Lee Wave

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Lee waves occur when there is a strong wind flow across a barrier such as a mountain range. A stable air layer is essential for the development of lee waves. This stability is typically characterized by a strong temperature inversion, where the temperature increases with height instead of decreasing. When the wind reaches the barrier, it ascends over the barrier but is deflected downward to lower levels of the atmosphere downwind due to the stable layer. This inversion at or above mountain top is often referred to as a “critical layer”, which enables amplification of the mountain waves in the layers between the mountaintop and the critical layer.

Adequate wind strength is necessary for the generation and maintenance of lee waves and their upward and downward motion of the stable air. This flow must also have strong wind shear in a deep stable layer with increasing winds with height above the mountain top and stability decreasing above the barrier. Lee waves are more likely when the wind direction is roughly perpendicular to the orientation of the barrier. In general, a wind direction within 30° of perpendicular to the barrier is sufficient for lee wave formation.

While not a strict requirement, the presence of moisture in the atmosphere can enhance the visibility and cloud formations associated with lee waves. Moisture can contribute to the formation of lenticular clouds, rotor clouds, or other cloud features that accompany lee waves. However, the stability of the air is primarily driven by temperature, so lee waves can still form in relatively dry air.

Image Source: COMETOpen a new window

Lee waves consist of alternating crests and troughs, similar to ocean waves. The crests represent the upward movement of air, while the troughs correspond to the downward movement of air. The distance between consecutive crests or troughs is known as the wavelength of the wave. Lee waves have a wavelength of 5-35 km and can persist for hundreds of kilometers downwind of the barrier. Lee waves are usually constrained within a few thousand feet of the mountain top and their amplitude decreases with height.

Lenticular clouds and rotor clouds are often associated with lee waves. Lenticular clouds form when moist air rises and cools as it ascends over the wave crest. Lenticular clouds can be stationary or slowly move with the wind, indicating the presence of a lee wave. Rotor clouds are turbulent clouds that can form beneath the crest of a lee wave. They result from the interaction between the wave and the airflow descending behind the wave crest.

It's important to note that the structure of a lee wave can vary depending on the specific atmospheric conditions, the shape of the mountain or obstruction, and other factors. The wave structure can be influenced by the interaction between the wave and the surrounding airflow, as well as the stability of the atmosphere and moisture content.

This is a depiction of lee waves and the formation of lenticular and rotor clouds. Depending on the moisture aloft lee waves can form alto- or cirro-lenticularis clouds in the wave crests. This leads to a well recognized cloud pattern with bands of lenticular clouds that mirror the profile of the barrier. Rotor clouds may also form beneath the crest of the lee waves.

Image Source: SkybraryOpen a new window

Dissipation

Lee waves will continue until there is sufficient change in either the wind speed or direction or if the stability profile of the atmosphere changes. Typical causes for the dissipation of lee waves can include the passage of a low pressure system and its associated cold front or rising pressures on the windward side of the barrier.

Duration

The duration of lee waves can vary widely depending on several factors, including atmospheric conditions, topography, and wind patterns. Lee waves can persist for relatively short periods, such as minutes or hours, or they can last for several hours or even days in some cases. Observing and forecasting the exact duration of lee waves can be challenging due to the complex interactions between atmospheric conditions and topography. The use of high-resolution models, detailed observations, and interpretation by meteorologists can help provide more accurate estimates of the duration of lee waves in specific situations.

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In general, lee waves in Canada are more common in regions with significant mountainous terrain. The western provinces, such as British Columbia and Alberta, which are home to the Rocky Mountains and the Coast Mountains, experience frequent lee wave activity. The Appalachian Mountains in the east also experience the formation of lee waves.

Additionally, the Canadian Prairies, located east of the Rocky Mountains, can also experience lee waves. In this region, lee waves can form when stable air flows over the Rocky Mountains and encounters the relatively flat terrain of the prairies.

This is a topographical map of Canada. As the bottom of the chart shows, the Rocky Mountains form the highest elevations in Canada, which are more prone to the development of lee waves. Other locations that are prone to lee waves include the coastal mountains and the Appalachian Mountains in eastern Canada.

Image Source: Canadian GeographicOpen a new window

Typically, lee waves occur more frequently in the winter when winds are strongest, and the atmosphere is most stable. Conversely they occur least frequently in the summer months when the atmosphere is less stable.

Here is a graph showing the frequency of PIREPS that report mountain wave turbulence. The graph clearly shows that most reports of mountain wave turbulence occurs in December and January, when the atmosphere is more stable and winds are strongest. There is significantly less that occur in July, August and September.

Image Source: COMET Open a new window

Detecting lee waves can be difficult to distinguish as they are not always visible to the naked eye. The main challenges in forecasting lee waves involve determining if the wind speed, shear, and stability of the atmosphere is going to generate sufficient wave action to cause turbulence. Typically, at least 20 knots at mountain top are the minimum wind speed necessary to develop lee waves. The forecaster must also assess the wind direction and whether or not it is perpendicular to the mountain range, if the atmosphere is stable near the mountaintop, and whether the wind shear is enough to produce lee waves.

Lee waves are not well handled in forecast models as the vertical resolution is not sufficient to resolve these features. Lenticular clouds and rotors are also very localized and require expertise in local topography in order to be able to forecast.

The clouds and weather panel for the GFA valid at 1200Z on October 28, 2022 shows isobars perpendicular to the Rockies, and strong surface winds expected across far-southern Alberta and into Saskatchewan, including forecast gusts of 45KT around CZPC (Pincher Creek, AB).

The icing and turbulence panel is where mentions of lee wave turbulence will outright be described, including location and altitude impacted. On October 28, severe lee wave and mechanical turbulence is forecast just south of CYYC to the USA border from the surface up to 10,000ft.

This is the atmospheric profile valid at 2100Z October 28, 2022 for CZPC (Pincher Creek, AB), which in this event was located in the area of the issued SIGMET and forecast lee wave turbulence. Peaks for the Rockies west of CZPC average around 8000ft, or 750mb. As described in the science explained section for necessary ingredients: we can see the inversion just above 750mb creating the needed “critical layer” (purple circle) and strong winds nearly perpendicular to the Rockies to both develop and maintain lee waves. Atmospheric instability exists below this inversion all the way to the surface, explaining the presence of possibly severe mechanical turbulence mentioned in the GFA and the risk of gusts to 45KT in the CZPC area.

Image Source: Pivotal Weather

This is a Graphical Turbulence Guidance (GTG) forecast valid at 2200Z on October 28 at an altitude of 7000ft. The narrow area of higher EDR along the leeside of the Rockies in Alberta captures the lee wave activity forecast that day, with the southern-most sections of the province showing the highest values. This area is collocated with the area of severe turbulence forecast in the GFA panel.

Image Source: Aviation Weather Center

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Visible satellite imagery on October 28, 2022 between 1610Z and 2300Z captures multiple signatures associated with lee wave activity. Stationary cloud bands parallel to the Rocky Mountains can be seen throughout the event. These clouds are a result of the rise and fall of air as it propagates east of the range, with enough moisture in the air to form cloud in the areas of upward motion. A higher, also stationary, cirrus deck is visible and especially pronounced just east of the Rockies in the southern-most section of Alberta. This is collocated with the issued SIGMET and area of strongest lee wave activity. Finally, the clear narrow gap seen immediately east of the Rockies is also known as the Foehn Gap, with its width defined by the wavelength of the associated lee waves.

Image Source: CIRAOpen a new window

Mid-level water vapor satellite imagery, valid 1610Z to 2340Z October 28, 2022 captures lee wave activity from a slightly different perspective. There is a dark band visible closest to the leeside of the Rockies across southern Alberta, with gravity waves just east of it. This is another perspective of the Foehn Gap seen in visible satellite imagery and captures the subsiding air on the immediate leeside of the mountain range, followed by gravity waves as air propagates eastward. The stationary and sharply defined blue and green area is upper-level orographic cirrus associated with vertically propagating wavesOpen a new window. Research has shown that the presence of this feature with a Foehn Gap increases the likelihood of turbulence in the area.

Image Source: CIRAOpen a new window

The METAR at CZPC valid at 2100Z within the active SIGMET captures the atmospheric instability shown in the atmospheric profile under the forecast section. Sustained surface winds at 32KT and gusts to 41KT verify the enhanced mixing present with stronger winds aloft, made possible by the instability in the area.

SIGMET G7 is a forecast SIGMET issued at 2000Z on October 28, 2022 for lee wave activity from the surface up to 12,000ft. The SIGMET is forecast rather than observed as no PIREPs were received to confirm the presence of turbulence, however satellite signatures, METARs, and forecast model data available to forecasters all presented classic signatures associated with its development.

The south view of the WEBCAM from CZPC, from 2020Z to 2120Z (see top right text in red for valid time) captures the signatures shown in satellite imagery from a ground-level perspective. The relatively clear gap between the Rockies, seen in the distance, and the stationary cirrus deck on the left-hand side of the image is the Foehn Gap seen in both visible and mid-level water vapor satellite imagery. Embedded within the stationary orographic cirrus deck on the left are multiple glimpses of the parallel bands of cloud associated with lee wave activity. Note how the western-most edge of this cirrus deck (burgundy line) remains clearly defined with only the north-to-south cloud coverage varying. This stationary feature is also captured in associated satellite imagery.

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