Chapter 10Chapter 10

Mid-latitude CyclonesMid-latitude Cyclones

Chapter 10Chapter 10

Mid-latitude CyclonesMid-latitude Cyclones

The Polar Front Theory was postulated in the earlypart 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 largeportions of a continent, they are familiar as

the systems that bring abrupt changesin 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 endof the cyclone’s life cycle

and takes place as the centerof the low pressure pulls backfrom the warm and cold fronts.

The figure depicts the typicalstructure of a mature cyclone

and the processes causing uplift.Shaded areas represent the

presence of cloud cover.The numbers representan approximation of theprecipitation 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 leadsto 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 variationsin 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 gradientsin the upper and middle atmosphere.

The effect of differing verticalpressure gradients on eitherside 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 conditionswhile others are calm and dry.

The figure shows a center of low surface pressure gradually movingnortheastward 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 examineslightning, thunder, and tornadoes.