Shortwave Trough

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This image is a 500hPa weather map depicting long wave and shortwave troughs in the atmosphere. The longwaves are represented by the red dashed lines whereas the shortwaves are represented by the blue dashed lines. There are many factors that can lead to the development of a shortwave trough, though they are strongest and most likely when a variety of elements overlap.

Strong upper-level winds, particularly in the jet stream, are essential for the formation and development of shortwaves. The presence of an upper-level jet stream provides the necessary dynamics and energy to initiate and sustain the development of shortwaves.
Large gradients in temperature and moisture (known as baroclinic zones) promote the development of shortwaves. Much like surface fronts, these contrasts in temperature and moisture in the mid and upper levels of the atmosphere create areas of instability and can serve as a trigger for the formation of shortwaves.
The counter-clockwise spinning motion of air (known as positive vorticity in upper levels), plays a role in the formation of shortwaves. Areas of increased vorticity can enhance the development and amplification of shortwaves.

Upward motion or lifting in the atmosphere is a critical ingredient for the formation of shortwaves. This vertical motion can result from various mechanisms, such as frontal boundaries, convergence zones, or interactions with other weather systems. The lifting promotes the development and intensification of shortwaves.

Image Source: NWS

This is an idealized depiction of a shortwave trough (double pink line) embedded in a longwave trough, enhancing lift from the surface, and supported further by divergence aloft.

The primary feature of a shortwave trough is its axis, a line or axis of low atmospheric pressure. It represents a region of relatively lower heights in the atmosphere compared to its surroundings. On weather maps, the trough axis appears as a line with a concave curvature, much like a surface trough. Shortwaves are typically associated with cyclonic or counter-clockwise circulation around the trough axis. The circulation pattern is a result of the pressure gradient within the trough.

Image Source: Adapted from University of Arizona’s diagram - ATMO336 - Spring 2009 (arizona.edu)Open a new window

Shortwaves often induce or enhance areas of atmospheric convergence near their trough axis. This convergence leads to the upward movement of air, which can result in the development of clouds and precipitation. The upward motion associated with the shortwave can contribute to the intensification of weather systems. Conversely, in the upper levels there is typically divergence that helps to enhance upward motion and can aid in the development of storms or other weather phenomena.

Shortwaves are frequently associated with surface frontal systems, such as cold fronts or warm fronts. The interaction between the shortwave and the fronts can lead to the formation or intensification of precipitation and changes in wind patterns.

Shortwave troughs, due to their ability to enhance upward motion, are often triggers for surface or mid-level-based convection. In any season, a passing shortwave can kick-start convection, such as by overpowering capping inversions, allowing once-trapped air to rise freely in a more unstable atmosphere.

This image shows an idealized shortwave trough acting as a trigger for summer convection. The upward motion caused by the shortwave enhances lift from the low to mid atmosphere, allowing humid air masses near the surface to be pulled into a part of the atmosphere where they may rise more freely. It is important to note that a shortwave is not always strong enough to trigger convection, this too depends on a variety of factors.

Image Source: Environment and Climate Change Canada

Dissipation

Shortwaves can dissipate or weaken through various processes in the atmosphere. The dissipation of shortwaves depends on several factors, including the environmental conditions and interactions with other weather systems.

As a shortwave moves into an area with less favorable conditions, it can gradually lose the energy needed to sustain itself and it weakens and dissipates. Environments leading to shortwave dissipation would be those that are more stable with less vertical wind shear, those that have a weaker temperature gradient, or less available moisture. Furthermore, loss of upper-level support, or the shortwave moving away from the jet stream can disrupt the energy supply and cause it to dissipate.

Shortwaves can interact with and be disrupted by other weather systems, such as larger-scale troughs or ridges. These interactions can lead to the merging or absorption of the shortwave by the larger-scale system, resulting in the dissipation of its distinct characteristics.

Even while travelling in the mid-levels of the atmosphere, shortwaves passing over or interacting with complex terrain, such as mountains or large bodies of water, can undergo modification or dissipation. The interaction with terrain can disrupt the airflow, alter the vertical motion patterns, or weaken and dissipate the shortwave.

Duration

The duration of shortwave troughs can vary depending on their size, strength and atmospheric conditions associated with the disturbance. Smaller-scale shortwaves that are weaker can last several hours. However, it is not uncommon for larger scale, more dynamic shortwaves to last several days.

The lifespan of a shortwave also depends on the dynamic interaction with other atmospheric features, such as fronts, upper-level winds, and the general flow pattern. Interaction with other systems can lead to the merging, amplification, or dissipation of shortwaves.

The climatology of shortwaves in Canada can vary across different regions and seasons due to the country's vast size and diverse geography. However, some general patterns can be observed:

Shortwaves can occur anywhere within Canada and often occur in conjunction with surface low pressure systems, and quite often follow the same tracks as low pressure systems as they are driven by the same overarching longwave pattern. Shortwaves are more intense in the shoulder seasons when the temperature gradients are largest, however, in the summer months shortwaves often act as a trigger for thunderstorm development. This image shows the main low pressure system tracks across Canada and their point of origin.

Image Source: SpringerLinkOpen a new window

Shortwaves provide many forecasting challenges for meteorologists due to their relatively small scale and their dynamic nature.

One of the main challenges in forecasting shortwaves is the timing and track of the system. Shortwaves usually move more quickly than larger-scale weather systems, and even a slight deviation in the forecast track can have a large impact on the associated change in weather, both at the surface and in the mid levels. This could potentially lead to a significant impact on the forecast severity of turbulence, of the development of thunderstorms and their severity, of precipitation amounts as well as precipitation typing.

Secondly, the exact intensity and mesoscale effects of shortwaves, depending on their location and their interaction with local terrain, can influence the severity and type of weather conditions produced.

Even in the presence of a shortwave, it is not always clear whether or not it will be able to overpower low level features and cause adequate lift to enhance/generate precipitation, or trigger significant convection. Forecasters must thoroughly analyze all elements and determine if the trough will be able to overcome elements that inhibit vertical motion.

Finally, sparse data, especially in Canada, makes it difficult to get an accurate representation of shortwave troughs for ingestion into weather models. Model resolution itself also plays a role in the challenge of shortwaves. Some mesoscale features produced by shortwaves cannot be properly resolved as these features are too small to be captured.

An upper-level shortwave trough is not always depicted in a GFA, especially when surface features exist that are also tied to the weather phenomena forecast to occur. Their depiction is only included when they are the dominant weather generator. In this example on February 8, the GFA panel valid at 0000Z shows a surface trough between CYOW (Ottawa) and CYUL that is forecast to generate occasional altocumulus castellanus (ACC) to an altitude of 24,000ft. While surface features such as troughs can certainly generate vigorous convection, it will often become deeper and longer lasting with the addition of upper-level support. Later sections will demonstrate how the presence of a shortwave trough assists the surface feature, making associated convection more distinguishable amidst continuous precipitation expected across a much wider area.

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The TAF for CYOW issued at 1143Z forecasts the passage of the surface trough starting at 2000Z, with lower conditions at 1 1/2SM visibility and BKN008 ceilings expected in snow. Similar to surface level features, the passing of upper features that enhance precipitation will be expressed in a TAF by periods of higher intensity or lower visibility in precipitation. Upper features, however, are harder to distinguish as they are not always accompanied by classic signatures such as wind shifts or changes in temperature/precipitation type. As the role of the TAF is to plan around changes in surface conditions, shortwave troughs and other upper features are not readily recognizable in this format.

In CYUL, the TAF issued at 1440Z (not shown here) showed 1 1/2SM of visibility in light snow between 2200Z and 0000Z. Like CYOW, forecasters early on captured the risk of possible lower conditions. By 2040Z, the TAF shown in this image, forecasters also had significant amounts of upstream observations and amended the forecast to capture possible further reductions in visibilities to 1/2SM in moderate snow showers.

This image is a capture from the SPC Mesoanalysis page on February 7 at 1900Z that is a graphical depiction of the process of surface convergence collocated with upper-level divergence (this process is explained in the second image under Necessary Ingredients in the Science Explained section). The red contours indicate convergence at low-levels of the atmosphere (850mb, approximately 5000ft) while purple contours indicate divergence in upper-levels (250mb, approximately 34,000ft). Though not perfectly overlaid, the significant upper-level divergence shown here and surface convergence along the surface trough (shown in the GFA) provides the support needed for the development of convection and associated tops forecast to 24,000ft. Radar imagery is also overlaid on this image behind the contours, with the most rigorous area nearly centered over the zone of maximum upper-level divergence generated by the shortwave trough.

Image Source: Storm Prediction Center Mesoscale Analysis PagesOpen a new window

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Mid-level water vapor satellite imageryOpen a new window is one of the best ways to visualize the passage of a shortwave trough. This GIF, valid between 1330Z and 2010Z on February 7 shows the rapid progression of the upper-level feature across eastern Ontario and into southern Quebec.

Image Source: CIRA

This still image of mid-level water vapor satellite imagery is shown to emphasize some of the signatures of a shortwave trough. Circles A, B, and C are areas of positive vorticity; counter-clockwise spinning air that can enhance a shortwave trough when they are in the same location. Dashed-line D is the approximate location of the surface trough at that time, while double dashed line E shows the approximate location of the upper-level shortwave trough. As discussed in the Science Explained section, forecasters will look at the relative location of surface and upper features to determine whether the surface weather is being supported from above. In this example, the surface feature (line D) is slightly ahead of the shortwave trough aloft (line E) which is ideal placement for upper-level support. The shortwave trough is being further strengthened by a region of positive vorticity (circle A) within it, which further supports lift from the surface, and likely serves to trigger or enhance convection.

Image Source: CIRA

The Corridor Integrated Weather System & Consolidated Storm Prediction for Aviation (CWIS/CoSPA) captured the rapid progression of the intense band of convective snow showers across Ontario and Quebec between 1855Z and 2055Z with tops verified as varying between 20,000ft and 26,000ft. The band is narrow and elongated along the axis the surface trough, supported by the upper-level shortwave trough following just behind. The surface trough jump-starts vertical motion from the surface, which the shortwave trough supports from higher in the atmosphere, providing additional divergence aloft which enhances vertical motion.

Image Source: MIT Lincoln Laboratory

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Since a shortwave trough passes higher in the atmosphere and not at the surface, it is not usually possible to read their passage in observations unless they are directly influencing precipitation passing over a particular station.

METARs for CYOW (Ottawa) capture the speed of the surface trough’s passage and how quickly conditions both deteriorated and improved within and behind it. The worst of the event lasted about 60 minutes (1928Z-2030Z), with conditions fluctuating significantly in that timeframe.

Image Source: OGIMET

The passage of the surface trough was noted similarly at CYUL, as ceilings and visibility both fell and improved as much and as quickly as at CYOW (Ottawa) two hours before, staying down for approximately 60 minutes. This consistency, when seen upstream of airports, helps forecasters determine onset, duration, and intensity of weather events and verify models as well.

Image Source: OGIMET

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