Extratropical cyclone

GOES-13 Imagery of an intense extratropical cyclone near theNE United States on March 26, 2014, with Wind gusts of101mph+ were reported.

This animation shows an extratropical cyclone developing overthe United States, starting late on October 25 and running throughOctober 27, 2010.

Extratropical cyclones, sometimes called mid-latitudecyclones or wave cyclones, are an everyday phenomenonwhich, along with anticyclones, drive the weather overmuch of the Earth. They are capable of producing any-thing from cloudiness and mild showers to heavy galesand thunderstorms. These types of cyclones are definedas synoptic scale low pressure weather systems that occurin the middle latitudes of the Earth (outside the tropics)not having tropical characteristics, and are connectedwith fronts and horizontal gradients in temperature anddew point otherwise known as “baroclinic zones”.[1]

1 Terminology

Extratropical cyclones encompass a class of storms withmany names. The term "cyclone" applies to numeroustypes of low pressure areas. The descriptor extratropicalsignifies that this type of cyclone generally occurs outsidethe tropics in the middle latitudes of Earth. The termmid-latitude cyclones may be used because of where theyform; post-tropical cyclones if extratropical transition hasoccurred.[1][2] Weather forecasters and the general publicoften describe them as “depressions” or “lows”. Termslike frontal cyclone, frontal depression, frontal low, ex-tratropical low, non-tropical low and hybrid low are oftenused as well.Extratropical cyclones are classified mainly as baroclinic,because they form along zones of temperature and dew-point gradient known as frontal zones. They can becomebarotropic late in their life cycle, when the distribution ofheat around the cyclone becomes fairly uniform with itsradius.[3]

2 Formation

Approximate areas of extratropical cyclone formation worldwide

Extratropical cyclones form anywhere within the extra-tropical regions of the Earth (usually between 30° and60° latitude from the equator), either through cyclogen-esis or extratropical transition. A study of extratropicalcyclones in the Southern Hemisphere shows that betweenthe 30th and 70th parallels, there are an average of 37cyclones in existence during any 6-hour period.[4] A sep-arate study in the Northern Hemisphere suggests that ap-proximately 234 significant extratropical cyclones formeach winter.[5]

1

2 2 FORMATION

An upper level jet streak. DIV areas are regions of divergencealoft, which will lead to surface convergence and aid cyclogene-sis.

2.1 Cyclogenesis

Main article: Cyclogenesis

Extratropical cyclones form along linear bands of tem-perature/dewpoint gradient with significant vertical windshear, and are thus classified as baroclinic cyclones. Ini-tially, cyclogenesis, or low pressure formation, occursalong frontal zones near a favorable quadrant of a maxi-mum in the upper level jetstream known as a jet streak.The favorable quadrants are usually at the right rear andleft front quadrants, where divergence ensues.[6] The di-vergence causes air to rush out from the top of the aircolumn. This in turn forces convergence in the low-levelwind field and increased upward motion within the col-umn. The increased upward motion causes atmosphericpressure at ground level to lower. This is because the up-ward air motion counteracts gravity, lessening the weightof the atmosphere in that location. The lowered pres-sure strengthens the cyclone (a low pressure system). Asthe cyclone strengthens, the cold front sweeps towardsthe equator and moves around the back of the cyclone.Meanwhile, its associated warm front progresses moreslowly, as the cooler air ahead of the system is denser,and therefore more difficult to dislodge. Later, the cy-clones occlude as the poleward portion of the cold frontovertakes a section of the warm front, forcing a tongue,or trowal, of warm air aloft. Eventually, the cyclone willbecome barotropically cold and begin to weaken.Atmospheric pressure can fall very rapidly when there arestrong upper level forces on the system. When pressuresfall more than 1 millibar (0.030 inHg) per hour, the pro-cess is called explosive cyclogenesis, and the cyclone canbe described as a (weather) bomb.[7][8][9] These bombsrapidly drop in pressure to below 980 millibars (28.94inHg) under favorable conditions such as near a naturaltemperature gradient like the Gulf Stream, or at a pre-ferred quadrant of an upper level jet streak, where up-per level divergence is best. The stronger the upper leveldivergence over the cyclone, the deeper the cyclone canbecome. Hurricane-force extratropical cyclones are mostlikely to form in the northern Atlantic and northern Pa-cific oceans in the months of December and January.[10]

On 14 and 15 December 1986, an extratropical cyclonenear Iceland deepened to below 920 hectopascals (27inHg),[11] which is a pressure equivalent to a category5 hurricane. In the Arctic, the average pressure for cy-clones is 980 millibars (28.94 inHg) during the winter,and 1,000 millibars (29.53 inHg) during the summer.[12]

2.2 Extratropical transition

Hurricane Florence in the north Atlantic after completing its tran-sition to an extratropical cyclone from a hurricane

Tropical cyclones often transform into extratropical cy-clones at the end of their tropical existence, usually be-tween 30° and 40° latitude, where there is sufficientforcing from upper-level troughs or shortwaves riding theWesterlies for the process of extratropical transition tobegin.[13] During this process, a cyclone in extratropicaltransition (known across the eastern North Pacific andNorth Atlantic oceans as the post-tropical stage),[14][15]will invariably form or connect with nearby fronts and/ortroughs 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 iscomplete, the storm may re-strengthen due to baroclinicenergy, depending on the environmental conditions sur-rounding the system.[13] The cyclone will also distort inshape, becoming less symmetric with time.[16][17][18]

During extratropical transition, the cyclone begins to tiltback into the colder airmass with height, and the cyclone’sprimary energy source converts from the release of la-tent heat from condensation (from thunderstorms near thecenter) to baroclinic processes. The low pressure systemeventually loses its warm core and becomes a cold-coresystem.[18][16]

The peak time of subtropical cyclogenesis (the midpointof this transition) in the North Atlantic is in the months ofSeptember and October, when the difference between the

3.1 Surface pressure and wind distribution 3

temperature of the air aloft and the sea surface temper-ature is the greatest, leading to the greatest potential forinstability.[19] On rare occasions, an extratropical cyclonecan transit into a tropical cyclone if it reaches an areaof ocean with warmer waters and an environment withless vertical wind shear.[20] An example of this happen-ing is in the 1991 Perfect Storm.[21] The process knownas “tropical transition” involves the usually slow develop-ment of an extratropically cold core vortex into a tropicalcyclone.[22][23]

The Joint TyphoonWarning Center uses the extratropicaltransition (XT) technique to subjectively estimate the in-tensity of tropical cyclones becoming extratropical basedon visible and infrared satellite imagery. Loss of centralconvection in transitioning tropical cyclones can causethe Dvorak technique to fail;[24] the loss of convectionresults in unrealistically low estimates using the Dvoraktechnique.[25] The system combines aspects of the Dvo-rak technique, used for estimating tropical cyclone inten-sity, and the Hebert-Poteat technique, used for estimatingsubtropical cyclone intensity.[26] The technique is appliedwhen a tropical cyclone interacts with a frontal boundaryor loses its central convection while maintaining its for-ward speed or accelerating.[27] The XT scale correspondsto the Dvorak scale and is applied in the same way, exceptthat “XT” is used instead of “T” to indicate that the sys-tem is undergoing extratropical transition.[28] Also, theXT technique is only used once extratropical transitionbegins; the Dvorak technique is still used if the systembegins dissipating without transition.[27] Once the cyclonehas completed transition and become cold-core, the tech-nique is no longer used.[28]

3 Structure

See also: Weather fronts

3.1 Surface pressure and wind distribu-tion

The windfield of an extratropical cyclone constricts withdistance in relation to surface level pressure, with the low-est pressure being found near the center, and the highestwinds typically just on the cold/poleward side of warmfronts, occlusions, and cold fronts, where the pressuregradient force is highest.[29] The area poleward and westof the cold and warm fronts connected to extratropicalcyclones is known as the cold sector, while the area equa-torward and east of its associated cold and warm fronts isknown as the warm sector.The wind flow around an extratropical cyclone iscounterclockwise in the northern hemisphere, and clock-wise in the southern hemisphere, due to the Coriolis ef-fect (this manner of rotation is generally referred to as

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

A clockwise spinning extratropical cyclone off southern Australia,in the southern hemisphere.

A typical counterclockwise spinning extratropical cyclone offnortheastern Japan on December 26, 2012, in the NorthernHemisphere.

cyclonic). Near this center, the pressure gradient force(from the pressure at the center of the cyclone comparedto the pressure outside the cyclone) and the Coriolis forcemust be in an approximate balance for the cyclone toavoid collapsing in on itself as a result of the differencein pressure.[30] The central pressure of the cyclone willlower with increasing maturity, while outside of the cy-clone, the sea-level pressure is about average. In mostextratropical cyclones, the part of the cold front aheadof the cyclone will develop into a warm front, giving the

4 5 MOTION

frontal zone (as drawn on surface weather maps) a wave-like shape. Due to their appearance on satellite images,extratropical cyclones can also be referred to as frontalwaves early in their life cycle. In the United States, an oldname for such a system is “warm wave”.[31]

In the northern hemisphere, once a cyclone occludes, atrough of warm air aloft—or “trowal” for short—will becaused by strong southerly winds on its eastern periph-ery rotating aloft around its northeast, and ultimately intoits northwestern periphery (also known as the warm con-veyor belt), forcing a surface trough to continue into thecold sector on a similar curve to the occluded front. Thetrowal creates the portion of an occluded cyclone knownas its comma head, due to the comma-like shape of themid-tropospheric cloudiness that accompanies the fea-ture. It can also be the focus of locally heavy precipita-tion, with thunderstorms possible if the atmosphere alongthe trowal is unstable enough for convection.[32]

3.2 Vertical structure

Extratropical cyclones slant back into colder air massesand strengthen with height, sometimes exceeding 30,000feet (approximately 9 km) in depth.[33] Above the sur-face of the earth, the air temperature near the center ofthe cyclone is increasingly colder than the surroundingenvironment. These characteristics are the direct oppo-site of those found in their counterparts, tropical cyclones;thus, they are sometimes called “cold-core lows”.[34] Var-ious charts can be examined to check the characteristicsof a cold-core system with height, such as the 700 mil-libars (20.67 inHg) chart, which is at about 10,000 feet(3,048 meters) in height. Cyclone phase diagrams areused to tell whether a cyclone is tropical, subtropical, orextratropical.[35]

4 Cyclone evolution

There are two models of cyclone development and life-cycles in common use—the Norwegian model and theShapiro-Keyser Model.[36]

4.1 Norwegian cyclone model

Of the two theories on extratropical cyclone structure andlife cycle, the older is the Norwegian Cyclone Model,developed during World War I. In this theory, cyclonesdevelop as they move up and along a frontal bound-ary, eventually occluding and reaching a barotropicallycold environment.[37] It was developed completely fromsurface-based weather observations, including descrip-tions of clouds found near frontal boundaries. This theorystill retains merit, as it is a good description for extratrop-ical cyclones over continental landmasses.

4.2 Shapiro-Keyser model

A second competing theory for extratropical cyclone de-velopment over the oceans is the Shapiro-Keyser model,developed in 1990.[38] Its main differences with the Nor-wegian Cyclone Model are the fracture of the cold front,treating warm-type occlusions and warm fronts as thesame, and allowing the cold front to progress through thewarm sector perpendicular to the warm front. This modelwas based on oceanic cyclones and their frontal structure,as seen in surface observations and in previous projectswhich used aircraft to determine the vertical structure offronts across the northwest Atlantic.

4.2.1 Warm seclusion

A warm seclusion is the mature phase of the extratropi-cal cyclone lifecycle. This was conceptualized after theERICA field experiment of the late 1980s, which pro-duced observations of intense marine cyclones that in-dicated an anomalously warm low-level thermal struc-ture, secluded (or surrounded) by a bent-back warm frontand a coincident chevron-shaped band of intense surfacewinds.[39] The Norwegian Cyclone Model, as developedby the Bergen School of Meteorology, largely observedcyclones at the tail end of their lifecycle and used the termocclusion to identify the decaying stages.[40]

Warm seclusions may have cloud-free, eye-like featuresat their center (reminiscent of tropical cyclones), signif-icant pressure falls, hurricane force winds, and moder-ate to strong convection. The most intense warm seclu-sions often attain pressures less than 950 millibars (28.05inHg) with a definitive lower to mid-level warm corestructure.[39] A warm seclusion, the result of a barocliniclifecycle, occurs at latitudes well poleward of the tropics.As latent heat flux releases are important for their devel-opment and intensification, most warm seclusion eventsoccur over the oceans; they may impact coastal na-tions with hurricane force winds and torrential rain.[38][41]Climatologically, the Northern Hemisphere sees warmseclusions during the cold season months, while theSouthern Hemisphere may see a strong cyclone eventsuch 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 re-sult in reintensification into a warm seclusion. For exam-ple, Hurricane Maria of 2005 reintensified into a strongbaroclinic system and achieved warm seclusion status atmaturity (or lowest pressure).[42]

5 Motion

Extratropical cyclones are generally driven, or “steered”,by deep westerly winds in a general west to east motionacross both the Northern and Southern hemispheres of

5

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

A February 24, 2007 radar image of a large extratropical cy-clonic storm system at its peak over the central United States.

the Earth. This general motion of atmospheric flow isknown as “zonal”.[43]Where this general trend is the mainsteering influence of an extratropical cyclone, it is knownas a "zonal flow regime".When the general flow pattern buckles from a zonal pat-tern to the meridional pattern,[44] a slower movement ina north or southward direction is more likely. Meridionalflow patterns feature strong, amplified troughs and ridges,generally with more northerly and southerly flow.Changes in direction of this nature are most commonlyobserved as a result of a cyclone’s interaction withother low pressure systems, troughs, ridges, or withanticyclones. A strong and stationary anticyclone can ef-fectively block the path of an extratropical cyclone. Suchblocking patterns are quite normal, and will generally re-sult in a weakening of the cyclone, the weakening of theanticyclone, a diversion of the cyclone towards the an-ticyclone’s periphery, or a combination of all three tosome extent depending on the precise conditions. It isalso common for an extratropical cyclone to strengthenas the blocking anticyclone or ridge weakens in thesecircumstances.[45]

Where an extratropical cyclone encounters another ex-tratropical cyclone (or almost any other kind of cyclonicvortex in the atmosphere), the two may combine to be-come a binary cyclone, where the vortices of the two cy-clones rotate around each other (known as the "Fujiwharaeffect"). This most often results in a merging of the twolow pressure systems into a single extratropical cyclone,

or can less commonly result in a mere change of direc-tion of either one or both of the cyclones.[46] The pre-cise results of such interactions depend on factors such asthe size of the two cyclones, their strength, their distancefrom each other, and the prevailing atmospheric condi-tions around them.

6 Effects

Preferred region of snowfall in an extratropical cyclone

6.1 General

Extratropical cyclones can bring mild weather with a littlerain and surface winds of 15–30 km/h (9.3–18.6 mph),or they can be cold and dangerous with torrential rainand winds exceeding 119 km/h (74 mph),[47] (some-times referred to as windstorms in Europe). The bandof precipitation that is associated with the warm front isoften extensive. In mature extratropical cyclones, an areaknown as the comma head on the northwest periphery ofthe surface low can be a region of heavy precipitation, fre-quent thunderstorms, and thundersnows. Cyclones tendto move along a predictable path at a moderate rate ofprogress. During fall, winter, and spring, the atmosphereover continents can be cold enough through the depth ofthe troposphere to cause snowfall.

6.2 Severe weather

Squall lines, or solid bands of strong thunderstorms, canform ahead of cold fronts and lee troughs due to the pres-ence of significant atmospheric moisture and strong up-per level divergence, leading to hail and high winds.[48]When significant directional wind shear exists in the at-mosphere ahead of a cold front in the presence of a strongupper level jet stream, tornado formation is possible.[49]Although tornadoes can form anywhere on Earth, thegreatest number occur in the Great Plains in the UnitedStates, because downsloped winds off the north-south ori-ented Rocky Mountains, which can form a dryline, aidtheir development at any strength.

6 9 REFERENCES

Explosive development of extratropical cyclones can besudden. The storm known in the UK as the "Great Stormof 1987" deepened to 953 millibars (28.14 inHg) with ahighest recorded wind of 220 km/h (140 mph), resultingin the loss of 19 lives, 15 million trees, widespread dam-age to homes and an estimated economic cost of £1.2 bil-lion (US$2.3 billion).[50]

Although most tropical cyclones that become extratropi-cal quickly dissipate or are absorbed by another weathersystem, they can still retain winds of hurricane or galeforce. In 1954, Hurricane Hazel became extratropi-cal over North Carolina as a strong Category 3 storm.The Columbus Day Storm of 1962, which evolved fromthe remains of Typhoon Freda, caused heavy damage inOregon and Washington, with widespread damage equiv-alent to at least a Category 3. In 2005, Hurricane Wilmabegan to lose tropical characteristics while still sportingCategory 3-force winds (and became fully extratropicalas a Category 1 storm).[51]

6.3 Climate and general circulation

In the classic analysis by Edward Lorenz (the Lorenz En-ergy Cycle),[52] extratropical cyclones (so-called atmo-spheric transients) acts as a mechanism in converting po-tential energy that is created by pole to equator tempera-ture gradients to eddy kinetic energy. In the process, thepole-equator temperature gradient is reduced (i.e. energyis transported poleward to warm up the higher latitudes).The existence of such transients are also closely relatedto the formation of the Icelandic and Aleutian Low —the two most prominent general circulation features inthe mid- to sub-polar northern latitudes.[53] The two lowsare formed by both the transport of kinetic energy andthe latent heating (the energy released when water phasechanged from vapor to liquid during precipitation) fromthe extratropical cyclones.

7 Historic storms

A violent storm during the Crimean War on November14, 1854, wrecked 30 vessels, and sparked initial inves-tigations into meteorology and forecasting in Europe. Inthe United States, the Columbus Day Storm of 1962, oneof many Pacific Northwest windstorms, led to Oregon'slowest measured pressure of 965.5 hPa (28.51 inHg),violent winds, and US$170 million in damage (1964dollars).[54] The “Wahine storm” was an extratropical cy-clone that struck Wellington, New Zealand on April 10,1968, so named after causing the inter-island ferry TEVWahine to strike a reef and founder at the entrance toWellington Harbour, resulting in 53 deaths. On Novem-ber 10, 1975, an extratropical storm on Lake Superiorcontributed to the sinking of the SS Edmund Fitzger-ald near the US/Canadian border, 15 NM northwest of

Cyclone Oratia showing typical comma shape of extratropicalcyclone over Europe in October 2000.

the entrance toWhitefish Bay.[55] A rapidly strengtheningstorm struck Vancouver Island on October 11, 1984, andinspired the development of moored buoys off the west-ern coast of Canada.[56] The Braer Storm of January 1993was the strongest extratropical cyclone known to occuracross the northern Atlantic ocean, with a central pres-sure of 914 millibars (27.0 inHg).[57] In 2012, HurricaneSandy transitioned to a post-tropical cyclone on the nightof October 29; a few minutes later it made landfall on theNew Jersey coast as an extratropical storm with windssimilar to a Category 1 hurricane and a wind field of over1,150 miles (1,850 km).In the Southern Hemisphere, a violent extratropical stormhit Uruguay onAugust 23–24, 2005, killing 10 people.[58]The system’s winds exceeded 100 mph (160 km/h) whileMontevideo, the country’s capital with 1.5 million inhabi-tants, was affected by tropical storm-force winds for over12 hours and by hurricane-force winds for nearly fourhours.[59] Peak gusts were registered at Carrasco Interna-tional Airport as 107 mph (172 km/h) and at the Harbourof Montevideo as 116 mph (187 km/h). The lowest re-ported pressure was 991.7 hPa (29.28 inHg). Extratrop-ical cyclones are common in this part of the globe duringfall, winter and spring months. The winds usually peak to80–110 km/h (50–68 mph), and winds of 116 mph (187km/h) are very uncommon.[59]

8 See also

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