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Mountain Waves / Standing Waves

Maps from nationalatlas.gov now called The National Map of the U.S. Geological Survey's (USGS) National Geospatial Program

The ridge of higher terrain along the north shore of Lake Superior, including Duluth, sometimes induces waves in the atmosphere similar to those produced by mountain ranges. Satellite and personal observations have identified a variety of wave-like features. These features include lenticular clouds sitting stationary above or just downstream of the ridgeline, wavelike patterns in stratocumulus clouds, and what looks like altostratus clouds with a sharp back edge along the ridgeline. If enough moisture is present, the rising motion in the ridge (not the trough) part of the wave lifts the air to form clouds.

A specific type of wave that meteorologists in Duluth are keen on identifying is called a standing wave. A standing wave is a mountain wave that remains nearly stationary relative to the mountain or hill that obstructs the wind flow and triggers the formation of the wave. It typically tilts upstream as it propagates vertically into the atmosphere. Air flows from northwest to southeast through the wave, sinking in the trough of the wave then rising in the ridge. Conditions favorable for a standing wave to form include an elongated hill with a rounded top rather than a sharp narrow peak, a gradual rise in elevation on the windward (upwind) side of the hill, and an abrupt drop in elevation on the leeward (downwind) side of the hill. The winds need to be near perpendicular to the hill and optimally 25 knots or greater in the lower to middle troposphere. A generally stable atmosphere is also needed in a layer deep enough to extend above the height of the hill. A stable atmosphere is important because buoyancy forces cause the air that sinks down the hill to rise back up in an attempt to balance the downward movement, setting off a vertical oscillation of the air.

A standing wave is just one of several kinds of waves that can result from the displacement of air as it flows over mountains or hills. The structures, types, and numbers of waves are dependent on many factors. These factors include the shape of the hill, the height of the hill, the length of the hill, the degree of stability of the air, the particular height and depth of the stable layers, the wind speed, and the wind speed shear (change in wind speed and direction with height). Different types of waves can exist at the same time. Some waves may propagate hundreds of miles downstream. Standing waves as indicated by their name remain nearly fixed relative to the terrain barrier that triggers them. They can drift upstream or downstream a bit over time.

Northwest wind cold air outbreaks crossing the shore of Lake Superior sometimes provide favorable conditions for the formation of a standing wave and also snow if enough moisture is in the air. Typically the wave shows up on the National Weather Service radar in Duluth as a band of snow above or just downwind of the ridge crest on the higher elevation scans. The snow lowers downstream until it reaches the ground over the south shore of Lake Superior from Bayfield County, WI through Douglas County, WI, and into northern Pine County, MN. Winds from the northwest blow most of the snow downwind. Winds flowing down the slope of the hill warm and dry the air so that any snow that tries to fall onto Duluth will likely be eroded before it reaches the ground.

Snow produced by the wave can often be distinguished from lake effect when precipitation on radar shows a band of snow extending southwest beyond the tip of the lake. Note on the topographic map near the top of the page that the ridge along the north shore extends beyond the tip of the lake. On a more rare occasion, the wave can develop far enough west or tilt upstream enough to produce snow in Duluth. From personal observation, the snow is very light with the heavier snow farther southeast. An exception occurred on January 25, 2010. Moderate to heavy snow fell for a few hours as far inland as the Duluth International Airport and a little beyond that location.

References and additional reading

Dan Miller, Science and Operations Officer, National Weather Service, Duluth, MN

Forecasting Staff, National Weather Service, Duluth, MN

Definition of Mountain Wave - American Meteorological Society Glossary of Meteorology

The "Mountain Waves and Downslope Winds" training module from the MetEd website

Brady, R. H. and J. S. Waldstreicher, 2001: Observations of Mountain Wave-Induced Precipitation Shadows over Northeast Pennsylvania, Wea. Forecasting, 16, 281-300.

Smith, R. B., 1979: The influence of mountains on the atmosphere. B. Saltzman, Ed., Adv. Geophys., 21, 87-230.
[ Specifically note pages 88 and 89. ]

Kirkwood, P. D., D. M. Gaffin, and S. S. Parker, 2002: An Unexpectedly Heavy and Complex Snowfall Event across the Southern Appalachian Region. Wea. Forecasting, 18, 224-235.