A Crash Course in Cyclogenesis

Cyclogenesis is the development and strengthening of surface low pressure areas in the middle latitudes of the globe. These surface low pressure systems are sometimes referred to as cyclones, where a cyclone is defined as a near circular air flow that is in the same direction as the rotating Earth, which is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. In the northern hemisphere, winds around low pressure centers blow counterclockwise and spiral inward. This inward flow at the surface (convergence) forces rising motion in the vicinity of cyclones. Clouds and precipitation form where air rises. Because cyclones often produce significant weather, which includes precipitation and strong winds, they are sometimes just referred to as "storms".

Much of the energy for the development of winter storms or cyclones is derived from the temperature difference (and more importantly the corresponding difference in air density) between cold air masses from the arctic and warm air masses from the tropics. From an energy standpoint, there is potential energy available along boundaries separating warm, light (low density) air from cold, heavy (high density) air. During cyclogenesis this potential energy is converted to kinetic energy (strengthening winds) as described in Energy available for cyclogenesis. Cyclogenesis, therefore, takes place where warm and cold air masses clash. The polar front is the generic name given to the boundary that separates warm tropical air masses from cold arctic air masses. On 500 mb height maps the polar front lies along the trough and ridge pattern with cold air to the north and warmer air to the south, particularly where the 500 mb winds are strong, because this indicates where there is a sharp warm/cold boundary. Explanation/Review. The 500 mb height is directly related to the temperature of the air in a vertical column between the ground and the 500 mb height level, such that warmer air results in higher 500 mb heights. This is because warmer air is more spread out (less dense) than colder air. When the 500 mb height lines are close together (tightly packed height contours on a 500 mb map), the air temperature is changing rapidly over a short distance. The polar front is located along this strong warm/cold boundary.

The figure below shows a typical 500 mb longwave pattern in black. The green "L" shows the most favored location for cyclogenesis, which is the development and strengthening of a surface low pressure area. Notice that surface cyclones most often develop under (or below in altitude) the location just downstream of a 500 mb trough. This is not a coincidence. Remember that there is an area of upper level divergence just downstream of 500 mb troughs. This favors upward vertical air motion and will work in tandom with the surface closed low pressure, which produces surface convergence and forced upward vertical air motion. The air flow at different levels in the atmosphere works together. At the ground surface there is horizontal convergence, forcing surface air to move upward through the troposphere. When the air reaches upper levels in the troposphere it diverges, completing the vertical circulation of rising air. The upward moving air, associated with surface low pressure the regions just downstream of 500 mb troughs, often results in the formation of clouds and precipitation. If the surface low gets strong enough, meteorologists will call it a winter storm. Finally, the surface low pressure gets "steered" or moved by the 500 mb winds, i.e., it moves in the direction of the 500 mb winds as indicated.

Typical mid-latitude winter weather system. Black lines represent the 500 mb height pattern. The positions of longwave troughs and ridges are shown in red. Green lines denote the position of a surface cyclone or low pressure area. Keep in mind that these features are at different altitudes. The black contours are part of 500 mb height maps, while the green lines represent a closed low on a surface weather map.

An air mass is a large area of air (typically thousands of square km) with nearly uniform properties of temperature and humidity. Air masses form when air remains stagnent or sits over a region for a few days or longer. These regions are dominated by persistent high pressure at the surface and light winds. After a few days, the air mass takes on the properties of the underlying surface. Concerning winter weather for the United States, cold air masses often develop over northern Canada or the northern Pacific Ocean, while warm air masses often develop over the Gulf of Mexico or the tropical Pacific Ocean.

Once air masses form, they may be put into motion by the large scale weather pattern. A 500 mb height map is a good way to view the large scale weather pattern. Cold air masses are often pushed southward where the 500 mb winds are from the north, while warm air masses are often pushed northward where the 500 mb winds are from the south. On surface weather maps, a weather front is simply the boundary between two different air masses. Where a cold air mass is invading an area currently covered by a warmer air mass, a cold front is indicated on a surface weather map. Conversely where a warm air mass is invading an area currently covered by a colder air mass, a warm front is indicated on a surface weather map. If neither the cold nor warm air is advancing, the frontal zone is marked by a stationary front. Weather fronts are marked on surface weather maps. Fronts often produce "weather" in the form of clouds and precipitation because air is forced to rise upward along the front. All fronts slope in the vertical so that the warmer (less dense) air mass sits on top of the colder (more dense) air mass. In other words, the warmer air mass is forced to rise over the colder air mass. Thus, the location of weather fronts on surface maps are important to see the boundary between and warm and cold air as well a region where air is forced to rise upward, which is necessary for clouds and precipitation to form.

In general, a rather narrow transition zone develops between two large bodies of air (air masses), which is called the frontal zone. Ideally, the polar frontal zone circles the northern hemisphere, marking a boundary between cold arctic air to the north and warmer subtropical air to the south. The shape of the idealized polar front would basically follow the wavy pattern of the 500 mb troughs and ridges. The polar front lies along where the 500 mb height contours are closely spaced together. Cyclogenesis most often occurs along this polar front, especially underneath areas located just downstream of 500 mb troughs because in these areas upper tropospheric divergence is occurring.

Polar Jet Stream

The polar jet stream a very fast moving current of air located near the top of the troposphere where air pressure is about 250 mb and the altitude is close 10 km. The polar jet stream occurs above the polar front. Energetically, the polar jet stream can be explained by the large pressure gradient (rapid change of the height of the 250 mb pressure surface over a short horizontal distance) that occurs above the polar front (see link provided in the next paragraph). The name jet stream was coined because high-flying military pilots encountered them during World War II. The polar jet stream generally flows from west to east, but follows a wavy path with troughs and ridges, just like the 500 mb flow. Today, commercial jets fly at altitudes near the jet stream. This is why flights from west to east are generally faster than flights from east to west. There is also a subtropical jet stream, which you may have heard mentioned in weather discussions, but we will not be able to cover that in this class.

Please take a look at A pictorial discussion of the polar jet stream. You should be able to explain why the polar jet stream is found above the polar front and why the wind speed above the polar front increases with increasing altitude such that the strongest winds (jet streams) are found near the top of the troposphere. Based on the discussion and figure above, you should be able to understand why a mid-latitude frontal zone produces a vertical wind shear pattern where wind speed increases with increasing altitude. Such a wind shear pattern can aid in the development of severe thunderstorms (assuming conditions are otherwise favorable for the development of thunderstorms).

The polar jet stream can be visualized from upper level height maps near the top of the troposphere, such as the 200 and 300 mb maps. Here is a Link to a current forecast of the 200 mb height (shown in contours) and wind speeds (shown with color shading) from Pivotal Weather. And here is a link to a current forecast of the 300 mb height pattern from Pivotal Weather. The "jet stream" shows up as concentrated regions of higher wind speeds.

Here is a link to A common type of the current jet stream map from Intellicast. On the map, arrows indicate the wind direction and the color shading indicates the wind speeds that are greater than 80 knots. The actual jet stream typically shows up as a narrow band of strong winds that gets concentrated above the polar front.

The position of the polar front (or warm/cold boundary) is not fixed but moves south and north within the middle latitudes in the winter. We have already seen evidence for this in the 500 mb height patterns. In 500 mb troughs, the polar front moves south as cold air masses plunge southward and in 500 mb ridges, the polar front moves northward as warm air masses push northward.

Due to time constraints we will not cover the remainder of this page this semester. Please feel free to read the remainder of the page if you are interested.

In addition within the 500 mb pattern, rising motion and precipitation is most likely just downwind of 500 mb troughs. In order for everything to work together and be consistent, it should make sense to you that surface low pressure (associated with rising motion) will be found underneath regions just downstream of 500 mb troughs. An idealized picture is shown below.

Typical mid-latitude winter weather system. Green wavy lines represent the 500 mb height pattern. The red circles represent the pattern of sea level isobars around a surface low and high pressure. Notice how the surface circulation brings cold air southward (under the 500 mb trough) and warm air northward under the 500 mb ridge. In this way middle latitude circulations transport energy from the equator toward the poles.

Ideally, surface low pressure will be located under the region of 500 mb divergence just downwind of a trough axis and surface high pressure will be located under the region of 500 mb convergence just downwind of a ridge axis. A good example showing both the 500 mb height pattern and the surface pressure pattern on the same map is provided. The 500 mb height pattern is color shaded, while the sea level pressure pattern is shown using contours. A 500 mb trough is located in the center part of the US with 500 mb ridges over the mountain west and the northeast. A surface low pressure area is located just downwind of the trough, while a surface high pressure area is located just downstream of the western 500 mb ridge.

Below is a series of three links to lecture notes that I used in a previous class. It shows an idealized representation of the lifecycle of a middle latitude surface cyclone from birth to death. The last page shows the energetics involved. Weather fronts are sloped in the vertical so that warm air is always lifted above colder air. This should help you to understand the precipitation patterns shown in the links below. Don't be concerned about the occluded front, just realize that it also forces lifing of air.

Finally, we need to put this all together to get a 3-D picture of what occurs during cyclogenesis. As mentioned above, developing surface low pressure areas, which are characterized by surface convergence and rising motion, will tend to form underneath where the upper level wind patterns (500 mb and above) are also favorable for rising motion, i.e., just downstream of upper level troughs where upper level divergence occurs.
Relationship between rising motion, surface convergene, and upper level divergence

You should now realize the following:

• If upper level divergence > surface convergence, surface low strengthens (pressure lowers)
• If upper level divergence = surface convergence, surface low remains steady
• If upper level divergence < surface convergence, surface low weakens (pressure rises)

My best attempt at drawing a 3-D representation of a mid-latitude winter weather pattern is given below.