Standing Wave May Have Produced Light Accumulating Snow in Duluth, Minnesota, April 7, 2007
This summary is only a limited analysis.
A standing atmospheric wave may have produced light snow in Duluth, Minnesota on the morning of April 7, 2007. Duluth is located at the western tip of Lake Superior. The city stretches along a ridge that runs along the north shore of the lake. A tenth of an inch of snow fell at the National Weather Service office six miles inland from the Lake Superior. This author located in central Duluth about one mile and a half inland measured two tenths of an inch. Widely scattered light snow showers not directly associated with the wave also contributed to the light accumulations. Snow with this kind of event usually falls downstream. One reason is that the event is typically detected with northwest winds through a deep layer of the troposphere. These northwest winds blow the snow southeast away from the city before the snow can reach the ground. A downslope wind direction also tends to dry out the air.
2. Definition of a Standing Wave
A standing wave is one that remains stationary relative to the barrier that induces the formation of the wave. It typically tilts upstream as it propagates vertically into the Troposphere, sometimes extending into the Stratosphere. Air flows through the wave, sinking in the trough and rising in the ridge. In the April 7, 2007 event, the ridge of higher elevation along the north shore of Lake Superior served as the obstruction. Conditions favorable for a standing wave to form include an elongated hill with a rounded top rather than a sharp narrow peak, winds perpendicular to the hill, winds 25 knots or greater in the lower to middle troposphere, and a generally stable atmosphere. A stable atmosphere is important because buoyancy forces cause the air displaced upward by the hill to be forced back to its previous level, setting off a wave-like 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 structure, type, and number of waves is 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 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 down stream.
3. Conditions for a Standing Wave on April 7, 2007
The two topographic criteria are easily met as the north shore ridge is definitely long and the top very broad. The side of the ridge facing the lake is steep but levels off beyond the crest. A large high pressure system extending from central Canada through most of the central U.S. (figure 4) contributed to stable conditions. Northwest to north winds provided air flow perpendicular to the ridge. Preceding the event, upper air charts valid at 00 UTC April 7 (6 PM CST April 6), 2007, for 925, 850, and 700 mb (figures 5, 6, and 7) showed 20 to 30 knot winds. Winds at the immediate surface were a little weaker (figure 4). Enough moisture lingered in the air upstream so that lift associated with the wave produced clouds and precipitation.
4. Why the Snow Fell in Duluth
On April 7, 2007, snow reached the ground in Duluth as shown by radar images. Figures 1, 2, and 3 show 0.5 degree reflectivity images from the National Weather Service's WSR-88D radar 10:35 UTC (4:35 AM CST), 11:24 UTC (5:24 AM CST), and 16:36 UTC (10:36 AM CST) respectively. Note how the snow extends east-northeast along the lake shore providing evidence that it is attached to the terrain.
Winds the previous evening, the evening of April 6, 2007, over the western tip of Lake Superior and the arrow head of Minnesota, were initially from the northwest at 00 UTC April 7, 2007 (6 PM CST April 6, 2007) from the surface to 850 mb (figures 4, 5, and 6). Winds were from the north at 700 mb (figure 7) and the northeast at 500 mb and above (figures 8 and 9). By 12 UTC April 7, 2007 (6 AM CST April 7, 2007) winds at 850 and 700 mb had veered to a northeast direction nearly parallel to the shore (figures 12 and 13). Winds at other levels, specifically the surface, 925 mb, 500 mb, and 300 mb, remained about the same (figures 10, 11, 14, and 15). The change in wind direction may have allowed the snow to get closer to the ground before encountering northwest winds near the surface. The winds were then not able to blow all of the snow southeast of the city before the snow reached the ground.
5. Tentative Conclusions
Snow produced by standing waves over the north shore ridge of Lake Superior rarely reaches the ground in Duluth, MN. Northwest winds blow the snow downwind before it reaches the surface. A switch in wind direction from northwest to northeast more than several thousand feet above the surface may have provided the snow a better chance to reach the ground. Remember that other factors also determine whether the snow reaches the ground, such as the adiabatic heating that occurs when air moves downhill toward the lake. The heating dries the air causing snow to sublimate before it reaches the ground. Other mechanisms possibly contributing to snow formation were not addressed.
6. Data Sources
Surface and upper air maps compliments of the National Centers for Environmental Prediction (NCEP).
Radar images compliments of the National Weather Service.
Lake Superior map compliments of nationalatlas.gov now called nationalmap.gov.
References and Consultation
Dan Miller, Science and Operations Officer, National Weather Service, Duluth, MN
Forecasting Staff, National Weather Service, Duluth, MN
“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
List of Figures