction, wind speed, and vertical wind shear, look at the Forecast Winds and Temperatures Aloft (FD) at altitudes near ridge level (for example, 9,000, 12,000, and 18,000 ft for the western United States; and 3,000, 6,000, and 9,000 ft for the lower terrain in the eastern United States). As a guideline for the existence of operationally important vertically propagating waves, one normally finds a value of 1.6 or less for the ratio of wind speeds 6,000 ft above the ridge to those at ridge-top level. For example, vertically propagating waves Figure 5-4. Schematic of the would be a concern if the atmosphere is strong shear zone associated with stable and the 18,000-ft winds are forecast a hydraulic jump in a mountain to be 50 kt, while the forecast winds are wave.
33 kt at 12,000 ft (ridge-top level in this example), a ratio of about 1.5
One can apply this criterion to observed data as well, using the 700 mb and 500 mb constant pressure charts. Remember, however, that these observations are taken only twice daily, at 122. and 002., so the data may be almost too old to be of value.
5.1.1.2 Charts For a measure of the stability of the atmosphere, look at the stability chart (one panel of a four-panel facsimile chart called the Composite Moisture Analysis). The same caveat on data age apples to this information observed at the same time as the data on the constant pressure charts.
Here, you are looking at the top numbers, the lifted indices, for a measure of the stability of the atmosphere. Positive numbers are stable, negative numbers are unstable; the larger the deviation from zero, the greater the stability or instability. You also can look at a radar summary chart;
echoes characterized as thunderstorms, or Figure 5-5. The Foehn cloud and RW+, are indicators of an unstable rotor clouds associated with a atmosphere. If you do not have access to jump at the downstream edge of a these charts in person, you merely have to region of strong downslope winds describe what you want to the Flight near Boulder, Colorado Service Station (FSS) briefer, who will read (photograph ©, 1991, R. Holle). the data for you.
5.1.1.3 Other Assistance Other important sources of information available during pre-flight planning include the Area Forecast (FA), AIRMETS and SIGMETS, Center Weather Advisories (CWA), Center Meteorological Impact Statements (MIS), and PIREPS. Indications of rotor clouds and/or altocumulus standing lenticulars (ACSL) in Aviation Routine Weather Reports (METARs) for an airport in or near a mountainous area are evidence that wave activity is present, as is a report of very strong surface winds in the absence of thunderstorms. It should be noted that automated observing systems are replacing human observers at many airports;
therefore, cloud-type information may not Additional hints of the presence of gravity waves, available in METARs, include pressure changes and wind gusts. Pressure jumps are indicated as PRJMP, followed by amount and time of observation. A rapid rise or fall in pressure would be noted as PRESRR or PRESFR, respectively. Wind gusts in the absence of other obvious physical mechanisms (such as fronts) also can indicate a gravity wave.
Visible GOES (Geostationary Observational Environmental Satellite) imagery also can give an indication of the likely existence of a mountain-induced gravity wave. In this case, there will be indications of clouds that have a stationary upstream edge over or near the known location of a mountain range, with the orientation of this upwind edge generally parallel to the orientation of the range.
Accurately forecasting the strength of turbulence associated with a wave is much more difficult than predicting the likelihood of wave development. The occurrence of mountain waves depends on complex interrelationships of terrain and atmospheric structure; therefore, the simple "rules" cited above should be applied with caution.
5.1.1.4 Summary Comments on Vertically Propagating Mountain Waves
• Large-scale winds at ridge level, blowing perpendicular (or nearly so) to the ridge-line, are normally required
• Wind speeds at ridge level are normally 20 kt or greater.
• Relatively weak vertical wind shear is present. Typically, the ratio of the wind speed 6,000 ft above ridge-top level to that at ridge-top level is less than 1.6 when this class of wave is of operational concern.
• The atmosphere is relatively stable. If there is a steep temperature lapse rate below 500 mb (an unstable atmosphere), with evidence of convection present, this type of atmospheric wave is unlikely to form.
• Vertically propagating waves are most winter and early spring months, when the winds at ridge level are strongest.
• In the generally prevailing westerly flow, these waves are most prevalent over mountain ranges that have a north-south orientation.
• Stronger ambient winds lead to a deeper wave.
• The greater the atmospheric stability, the shallower the wave (for a given wind speed).
5.1.2 Trapped Lee Waves In the preceding section, we discussed an important type of mountain wave that propagates (l.e., transports its energy) vertically. Now we want to 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 (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.
Trapped Lee Waves Height (km) Air motion Figure 5-6. Computer simulation of trapped lee waves behind a 300-meter-high mountain.