Extratropical cyclone

An extratropical cyclone, sometimes called a mid-latitude cyclone or a cyclone, is a synoptic scale low pressure weather system that has neither tropical or polar characteristics, and is connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones".^[1] Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing weather ranging from cloudiness and mild showers, to heavy gales and thunderstorms.

A ficticious synoptic chart of an extratropical cyclone affecting the UK. The blue arrows between isobars indicate the direction of the wind, while the "L" symbol denotes the center of the "low". Note the occluded, cold and warm frontal boundaries.

An image from a weather satellite of the Great Blizzard of 1993. This is a classic extratropical cyclone structure, with the low pressure area over Georgia and Alabama, the cold front extending down into Central America, and an occlusion extending up into southern Canada.

Terminology

An extratropical cyclone is a class of storms described by many different names. While they are occasionally referred to just as "cyclones", this is incorrect, as cyclone is a broader term covering many other types of weather phenomena. The descriptor "extratropical" signifies that this type of cyclone generally occurs outside of the tropics, in the middle latitudes of the planet. These systems may also be described as "mid-latitude cyclones" due to their area of formation, or "post-tropical cyclones" where extratropical transition has occured.^[1][2] They are often described as "depressions" or "lows" by weather forecasters and the general public.

Although extratropical cyclones are almost always classified as baroclinic because they form along zones of temperature and dewpoint gradient, they can sometimes become barotropic late in their life cycle when the distibution of heat around the cyclone becomes fairly uniform with radius.

Formation

 

Approximate areas of extratropical cyclone formation worldwide

 

An upper level jet streak. DIV areas are regions of divergence aloft, which will lead to surface convergence and aid cyclogenesis.

Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the equator) in one of two ways; either through cyclogenesis or extratropical transition. A study of extratropical cyclones in the Southern Hemisphere shows that on average, there are an average of 37 in existance during any 6 hour period between the 30th and 70th parallels.^[3] A seperate study in the northern hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.^[4]

Cyclogenesis

Main article: Cyclogenesis

Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical wind shear, and are thus classified as baroclinic cyclones. Initially, cyclogenesis, or low pressure formation, occurs along frontal zones near a favorable quadrant of the upper level jetstream, usually being the right rear and left front quadrants, where divergence ensues. This causes air to rush out from the top of the air column which in turn forces convergence in the low-level wind field and increased upward motion within the column. The increased upward motion causes surface pressures to lower as the upward air motion counteracts gravity, lessening the weight of the atmosphere (surface pressure) in that ___location, and thus strengthening the cyclone. As the cyclone strengthens, the cold front sweeps towards the equator and moves around the back of the cyclone. Meanwhile, its associated warm front progresses more slowly, as the cooler air ahead of the system is denser, and therefore more difficult to dislodge. Later, the cyclones occlude as the poleward portion of the cold front overtakes a section of the warm front, forcing a tongue, or trowal, of warm air aloft. Eventually, the cyclone will become barotropically cold and begin to weaken.

A rapidly-falling atmospheric pressure is possible due to strong upper level forces on the system, and such a cyclone is sometimes referred to as a bomb.^[5][6] These bombs rapidly drop in pressure to below 980 millibars (980 hPa, 28.94 inHg) under favorable conditions such as near a natural temperature gradient like the the Gulf Stream, or at a preferred quadrant of an upper level jet streak, where upper level divergence is best. The stronger the upper level divergence over the cyclone, the deeper the cyclone can become. Hurricane-force extratropical cyclones are most likely to form in the northern Atlantic and northern Pacific oceans in the months of December and January.^[7] The lowest pressure measured from an extratropical cyclone in the United States was 951.7 hPa on March 1, 1914 in Bridgehampton, New York. Between January 4 and January 5, 1989, an extratropical cyclone south of Atlantic Canada deepened to 928 hPa.^[8] In the Arctic, the average pressure for cyclones is 988 hPa (29.18 inHg) during the winter, and 1000 hPa (29.53 inHg) during the summer.^[9] A rapidly-falling atmospheric pressure is possible due to strong upper level forces on the system, and such a cyclone is sometimes referred to as a bomb.^[5]

Extratropical transition

 

Cyclone Florence in the north Atlantic after completing its transition to an extratropical cyclone from a hurricane.

Tropical cyclones often transform into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude, where there is sufficient forcing from upper-level troughs or shortwaves riding the Westerlies for the process of extratropical transition to begin. During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat from condensation (from thunderstorms near the center) to baroclinic processes. The low pressure system eventually loses its warm core and becomes a cold-core system. During this process, a cyclone in extratropical transition (known in Canada as the post-tropical stage)^[10] will invariably form or connect with nearby fronts and/or troughs consistent with a baroclinic system. Due to this, the size of the system will usually appear to increase, while the core weakens. However, after transition is complete, the storm may re-stengthen due to baroclinic energy, depending on the environmental conditions surrounding the system. The cyclone will also distort in shape, becoming less symmetric with time.

On rare occasions, an extratropical cyclone can transition into a tropical cyclone if it reaches an area of ocean with warmer waters and an environment with less vertical wind shear. The peak time of subtropical cyclogenesis (the midpoint of this transition) is in the months of September and October, when the difference between the temperature of the air aloft and the sea surface temperature is the greatest, leading to the greatest potential for instability.^[11]

Structure

Surface pressure/Wind distribution

 

QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds are on the outside of the occlusion.

The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the highest winds and lowest pressure being found near the center; typically just on the cold/poleward side of warm fronts and occlusions and just behind cold fronts (where pressure gradient force is highest.) ^[12] Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure. The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the sea-level pressure is not very low; its typical value is below 1013.2 mbar (1013.2 hPa, 29.92 inHg), which is the average sea level pressure for Earth. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather maps) a wave-like shape. Due to this appearance in their early life cycles, extratropical cyclones can also be referred to as frontal waves. In the United States, an old name for such a system is "warm wave".^[13]

Rotation

The wind flow around a large cyclone, generally referred to as cyclonic, is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect.

Vertical structure

Extratropical cyclones slant back into colder air masses and strengthening with height, sometimes exceeding 30,000 feet (approximately 10 km) in depth.^[14] Above the surface of the earth, the air temperature near the center of the cyclone is increasingly colder than the surrounding environment. These characteristics are the direct opposite of those found in their tropical cyclones, and they are sometimes called "cold-core lows" for these reasons.^[15] Various charts can be examined to check the characteristics of a cold-core system with height, such as the 700 mbar (hPa) chart, which is at about 10,000 feet or 3,000 meters in height. Cyclone phase space diagrams are used to tell whether a cyclone is tropical, subtropical, or extratropical.^[16]

Cyclone evolution

File:Cyclonemodels.gif

Norwegian cyclone and Shapiro-Keyser model differences in frontal structure.

There are two models of cyclone development and lifecycles in common use - The Norwegian model and the Shapiro-Keyser Model. ^[17]

Norwegian cyclone model

Of the two theories on extratropical cyclone structure and life cycle, the oldest is the Norwegian Cyclone Model, developed during World War I. In this theory, cyclones develop as they move up and along a frontal boundary, eventually occluding and ending up in a barotropically cold envirnoment.^[18] It was developed completely from surface-based weather observations, including descriptions of clouds found near frontal boundaries. This theory still retains merit, as it is a good description for extratropical cyclones over continental landmasses.

Shapiro-Keyser model

A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990.^[19] 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. This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used planes to determine the vertical structure of fronts across the northwest Atlantic.

Warm seclusion

A warm seclusion is the mature phase of the extratropical cyclone lifecycle conceptualized after the ERICA field experiment of the late 1980s. The experiment produced observations of intense marine cyclones that indicated an anomalously warm low-level thermal structure, secluded (or surrounded) by a bent-back warm front and a coincident chevron-shaped band of intense surface winds.^[20] The Norwegian Cyclone Model, as developed by the Bergen School of Meteorology, largely observed cyclones at the tail end of their lifecycle and used the term occlusion to identify the decaying stages.^[21]

Warm seclusions may have cloud-free, eye-like features at their center (reminiscent of tropical cyclones), significant pressure falls, hurricane force winds, and moderate to strong convection. The most intense warm seclusions often attain pressures less than 950 mbar with a definitive lower to mid-level warm core structure.^[20] A warm seclusion is the result of a baroclinic lifecycle and occurs at latitudes well poleward of the tropics. The process known as "tropical transition" involves the usually slow development of an extratropically cold core vortex into a tropical cyclone.^[22][23]

As latent heat flux releases are important for their development and intensification, a vast majority of warm seclusion events occur over the world's oceans and may impact coastal nations with hurricane force winds and torrential rain.^[24][25] Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months, while the Southern Hemisphere may see a strong cyclone event such as this during all times of the year.

In all tropical basins, except the Northern Indian Ocean, the extratropical transition of a tropical cyclone may result in reintensification into a warm seclusion. For example, Hurricane Maria of 2005 reintensified into a strong baroclinic system and achieved warm seclusion status at maturity (or lowest pressure).^[26]

Motion

 

A zonal flow regime. Note the dominant west-to-east flow as shown in the 500 hPa height pattern.

Extratropical cyclones are generally driven, or "steered", by deep westerly winds in a general west to east motion across both the Northern and Southern hemispheres of the Earth. This general motion of atmospheric flow is known as "zonal".^[27] Where this general trend is the main steering influence of an extratropical cyclone, it is known as a "zonal flow regime".

When the general flow pattern buckles from a zonal pattern to the meridional pattern,^[28] a slower movement in a north or southward direction is more likely. Meridional flow patterns feature strong, amplified troughs and ridges, generally with more northerly and southerly flow.

Changes in direction of this nature are most commonly observed as a result of a cylone's interaction with other low pressure systems, troughs, ridges, or with anticyclones. A strong and stationary anticyclone can effectively block the path of an extratropical cyclone. Such blocking patterns are quite normal, and will generally result in a weakening of the cyclone, the weakening of the anticyclone, a diversion of the cyclone towards the anticyclones periphery, or a combination of all three to some extent depending on the precise conditions. It is also common for an extratropical cyclone to strengthen as the blocking anticyclone or ridge weakens in these circumstances.^[29]

Where an extratropical cyclone encounters another extratropical cyclone (or almost any other kind of cyclonic vortex in the atmosphere), the two may combine to become a "Binary cyclone", where the vorticies of the two cyclones rotate around each other (known as the "Fujiwhara effect".) This most often results in a merging of the two low pressure systems into a single extratropical cyclone, or can less commonly result in a mere change of direction of either one or both of the cyclones.^[30] The precise results of such interactions depend on a number of factors including the size of the two cyclones, their strength, the distance between the two cyclones, and the prevailing atmospheric conditions around them.

Effects

General

 

Preferred region of snowfall in an extratropical cyclone.

Extratropical cyclones can bring mild weather with a little rain and surface winds of 7-15 knots, or they can be cold and dangerous with torrential rain and winds exceeding 64 knots,^[31] (sometimes referred to as windstorms in Europe.) The band of precipitation that is associated with the warm front is often extensive. Cyclones tend to move along a predictable path at a moderate rate of progress. During late fall, winter, and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall.

Severe Weather

Squall lines, or solid lines of strong thunderstorms, can form ahead of cold fronts and lee troughs due to the presence of significant atmospheric moisture coupled with strong upper level divergence, leading to hail and high winds.^[32] When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper level jet stream, tornado formation is possible.^[33] While tornadoes can form anywhere on Earth, the greatest number occur in the Great Plains in the United States, as downsloped winds off the north-south oriented Rocky Mountains, also known as a dryline, aids their development from as weak as F0 to as powerful as F5.

Explosive development of extratropical cyclones can be sudden. The bomb known in the UK as the "Great Storm of 1987" deepened to 953 mbar with a highest recorded wind of 119 knots (61 m/s), resulting in the loss of 19 lives' 15 million trees, widespread damage to homes and an estimated economic cost of £1.2 billion.^[34][35]

While most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or gale force. In 1954, Hurricane Hazel became extratropical over North Carolina while a strong Category 3 storm. The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage well north in Oregon and Washington states with widespread damage equivalent to a Category 3 or higher hurricane. More recently, Hurricane Wilma in 2005 began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical while still a Category 1 storm). ^[36]

See also

Template:Tcportal

• Surface weather analysis
• Cyclogenesis
• Tropical cyclones
• Tropical cyclogenesis
• Nor'easter
• European windstorm

References

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8. JeffMasters (2006-02-15). "Flying into a record Nor'easter". JeffMasters' Blog on Wunderground.Com. Retrieved 2006-10-04. {{cite web}}: Check date values in: |date= (help); External link in |publisher= (help)
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15. Robert Hart (2003-02-18). "Cyclone Phase Analysis and Forecast: Help Page". Florida State University Department of Meteorology. Retrieved 2006-10-03. {{cite web}}: Check date values in: |date= (help); External link in |publisher= (help)
16. Robert Hart (2006-10-04). "Cyclone phase evolution: Analyses & Forecasts". Florida State University Department of Meteorology. Retrieved 2006-10-03. {{cite web}}: Check date values in: |date= (help); External link in |publisher= (help)
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24. David M. Schultz ; Heini Wernli (2001-01-05). "Determining Midlatitude Cyclone Structure and Evolution from the Upper-Level Flow". Cooperative Institute for Mesoscale Meteorological Studies. Retrieved 2006-11-01. {{cite web}}: Check date values in: |date= (help); External link in |publisher= (help)
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29. Anthony R. Lupo ; Phillip J. Smith (1997-05-02). "The Interactions between a Midlatitude Blocking Anticyclone and Synoptic-Scale Cyclones That Occurred during the Summer Season". Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana. Retrieved 2006-10-21. {{cite web}}: Check date values in: |date= (help); External link in |publisher= (help)
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