Wave Soaring Weather
Where there is wind and stable air, there is the likelihood of waves in the atmosphere. Most of the waves that occur throughout the atmosphere are of no use to the glider pilot. However, mountains or ridges often produce waves downstream, the most powerful of which have lifted gliders to 49,000 feet. Indirect measurements show waves extending to heights around 100,000 feet. If the winds aloft are strong and widespread enough, mountain lee waves can extend the length of the mountain range. Pilots have achieved flights in mountain wave using three turn points of over 2,000 kilometers. Another type of wave useful to soaring pilots is generated by thermals, which were discussed in the previous section.
A common analogy to help visualize waves created by mountains or ridges uses water flowing in a stream or small river. A submerged rock causes ripples (waves) in the water downstream, which slowly dampen out. This analogy is useful, but it is important to realize that the atmosphere is far more complex, with vertical shear of the wind and vertical variations in the stability profile. Wind blowing over a mountain does not always produce downstream waves.
Mountain wave lift is fundamentally different from slope lift. Slope soaring occurs on the upwind side of a ridge or mountain, while mountain wave soaring occurs on the downwind side. (Mountain wave lift sometimes tilts upwind with height. Therefore, at times near the top of the wave, the glider pilot may be almost directly over the mountain or ridge that produced the wave). The entire mountain wave system is also more complex than the comparatively simple slope soaring scenario.
Mechanism for Wave Formation
Waves form in stable air when a parcel is vertically displaced and then oscillates up and down as it tries to return to its original level, illustrated in Figure 9-24. In the first frame, the dry parcel is at rest at its equilibrium level. In the second frame, the parcel is displaced upward along the DALR, where it is cooler than the surrounding air. The parcel accelerates downward toward its equilibrium level but it overshoots the level due to momentum and keeps going down. The third frame shows that the parcel is now warmer than the surrounding air, and starts upward again. The process continues with the motion damping out. The number of oscillations depends on the initial parcel displacement and the stability of the air. In the lower part of the figure, wind has been added, illustrating the wave pattern that the parcel makes as it oscillates vertically. If there were no wind, a vertically displaced parcel would just oscillate up and down, while slowly damping, at one spot over the ground, much like a spring. [Figure 9-24]
The lower part of Figure 9-24 also illustrates two important features of any wave. The wavelength is the horizontal distance between two adjacent wave crests. Typical mountain wavelengths vary considerably, between 2 and 20 miles. The amplitude is half the vertical distance between the trough and crest of the wave. Amplitude varies with altitude and is smallest near the surface and at upper levels. As a note, mountain lee waves are sometimes simply referred to as mountain waves, lee waves, and sometimes, standing waves.
In the case of mountain waves, it is the airflow over the mountain that displaces a parcel from its equilibrium level. This leads to a two-dimensional conceptual model, which is derived from the experience of many glider pilots plus postflight analysis of the weather conditions. Figure 9-25 illustrates a mountain with wind and temperature profiles. Note the increase in windspeed (blowing from left to right) with altitude and a stable layer near mountaintop with less stable air above and below. As the air flows over the mountain, it descends the lee slope (below its equilibrium level if the air is stable) and sets up a series of oscillations downstream. The wave flow itself usually is incredibly smooth. Beneath the smooth wave flow is what is known as a low-level turbulent zone, with an embedded rotor circulation under each crest. Turbulence, especially within the individual rotors, is usually moderate to severe, and can occasionally become extreme. [Figure 9-25]
This conceptual model is often quite useful and representative of real mountain waves, but many exceptions exist. For instance, variations to the conceptual model occur when the topography has many complex, three-dimensional features, such as individual higher peak, large ridges, or spurs at right angles to the main range. Variations can occur when a northsouth range curves to become oriented northeast-southwest. In addition, numerous variations of the wind and stability profiles are possible.
Turbulence associated with lee waves deserves respect. Low-level turbulence can range from unpleasant to dangerous. Glider pilots refer to any turbulence under the smooth wave flow above as “rotor.” The nature of rotor turbulence varies from location to location as well as with different weather regimes. At times, rotor turbulence is widespread and fairly uniform; that is, it is equally rough everywhere below the smooth wave flow. At other times, uniformly moderate turbulence is found, with severe turbulence under wave crests. On occasion, no discernable turbulence is noted except for moderate or severe turbulence within a small-scale rotor under the wave crest. Typically, the worst turbulence is found on the leading edge of the primary rotor. Unfortunately, the type and intensity of rotor turbulence are difficult to predict. However, the general rule of thumb is that higher amplitude lee waves tend to have stronger rotor turbulence.
Clouds associated with the mountain wave system are also indicated in Figure 9-25. A cap cloud flowing over the mountain tends to dissipate as the air forced down the mountain slope warms and dries. The first (or primary) wave crest features a roll or rotor cloud with one or more lenticulars (or lennies, using glider slang) above. Wave harmonics farther downstream (secondary, tertiary, etc.) may also have lenticulars and/or rotor clouds. If the wave reaches high enough altitudes, lenticulars may form at cirrus levels as well.
It is important to note that the presence of clouds depends on the amount of moisture at various levels. The entire mountain wave system can form in completely dry conditions with no clouds at all. If only lower level moisture exists, only a cap cloud and rotor clouds may be seen with no lenticulars above, as in Figure 9-26A. On other days, only mid-level or upperlevel lenticulars are seen with no rotor clouds beneath them. When low and mid levels are very moist, a deep rotor cloud may form, with lenticulars right on top of the rotor cloud, with no clear air between the two cloud forms.
In wet climates, the somewhat more moist air can advect (meaning to convey horizontally by advection) in, such that the gap between the cap cloud and primary rotor closes completely, stranding the glider on top of the clouds, as in Figure 9-26B. Caution is required when soaring above clouds in very moist conditions.
Suitable terrain is required for mountain wave soaring. Even relatively low ridges of 1,000 feet or less vertical relief can produce lee waves. Wave amplitude depends partly on topography, shape, and size. The shape of the lee slope, rather than the upwind slope, is important. Very shallow lee slopes are not conducive to producing waves of sufficient amplitude to support a glider. A resonance exists between the topography width and lee wavelength that is difficult to predict. One particular mountain height, width, and lee slope is not optimum under all weather conditions. Different wind and stability profiles favor different topography profiles. Hence, there is no substitute for experience at a particular soaring site when predicting wave-soaring conditions. Uniform height of the mountaintops along the range is also conducive to better organized waves.
The weather requirements for wave soaring include sufficient wind and a proper stability profile. Windspeed should be at least 15 to 20 knots at mountaintop level with increasing winds above. The wind direction should be within about 30° perpendicular to the ridge or mountain range. The requirement of a stable layer near mountaintop level is more qualitative. A sounding showing a DALR, or nearly so, near the mountaintop would not likely produce lee waves even with adequate winds. A well-defined inversion at or near the mountaintop with less stable air above is best.
Weaker lee waves can form without much increase in windspeed with height, but an actual decrease in windspeed with height usually caps the wave at that level. When winds decrease dramatically with height, for instance, from 30 to 10 knots over two or three thousand feet, turbulence is common at the top of the wave. On some occasions, the flow at mountain level may be sufficient for wave, but then begins to decrease with altitude just above the mountain, leading to a phenomenon called “rotor streaming.” In this case, the air downstream of the mountain breaks up and becomes turbulent, similar to rotor, with no lee waves above.
Lee waves experience diurnal effects, especially in the spring, summer, and fall. Height of the topography also influences diurnal effects. For smaller topography, as morning leads to afternoon and the air becomes unstable to heights exceeding the wave-producing topography, lee waves tend to disappear. On occasion, the lee wave still exists but more height is needed to reach the smooth wave lift. Toward evening as thermals again die down and the air stabilizes, lee waves may again form. During the cooler season, when the air remains stable all day, lee waves are often present all day, as long as the winds aloft continue. The daytime dissipation of lee waves is not as notable for large mountains. For instance, during the 1950s Sierra Wave Project (see www.soaringmuseum.org), it was found that the wave amplitude reached a maximum in mid to late afternoon, when convective heating was a maximum. Rotor turbulence also increased dramatically at that time.
Topography upwind of the wave-producing range can also create problems, as illustrated in Figure 9-27. In the first case [Figure 9-27A], referred to as destructive interference, the wavelength of the wave from the first range is out of phase with the distance between the ranges. Lee waves do not form downwind of the second range, despite winds and stability aloft being favorable. In the second case [Figure 9-27B], referred to as constructive interference, the ranges are in phase, and the lee wave from the second range has a larger amplitude than it might otherwise.
Wave flight is a unique soaring experience and requires planning, equipment and Federal Aviation Administration (FAA) notification for the flight. The Soaring Society of America (SSA) Awards and Badges offers soaring pilots Lennie Awards for completing and documenting a wave flight. [Figure 9-28]
Isolated small hills or conical mountains do not form classic lee waves. In some cases, they do form waves emanating at an angle to the wind flow similar to water waves created by the wake of a ship. A single peak may require only a mile or two in the dimension perpendicular to the wind for high amplitude lee waves to form, though the wave lift is confined to a relatively small area in these cases.