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Difference between revisions of "Extratropical Cyclone Models"

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(Shapiro-Keyser model)
(The Norwegian Cyclone Model)
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==The Norwegian Cyclone Model==
 
==The Norwegian Cyclone Model==
Early in the 20th century, Norwegian meteorologists formulated a model for a 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.  
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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.  
 
[[File:Cyclo 2.png|none|thumb|300px|Wave forms on front (image source: NOAA)]]
 
[[File:Cyclo 2.png|none|thumb|300px|Wave forms on front (image source: NOAA)]]
 
As the pressure within the disturbance decreases, the disturbance assumes the appearance of a cyclone and forces poleward and equatorward movements of warm and cold air.  
 
As the pressure within the disturbance decreases, the disturbance assumes the appearance of a cyclone and forces poleward and equatorward movements of warm and cold air.  

Revision as of 13:52, 13 February 2019

Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Weather Phenomena

Description

This article describes in basic terms the 2 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 and forces poleward and equatorward movements of warm and cold air.

Wave intensifies (image source: NOAA)

As the cyclone intensifies, the cold air streams equatorward faster than the warm air streams poleward, allowing the cold front to overtake the warm front to produce a more complicated frontal structure called an occluded front. 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 towards 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 extratropical cyclones over continental landmasses.

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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