Chapter 10 Mid-latitude Cyclones Chapter 10 Mid-latitude Cyclones

The Polar Front Theory was postulated in the early part of the twentieth century to describe the formation, development, and dissipation of mid-latitude cyclones. Mid-latitude cyclones are large systems that travel great distances and often bring precipitation and sometimes severe weather to wide areas. Lasting a week or more and covering large portions of a continent, they are familiar as the systems that bring abrupt changes in wind, temperature, and sky conditions.

Cyclogenesis is the formation of a mid-latitude cyclone. Initially, the polar front separates the cold easterlies and the warmer westerlies. As cyclogenesis begins, a “kink” develops along the boundary. The cold air north of the front begins to push southward behind the cold front, and air behind the warm front advances northward, creating a counterclockwise rotation around a weak low-pressure system.

With further intensification, the low pressure deepens even further and distinct warm and cold fronts emerge from the original polar front. Convergence associated with the low pressure can lead to uplift and cloud formation, while linear bands of deeper cloud cover develop along the frontal boundaries. Occlusion represents the end of the cyclone’s life cycle and takes place as the center of the low pressure pulls back from the warm and cold fronts.

The figure depicts the typical structure of a mature cyclone and the processes causing uplift. Shaded areas represent the presence of cloud cover. The numbers represent an approximation of the precipitation probability.

Rossby waves are waves in the mid-latitude westerlies having wavelengths on the order of thousands of kilometers. In the figure moving from points 1 to 3, the air rotates counterclockwise. Between points 3 and 5, it rotates clockwise. The rotation of a fluid is referred to as its vorticity. The figure shows vorticity changes in the moving air, relative to the surface.

The overall rotation of air, or its absolute vorticity, has two components: relative vorticity, or the vorticity relative to Earth’s surface, and Earth vorticity, which is due to Earth’s daily rotation about its axis. Counterclockwise rotation in the Northern Hemisphere is said to have positive vorticity while air rotating clockwise possesses negative vorticity.

The figure represents change in vorticity along a Rossby wave trough. As the air flows from positions 1 to 3, it undergoes little change in direction and thus has no relative vorticity. From positions 4–6, it turns counterclockwise and thus has positive relative vorticity. The air flows in a constant direction from positions 7–9. Thus, the trough has three segments based on vorticity, separated by two transition zones.

Divergence in the upper atmosphere, caused by decreasing vorticity, draws air upward from the surface and provides a lifting mechanism for the intervening column of air. This can initiate and maintain low-pressure systems at the surface. Conversely, increasing upper-level vorticity leads to convergence and the sinking of air, which creates high pressure at the surface. Surface low-pressure systems resulting from upper-tropospheric motions are called dynamic lows and are distinct from thermal lows caused by localized heating of air from below.

Temperature distributions in the lower atmosphere lead to variations in upper-level pressure. Above A the entire column of air in the lower atmosphere is warm, so the pressure drops relatively slowly with height. At B cold air occupies the lowest 500 m, with warmer air aloft. This leads to slightly lower pressure at the 1 km level. At C cold air occupying the lowest 1000 km causes a greater rate of pressure decrease with altitude, and lower pressure at the 1 km level. Thus, the existence of sloping frontal boundaries establishes horizontal pressure gradients in the upper and middle atmosphere.

The effect of differing vertical pressure gradients on either side of warm and cold fronts leads to upper-tropospheric troughs and ridges.

Speed divergence and convergence occur when air moving in a constant direction either speeds up or slows down. Speed divergence occurs where contour lines come closer together in the downwind direction. In the top figure, the wind speed, indicated by the length of the arrows, increases in the direction of flow and causes speed divergence. Speed convergence occurs when faster- moving air approaches the slower- moving air ahead (bottom).

Diffluence and confluence occur when air stretches out or converges horizontally due to variations in wind direction. In the top figure, a certain amount of air is contained in the shaded area between points 1 and 3. As it passes to the region between points 2 and 4, the same amount of air occupies a greater horizontal area. This is diffluence, a pattern that commonly appears wherever vorticity changes cause divergence to occur. Confluence is shown in the bottom.

In the figure, the height contours exhibit a zonal pattern with a minimum of north–south displacement. Because they have no pronounced vorticity changes, zonal patterns hamper the development of intense cyclones and anticyclones. They are therefore more often associated with a large-scale pattern of light winds, calm conditions, and no areas of widespread precipitation.

The pattern above shows a strong meridional component, which can lead to the formation of major cyclones and anticyclones. Some areas will experience cloudy and wet conditions while others are calm and dry.

The figure shows a center of low surface pressure gradually moving northeastward relative to a Rossby wave. In doing so, it moves away from the region of maximum divergence aloft and evolves as it travels. It goes through the various stages of its life cycle, typically occluding and dissipating as it approaches the upper-level ridge.

In the conveyor belt model, the warm conveyor belt originates near the surface in the warm sector and flows toward and over the wedge of the warm front. The cold conveyor belt lies ahead of the warm front. It enters the storm at low levels as an easterly belt flowing westward toward the surface cyclone. The dry conveyor belt originates in the upper troposphere as part of the generally westerly flow.

A barotropic atmosphere (a) exists where the isotherms (the dashed lines showing the temperature distribution) and height contours (solid lines) are aligned in the same direction. No temperature advection occurs when the atmosphere is barotropic. A baroclinic atmosphere occurs where the isotherms intersect the height contours. Cold air advection (the horizontal movement of cold air) is occurring in (b), while warm air advection is occurring in (c). (a) (b) (c)

The next chapter examines lightning, thunder, and tornadoes.