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==Description==
 
==Description==
  
This article describes in basic terms the 2 most commonly quoted models for extratropical cyclone development (cyclogenesis).
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This article describes in basic terms the two most commonly quoted models for extratropical cyclone development ([https://ocean.weather.gov/articles/types_of_cyclo.pdf cyclogenesis]).
  
 
==The Norwegian Cyclone Model==
 
==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.  
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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|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.  
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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.
 
[[File:Cyclo 3.png|none|thumb|300px|Wave intensifies (image source: NOAA)]]
 
[[File:Cyclo 3.png|none|thumb|300px|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.  
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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.  
 
[[File:Cyclo 4.png|none|thumb|300px|A mature low pressure system (image source: NOAA)]]
 
[[File:Cyclo 4.png|none|thumb|300px|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.
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The separation of the cyclone from the warm air toward the Equator, however, eventually leads to the storm's decay and dissipation.
 
[[File:Cyclo 5.png|none|thumb|300px|Dissipating stage of cyclone (image source: NOAA)]]
 
[[File:Cyclo 5.png|none|thumb|300px|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.
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The Norwegian model still retains merit, as it is a good description for some cyclogenesis events.
  
 
==Shapiro-Keyser model==
 
==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|sting jet]] phenomenon - a short duration flow of strong winds that can reach 100 kts at surface level.
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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 fracture) and a back-bent front forms behind the low centre. The back-bent front can also be associated with the [[Sting Jet|sting jet]] phenomenon - a short duration flow of strong winds that can reach 100 kts at surface level.
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The center of the low is within a pocket of low-level warm air. This is called seclusion as the pocket of warm air has been encircled by cold air wrapping around the storm. The spiraling occluded front does not connect to the center of the low. The occlusion process is not due to the cold front catching up to the warm front; rather it is due to warm and cold air flows wrapping into the cyclone’s circulation.
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These are often the rapidly developing cyclones known as “[https://www.skybrary.aero/index.php/Explosive_Cyclogenesis bombs]”.
  
 
[[File:Shapiro-Keyser_Cyclone.png|600px|thumb|none|Keyser-Shapiro Cyclone model - Source: NOAA, 2006 (taken from wikimedia commons)]]
 
[[File:Shapiro-Keyser_Cyclone.png|600px|thumb|none|Keyser-Shapiro Cyclone model - Source: NOAA, 2006 (taken from wikimedia commons)]]
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These models are only looking at low-level fronts and circulations. The [[Jet Stream|jet stream]] and weather systems at upper-levels of the atmosphere are intimately tied in to cyclogenesis at the surface. The pressure falls at the surface are due to divergence or removal of air aloft. When this divergence aloft ceases, the surface pressures will start to rise and the cyclone will weaken and eventually dissipate.
  
 
==Related Articles==
 
==Related Articles==
 
*[[Explosive Cyclogenesis]]
 
*[[Explosive Cyclogenesis]]
 
*[[Sting Jet]]
 
*[[Sting Jet]]
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*[[Jet Stream]]

Latest revision as of 23:30, 3 April 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 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 fracture) and a back-bent front forms behind the low centre. The back-bent front can also be associated with the sting jet phenomenon - a short duration flow of strong winds that can reach 100 kts at surface level.

The center of the low is within a pocket of low-level warm air. This is called seclusion as the pocket of warm air has been encircled by cold air wrapping around the storm. The spiraling occluded front does not connect to the center of the low. The occlusion process is not due to the cold front catching up to the warm front; rather it is due to warm and cold air flows wrapping into the cyclone’s circulation.

These are often the rapidly developing cyclones known as “bombs”.

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

These models are only looking at low-level fronts and circulations. The jet stream and weather systems at upper-levels of the atmosphere are intimately tied in to cyclogenesis at the surface. The pressure falls at the surface are due to divergence or removal of air aloft. When this divergence aloft ceases, the surface pressures will start to rise and the cyclone will weaken and eventually dissipate.

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