8.3: Mid-latitude Frontal Cyclones

Cyclogenesis and Life Cycle

Early weather forecasters observed a pattern that occurred whenever significant precipitation occurred in the Northern Hemisphere mid-latitudes. First, pressure would begin to drop, then the winds would increase, then precipitation would occur, and finally the skies would clear again. The wind direction would also change in a predictable manner, similar to a tropical cyclone. Based on this pattern, meteorologists in Bergen, Norway, developed a model to explain the life cycle of a "mid-latitude cyclone". According to this Norwegian model, a mid-latitude cyclone develops along the polar front. The polar front is a stationary front that separates cold polar air from warmer subtropical air at around 60° latitude. It is a semi-continuous boundary, along which mid-latitude cyclones form and move as a series of waves. For this reason, a developing storm is sometimes referred to as a wave cyclone.

Figure \(\PageIndex{1}\) shows a portion of the polar front as a stationary front, with cold air to the north and warmer air to the south flowing parallel to the front in opposite directions. These winds moving in opposite directions can set up a rotational motion along the front. This is similar to how a pen turns when you place it between your palms and move your left and right hands in opposite directions.

Figure \(\PageIndex{1}\): Part 1 of cyclogenesis consists of a stationary front with opposite-moving winds on either side. Cooler winds are to the north heading west, while warmer winds are to the south heading east. (Public Domain; Pflatau via Wikimedia Commons).

Under the right conditions, a frontal wave will begin to form along the front, with a cold front pushing southward and a warm front moving northward. The lowest pressure lies at the junction of the two fronts. The southward-moving cold front pushes warmer, less dense air upward, while the warm front overruns and moves over the colder air ahead of it. This creates rising motion in the column, and a narrow band of precipitation forms. The surface low-pressure system is steered by winds aloft, typically moving eastward or northeastward, and it gradually develops into a fully mature cyclone 12 to 24 hours after its incipient stage.

Figure \(\PageIndex{2}\): Part 2 of cyclogenesis includes the formation of a frontal wave or a frontal boundary in front of the low-pressure region. (Public Domain; Pflatau via Wikimedia Commons).

The central pressure lowers and the pressure gradient increases, causing a stronger cyclonic (counterclockwise) flow inward toward the low’s center, as illustrated in Figure \(\PageIndex{2}\). The precipitation band widens ahead of the warm front and narrows ahead of the cold front. The region of warmer air between the cold and warm fronts here is called the warm sector. Here, the weather is generally partly cloudy, with scattered showers possible if the air is conditionally unstable.

Where does the mid-latitude cyclone get its energy from? As warmer and colder air masses attempt to regain equilibrium, warm air rises over colder air, converting potential energy into kinetic energy (motion). As warmer air rises, it condenses into clouds, releasing latent heat into the system. Near the surface, winds converge inward toward the low’s center. Wind speeds may increase as a result of a stronger pressure gradient force near the center, increasing the kinetic energy in the system, as illustrated in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Part 3 of cyclogenesis results in a newly developed cyclone with a significantly lower pressure in the center, surrounded by a counter-clockwise wind pattern. (Public Domain; Pflatau via Wikimedia Commons).

As the cyclone moves eastward, the central pressure continues to decrease, and winds increase during its mature stage. As the faster-moving cold front advances toward the warm front, it reduces the warm sector's size. In the Norwegian cyclone model, the cold front overtakes the warm front, and the cyclone becomes occluded, as illustrated in Figure \(\PageIndex{4}\). Cloud cover and precipitation extend over a wide area, and the storm is typically most intense during this stage.

The point where the cold front, warm front, and occluded front intersect is known as the triple point. Occasionally, a secondary low may form at this triple point, move eastward, and intensify into another cyclone.

Figure \(\PageIndex{4}\): Part 4 of cyclogenesis results in the formation of an occluded front and "triple point" where the cold, warm, and occluded fronts intersect. (Public Domain; Pflatau via Wikimedia Commons).

Eventually, as occlusion advances, the low-pressure center will begin to dissipate because cold air exists on both sides of the occluded front, as illustrated in Figure \(\PageIndex{5}\). The warm, rising air is removed from the center of the storm, thereby cutting it off from its primary energy source. Eventually, the old storm dissipates and gradually fades away.

Figure \(\PageIndex{5}\): Part 5 of cyclogenesis includes the continued formation of an occluded front and the eventual dying out of the cyclone. (Public Domain; Pflatau via Wikimedia Commons).

This sequence of a developing mid-latitude cyclone is similar to a whirling eddy in a river that forms behind a stick or log, moves along with the river, and quickly disappears downstream. This entire life cycle can last from several days to a little more than a week. Cyclones at various stages of development can be observed simultaneously along the polar front; this succession of storms is known as a cyclone “family”. As mentioned before, some cyclones form from dying previous cyclones and become a part of the succession. This polar-front model of development for a mid-latitude cyclone is somewhat simplified; in fact, very few storms follow this model exactly. However, it is a good foundation for understanding storm structure.