Mountain and Lee Waves in Satellite Imagery

Dr. Brad Muller

The source of much of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR) pursuant to a Cooperative Agreements with the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. ©1997-2007 University Corporation for Atmospheric Research. All Rights Reserved.




Mountain 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

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 mountain waves:

(Image from Gary Ellrod's COMET site)
http://www.comet.ucar.edu/class/satmet/common/html/website_index/ellrod/mwavlo.gif


Vertically Propagating Waves

Vertically propagating waves occur when
-Wind speed above the mountain does not increase significantly.
-Stability increases above the mountain top stable layer.


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:

(Image from Gary Ellrod's COMET site)
http://www.comet.ucar.edu/class/satmet/common/html/website_index/ellrod/mwavhi.gif


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):