Training Module 5:

Cyclogenesis -
The Development of
Mid-Latitude Cyclones

Adapted from An Introduction to Satellite Image Interpretation, Eric D. Conway and the Maryland Space Grant Consortium, ©1997, Johns Hopkins University Press, Baltimore, 255 pp with Interactive CD-ROM.

For more information about this book and how to order copies go to the JHU Press On-line Catalog


 

The images listed below all contain examples of storms known as mid-latitude cyclones. These storm systems affect all parts of the United States, especially in the winter as they charge across North America, often bringing extreme cold, heavy snow, ice storms, freezing rain, thunder storms (even thunder snow!), and other dangerous winter weather. These storms are often easily recognized by their traditional comma shaped cloud system, and can often be tracked days ahead of time, making them a great way to introduce yourself and your students to the use of satellite remote sensing to study the Earth's environment.

This web page provides some background information about the development and morphology of the mid-latitude cyclone and provides some examples of notable storm systems for your observations.

Directions

Once you are familiar with the material and information in this section, browse the images to locate the mid-latitude cyclone(s) in each. For each cyclone, try to determine where the central low pressure is, where each of the fronts are, and which stage of cyclone development the storm exhibits.


Full-sized imagery for this activity

Blizzard of 93 [blizard1.jpg]
Blizzard of 93 [blizard2.jpg]
Blizzard of 79 [bliz_79.gif]
4 Systems in the Pacific Ocean [cyclo.gif]
Blizzard of 96 [bliz_96.jpg]


Introduction

In the mid-latitudes, fronts usually occur as parts of larger storm systems known as mid-latitude cyclones. A mid-latitude cyclone is a weather system that includes a well-defined surface low-pressure area and associated warm, cold, and occluded fronts.

Cyclogenesis is a term that refers to the development of such a weather system. Mid-latitude cyclogenesis often occurs when an upper trough of intermediate wavelength (2500­5000 km, or 1500­3000 miles) interacts with a surface frontal zone. The low-pressure system grows in the presence of vertical wind shear (winds increasing with height) and thermal instability (convection). The factors that lead to lowering of the pressure at the surface are:

When these conditions exist together, a storm system is most likely to develop. The most common type of cyclogenesis is referred to as leaf-to- comma cyclogenesis. This type of storm development begins with a cloud system known as a leaf cloud, and concludes with a comma-shaped cloud system. An illustration of the cloud patterns that form during this type of cyclogenesis can be in the following diagrams. It should be noted that there are many types of cyclogenesis, and each storm is unique when studied in detail.

Leaf stage

Storm systems often start out as a cloud formation known as a leaf cloud. This feature is usually found on the east side of an upper-level trough, and is often elongated and leaf-shaped. Leaf clouds have well-defined borders and contain vertically deep and thick clouds. They show up very clearly on both IR and VIS imagery.

The poleward side of a leaf cloud has a very distinct border that often forms a flat S shape. Leaf clouds can often be identified by a curved notch on the western or southwestern edge of the leaf. This pattern is caused by the jet stream pushing into the western edge of the cloud system. The first illustration depicts a typical leaf cloud formation. The highest cloud tops in a leaf cloud are often located over the eastern portion of the leaf. Cloud top height decreases westward, with middle-level clouds appearing over the westward portion of the notch. Low clouds are found along the top portion of the notch. Usually a cold front is located along the equatorward border of the leaf or within the leaf cloud structure. The leaf cloud is a significant region of clouds and precipitation, even if cyclogenesis does not occur. The western edge of the leaf has a well-defined border and associated notch. This leaf has formed on the eastern side of a relatively high-amplitude jet stream trough.

Open comma stage

Within a leaf cloud, the air is rotating or spinning around a point of maximum vorticity (the region of maximum spin). As the air circulates around this point, the cloud system becomes distorted. If the whole system remains stationary, the clouds will gradually develop into a comma shape. If the system moves eastward, the cloud system will undergo further distortion into a cloud pattern called a comma cloud, shown in the second and third illustrations. Comma clouds appear in all sizes and shapes, depending on the development of the vorticity pattern. They range in size from small thunderstorms to large-scale mid-latitude cyclones, and they can change very rapidly.

Comma clouds have many characteristics that are important to identify when studying these storms. The back edge of a comma cloud, the part of the comma that is most easily identified, has a well-defined S shape. The point at which the back edge curvature changes from cyclonic (counterclockwise) to anticyclonic (clockwise) is called the inflection point. The front edge of a comma cloud is less clearly defined and tends to become very ragged as upper-level winds spread the clouds out. A comma cloud usually has the beginnings of a dry slot, also known as the surge region, where the jet stream crosses the system. This region usually forms as the notch in the leaf cloud expands. Here the jet causes the clouds to move more rapidly with respect to the other clouds, and they become more advanced downstream. The comma head generally lies to the west of the maximum winds. It tends to lag behind and to show the greatest tendency to rotate. The tail of the comma extends from the surge region southward. It generally lies more parallel to the axis of the maximum winds. A cold front is usually located along the tail.

As the comma develops, pressures in the storm system usually fall. The surface low migrates toward the western edge of the cloud mass, near where the inflection point and the jet stream are located. A warm front (often hard to see in satellite imagery) extends eastward from the inflection point.

Mature (occluded) stage

As the storm continues to develop, the low-pressure circulation may become cut off or isolated from the jet stream that it was originally associated with. Without the momentum from the jet stream and the associated temperature gradients that occur along the jet stream, the storm system loses its ability to deepen. The central pressure in the storm stops falling and may even begin to increase. The clouds in the comma system spiral around the center of the storm, and the cold front begins to overtake the warm front. It is at this stage that the system becomes occluded. The point at which an occluded, a cold, and a warm front all come together is referred to as the triple point; it is often seen in satellite imagery near where the jet stream cuts across the system.

Shearing stage

As the storm system continues to weaken, upper-level winds tend to tear it apart. The comma head is often cut off from the tail, and the cloud system loses its \organizational pattern. The comma head and its associated low pressure may lag behind and continue to rotate. This is referred to as the start of a cut-off, cold-core low. Such lows weaken slowly with time and tend to be very persistent. Some may last a week or more.