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Extratropical Cyclone Models

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Revision as of 14:06, 13 February 2019 by Editor.1 (talk | contribs) (The Norwegian Cyclone Model)
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Category: Weather Weather
Content source: SKYbrary About SKYbrary
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Tag(s) Weather Phenomena

Description

This article describes in basic terms the two most commonly quoted models for extratropical cyclone development (cyclogenesis).

The Norwegian Cyclone Model

Early in the 20th century, Norwegian meteorologists formulated a model for an extratropical cyclone that develops as a disturbance along the boundary (front) between the polar and mid-latitude air masses. The disturbance distorts the front into a wavelike configuration.

Wave forms on front (image source: NOAA)

As the pressure within the disturbance decreases, the disturbance assumes the appearance of a cyclone with its characteristic counterclockwise flow. Southerly winds take warm air northward ahead of the storm and northerly winds bring cold air south behind it. Besides rotating counterclockwise, the air also flows inward toward the center of the cyclone.

Wave intensifies (image source: NOAA)

As the cyclone intensifies, the cold air streams toward the equator faster than the warm air streams toward the pole, allowing the cold front to overtake the warm front thus forcing the warm air aloft. This produces a more complicated frontal structure called an occluded front, which separates two relatively cold air masses. The occlusion process may be followed by further storm intensification.

A mature low pressure system (image source: NOAA)

The separation of the cyclone from the warm air toward the Equator, however, eventually leads to the storm's decay and dissipation.

Dissipating stage of cyclone (image source: NOAA)

The Norwegian model still retains merit, as it is a good description for some cyclogenesis events.

Shapiro-Keyser model

A competing theory for extratropical cyclone development over the oceans, the Shapiro-Keyser model, was developed in 1990 and based on data from surface observations and aircraft to determine vertical structure of fronts in the northwest Atlantic. Its main differences with the Norwegian Cyclone Model are the fracture of the cold front, treating warm-type occlusions and warm fronts as the same, and allowing the cold front to progress through the warm sector perpendicular to the warm front. With this model, a weakness appears along the poleward portion of the cold front near the low centre (frontal factor) and a back-bent front forms behind the low centre. The back-bent front is also associated with the sting jet phenomenon - a short duration flow of strong winds that can reach 100 kts at surface level.

Keyser-Shapiro Cyclone model - Source: NOAA, 2006 (taken from wikimedia commons)

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