waves occur when there is a stable layer
near or just above mountain top height, and
the air flow is more or less perpendicular
to the mountains.
Types of Mountain Waves
There are two main types of mountain waves: trapped
waves and vertically propagating waves.
Trapped waves occur when wind speed
above the mountain increases sharply with height and
when stability decreases
above the mountain top stable layer.
VIS image, trapped waves:
Airflow over mountains:
Mountain waves are not always turbulent, but
they can be. The horizontal spacing of the waves
is related to the wind speed over the mountains.
Higher speeds yield longer wavelengths (greater
spacing). Faster wind speeds also are related to
stronger mountain wave turbulence as shown in the
following graph relating aircraft
turbulent intensities to wavelength of trapped
propagating waves occur when
-Wind speed above the mountain does not increase
-Stability increases above the mountain top stable
Vertically propagating waves can become very turbulent
in the upper troposphere and lower stratosphere in the
so-called "breaking wave" region, and also in the "hydraulic
jump" region of the wave. They are closely related to
downslope wind storms such as Chinooks (see image below).
Water vapor image, vertically propagating waves:
Vertically propagating waves often produce orographic
cirrus on their downwind side. According to Ellrod,
these waves are not
turbulent when the clouds are next to the mountains.
However, if they are downwind with a "Foehn gap" between the
mountains and the clouds (due to strong subsidence in the
lee of the mountains) then they are turbulent:
VIS image, vertically propagating waves with a Foehn gap:
The following figure shows the path of an aircraft flight
through a vertically propagating mountain wave and
associated turbulence measurements:
This image shows a hydraulic jump and rotor cloud east of
the Sierra Nevada Mountains over the Owens Valley:
Animations can help sort out whether or not there are
mountain waves--features are nearly stationary when mountain
wave effects are occuring. These are vertically
propagating waves (and also some gravity waves):