Cyclones: Occluded

Occluded cyclones represent the late-mature stage of short-lived, rapidly moving extratropical cyclones that, with much slower-moving anticyclones, are responsible for the day-to-day weather changes in the middle latitudes. In redistributing energy and moisture, occluded cyclones are an integral component of the general circulation of the atmosphere. Juxtaposition of contrasting air masses with different temperatures and densities that resist mixing promotes the establishment of a front. A wave development on this front initiates cyclonic circulation and the genesis of a low-pressure region. The front develops discrete warm and cold portions separated by a warm sector. Through time, the cold front moves more rapidly than the warm front and eventually overtakes the warm one. Then, occlusion takes place. Precipitation associated with occluded cyclones may be of moderate to heavy intensity, resulting in significant amounts of rainfall, especially during the partially occluded phase.

Jet Streams and Surface Weather

In the midlatitudes, the general airflow pattern is from west to east, termed the Westerlies, and this dominates throughout the depth of the troposphere. At upper-troposphere levels, the airflow commonly resembles a series of horizontal wavelike oscillations, referred to as Rossby waves, that encompass most of the midlatitude region. The nature of airflow through an upper-level Rossby wave has important ramifications for the development of weather systems at the surface.

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Figure 1 Upper airflow and surface extratropical cyclone formation

Source: Author.

The fastest-flowing airflow usually occurs in the trough of a Rossby wave, but as the air exits the trough, changes in the flow along the upper wave lead to divergence and convergence. When the air in the upper troposphere diverges (spreads out), this draws air upward from below and encourages the surface genesis of a low-pressure center accompanied by cloud development and precipitation. A zone of divergence exists just downwind of an upper-level trough (Figure 1). In contrast, immediately downwind of an upper-level ridge, convergence promotes subsidence, which inhibits cloud development and induces pressure to rise at the surface and anticyclone formation to occur.

Low-pressure development at the surface originates with wave development on the front that separates two air masses of markedly different temperature and humidity. Cyclogenesis continues through stages to produce a mature extratropical cyclone—a synoptic-scale feature some 1,500 to 3,000 kilometers in diameter.