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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 9-26]
The lower part of Figure 9-26 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
Figure 9-25. Three-dimensional effects of oblique winds and
bowls.
9-20
9-21
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 along with post-flight analysis of the
weather conditions. Figure 9-27 illustrates a mountain
with wind and temperature profiles. Note the increase
in wind speed (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 is usually incredibly
smooth. Beneath the smooth wave flow is what is
known as a low-level turbulent zone, with an imbedded
rotor circulation under each crest. Turbulence,
especially within the individual rotors is usually moderate
to severe, and on occasion can become extreme.
[Figure 9-27]
Equilibrium
DALR
T
Parcel
Motion
Height (Thousands of Feet) Temperature
Temperature
30
25
20
15
10
5
Wind
Wind
Stable
Layer
Tropopause
CCSL
ACSL Lee Wave Region
Lower Turbulent Zone
Roll Cloud
Main Downdraft Main Update
Horizontal Distance
Cap Cloud
Figure 9-26. Parcel displaced vertically and oscillating around its equilibrium level.
Figure 9-27. Mountain lee wave system.
9-22
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, threedimensional
features, such as individual higher peak,
large ridges or spurs at right angles to the main range.
Variations can occur when a north-south range curving
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 too
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
is 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 9-27. Acap 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 terminology)
above. Wave harmonics further downstream (secondary,
tertiary, etc.) may also have lennies and/or rotor
clouds. If the wave reaches high enough altitudes,
lennies 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
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Glider Flying Handbook(120)
