Mid-latitude Cyclones

Cyclone: a weather system characterized by relatively low surface pressure compared with the surrounding air; surface winds blow counter-clockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere) and inward that is associated with convergence, and therefore rising motion, cloudiness, and precipitation.

The formation, strengthening, or regeneration of a cyclone is called cyclogenesis, while the weakening or dissipation of a cyclone is called cyclolysis. The former is associated with deepening while the latter is associated with filling of a low pressure center.

The original description of the structure and life cycle of a midlatitude low-pressure system, first proposed during World War I by researchers at the Norwegian School of Meteorology at Bergen. This conceptual model is therefore referred to as the Norwegian cyclone model.

Polar Front Theory: As the midlatitude cyclone progresses through its life cycle, it is supported and steered by the upper-level circulation toward the east and northeast. The storm typically begins as a wave along the polar front and deepens as the surface air pressures continue to drop. As a results, winds strengthen and frontal weather develops. The storm finally occludes as the faster moving cold front "catches up" with the slower moving warm front and the upper-level low center becomes vertically stacked over the surface low center.

Figure 12.1, page #314 (Ahrens)

Warm sector: the region between cold and warm front where temperatures are relatively higher than behind the cold front as well as the ahead of the warm front in the presence of an occlusion.

Cyclonic circulation causes a meridional wave to form along the front, known as wave cyclone, during mature stage of a cyclone. The pattern of cloudiness associated with a wave cyclone is called comma cloud. The head of the comma stretches from the low center to the northwest and its tail follows along the cold front.

Figure 12.2, page #315 (Ahrens)

Figure 12.6, page #319 (Ahrens)

Figure 12.7, page #319 (Ahrens)

Barotrophic atmosphere: density depends only on the pressure, the streamlines and the isotherms are parallel to each other.

Baroclinic atmosphere: density depends on the pressure and temperature, the steamlines and the isotherms cross each other. Baroclinic instability is a necessary condition for the development of a mid-latitude cyclone in which a meridional (north-south) temperature gradient, a strong vertical wind shear, temperature advection, and divergence aloft occur.

Figure 12.10, page #323 (Ahrens)

Vorticity: a measure of the spin of a small air parcel. The rotational of the Earth around its axis is counter-clockwise and is called rotational vorticity , while the vorticity that is generated from movement of air flow (curvature) and change of wind speed over a horizontal distance (shear) is the relative vorticity . The sum of the rotational and relative vorticity is called absolute vorticity . The vorticity maximum is often observed at mid-troposhere (500 mb map). The divergence is in the right, while the convergence is in the left side of the vorticity maximum.

Figure 12.20, page #330 (Ahrens)

Figure 12.21, page #330 (Ahrens)

Figure 12.22, page #331 (Ahrens)

Figure 12.24, page #332 (Ahrens)

The environmental conditions at around a cyclone differ substantially from one location to another. For example, air temperatures are lowest to the northwest of a cyclone center where strong and gusty northwest winds advect cP or A air southward and eastward. A tendency to clear sky is evident to the west of the center where precipitation tapers off to showers as the cyclone moves toward northeast, while the leading of cold air located south of the cyclone center is accompanied by a narrow band of showers, and embedded thunderstorms. To the southwest, skies are generally clear associated with sinking motion, while skies are partly cloudy with possible convective showers during the afternoon to the southeast of the center where south and southwest winds advect milder mT air northward from over Gulf of Mexico. To the north and northeast, a zone of extensive overrunning as mT air surges over a wedge of cool air maintained by east and northeast winds at the surface. Skies are cloudy, and precipitation is steady and substantial to the northeast.

Figure 12.12, page #325 (Ahrens)

A midlatitude cyclone's life cycle typically last three days. However, when conditions are favorable, the cyclone can progress from its birth to maximum intensity in 24 hr or less. A rapid intensification of a cyclone whose pressure drops by at least 24 mb in 24 hr is named bomb. Only a few cyclones satisfy the condition for the bomb, which is usually observed when a cyclones moves off the East coast of North America or Asia over warm ocean currents of Gulf Stream or Kuroshio.

Under unfavorable conditions, the cyclone never progress to a fully occluded system, instead spending its lifespan as a weak cyclone rippling along a statinary front.

Conveyor belt conceptual model: the depiction of three-dimensional structure of a cyclone in terms of three airstreams. The warm conveyor belt originates in the cyclone's warm sector near the Earth's surface and rises rapidly as it passes over the warm front and eventually joins the westerly flow aloft. The cold conveyor belt originates close to the surface east of the cyclone and north of the warm front in colder air. It passes under the warm conveyor belt and rises near the surface center, turning clockwise to join the westerly flow aloft and forming the head of the comma cloud. The dry airstream originates aloft and descends both behind the cold front and near the cyclone center. The latter branch forms the dry slot in the comma cloud before rising to join the westerly flow aloft.

Figure 12.13, page #326 (Ahrens)

The track followed by a cyclone is critical to the type of weather experienced at a given location. On the warm side of the storm track, winds veers (turn clockwise) with time and a warm front is followed by a cold front. A given location, say St. Louis, experiences warm advection and falling pressure first followed by cold advection and rising pressure. The weather condition changes from steady rain before the warm frontal passage to partial clearing with the warm frontal passage to scattered showers just before the cold frontal passage and to clearing after the cold frontal passage.

On the cold side of a storm track, winds back (turn counter-clockwise) with time without any frontal passages. A given location experiences no advection and falling pressure fall first followed by cold advection and rising pressure. The weather condition changes from steady rain or snow to clearing as wind shifts slowly from east or northeast to the northwest.

The principal storm tracks across the US are born at 6 different locations: Alberta, Gulf of Alaska, Colorado, Gulf of Mexico, Texas-Arkansas, and Carolinas. In summer, a few organized cyclones occur because of northward shift of polar jet, while cyclogenesis is more frequent as the polar jet moves southward in winter.

Alberta cyclones occur most frequently since it is present all year around, but southern storms, such as Colorado- and Gulf-track storms bring the heaviest precipitation since they are closer the source of moisture. The precipitation produced for a particular location also depends on the speed of the cyclone such that a slow moving cyclone will leave more precipitation than a fast moving cyclone.

Figure 12.5, page #317 (Ahrens)

Cold-core cyclone: a weather system characterized by deepening low pressure center with altitude. The thickness between pressure surfaces, which is directly proportional to the layer temperature, is lowest at the center of the low. The low pressure center aloft is located left that at the surface in a wave cyclone.

Warm-core cyclone: a weather system characterized by the thermal lows that are stationary, have no fronts and are associated with very hot, dry air. The shallow near-surface cyclonic circulation weakens and then reverses with altitude. Hurricanes are a warm-core cyclone.

Polar low: a low pressure system that develops over polar water behind (or poleward side of) a polar front. Its diameter is 1000 to 500 km or even less, making it smaller than the mid-latitude cyclone. Polar low has a hurricane like center (eye), comma shape cloud band, and relatively warmer core with strong winds and heavy showery precipitation (snow). It forms in winter (November through March). During this time, the sun is low on the horizon or absent, and therefore the air next to the snow- and ice-covered ground cools rapidly and become frigid cold. If this air meets with relatively warm air above a warm ocean current, an arctic front forms.

Figure 12.28, page #335 (Ahrens)

Anticyclone: a weather system characterized by relatively high surface pressure compared with the surrounding air; surface winds blow clockwise in the Northern Hemisphere (counter-clockwise in the Southern Hemisphere) and outward that is associated with divergence, and therefore sinking motion, and a fair weather.

Cold-core anticyclone: a shallow weather system that coincides with the dome of continental polar or arctic air. They are responsible for the frigid temperatures over the continental US in winter. They are shallow systems in which the clockwise circulation weaken with altitude and often reverses.

Warm-core anticyclones: a weather system characterized by strengthening high pressure center with altitude. The thickness between pressure surfaces is highest at the center of the high. The semiperminent subtropical anticyclones, such as Bermuda-Azores high, are examples of warm-core anticyclones which are accompanied by subsiding, warm, dry air.