The extratropical cyclone, also known as "extratropical depression" or "frontal cyclone", is an approximately elliptical baroclinic cyclone active in the middle and high latitudes of the extratropical zone.The extratropical cyclone activity is often accompanied by cold air invasion, and the weather phenomena such as cooling, sandstorm, snow, frost, strong wind and rainstorm follow[16]。
Structurally speaking, an extratropical cyclone is a cold center system, that is, the central pressure of an extratropical cyclone is lower than that around it, and it has the nature of a cold center.In terms of scale, extratropical cyclones are generally larger than tropical cyclones, with diameters ranging from hundreds of kilometers to 3000 kilometers, with an average diameter of 1000 kilometers.
Extratropical cyclones appear along with the front. Sometimes 2-5 extratropical cyclones will form on the same front, moving from west to east in turn, called "cyclone family".The whole life cycle of extratropical cyclones from generation, development to extinction is generally 2-6 days.
The extratropical cyclone is one of the important weather systems that cause large-scale weather changes, and has an important impact on the weather changes in the middle and high latitudes.Huanghuai cyclone、Jianghuai cycloneThey are collectively called extratropical cyclones.
From the structural point of view, the extratropical cyclone is a cold center system, that is, the central pressure of the extratropical cyclone is lower than the surrounding, and it has the nature of a cold center.In terms of scale, extratropical cyclones are generally larger than tropical cyclones, with diameters ranging from hundreds of kilometers to 3000 kilometers, with an average diameter of 1000 kilometers.[15]
For a mature extratropical cyclone, its underlying structure generally consists of 1-2cold frontAnd oneWarm frontIt is formed that between the warm front and the first cold front there is a warm zone, while behind the cold front there is a cold zone, and in front of the warm front and behind the center of the system there is a relatively mild cold warm transition zone.The extratropical cyclone is generally a high altitude trough line at high altitude, and the strength of the trough line decreases with the increase of the pressure layer.
Causes
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Generated alone
Causes of extratropical cyclone formation
Some extratropical cyclones develop from a wave on the front.Waves are formed on the front for some reasons, and a closed isobaric line appears near the top of the wave, and then gradually develops to form a complete cyclone.Whether it is a westerly extratropical cyclone or a frigid extratropical cyclone, an initial cause of its development is the emergence of baroclinic state. For extratropical cyclones, a necessary condition for the emergence of baroclinic is the emergence of convergence at the bottom and the sliding hedge of cold and warm currents.Therefore, it can be considered that the causes for the formation of extratropical cyclones are as follows: first, there is a trough line developing or a transverse trough maintaining in the westerly belt, which leads to the formation of a relatively stable quasi stationary front of the cold and warm air masses in the junction area.Then, there is an obvious fast cold advection moving southward on the north side, or a fast warm advection moving upward on the south side. (Here, the cold advection is taken as an example) With the cold advection moving southward, the quasi-static stop front begins to appear abnormal distortion, and at the same time, the trough lines in the height field continue to develop, providing the middle and upper divergence space (jet stream) for the warm and humid air masses lifted by the cold advection.
Evolved from tropical cyclone
An extratropical cyclone can also beTropical cycloneWhen the tropical cyclone moves north to the temperate zoneWesterly troughIt loses the characteristics of tropical cyclone due to its influence and becomes an extratropical cyclone.On the other hand, depending on its location and intensity, the impact range of large extratropical cyclones may exceed that of extratropical regions, and subtropical regions may also be affected.
Most tropical extratropical cyclones are converted from tropical systems, of which TC accounts for more than 85%, so TC is taken as an example.As we all know, TC is a warm core structure composed ofCISK mechanismA mesoscale system that provides main energy. When it encounters a strong cold advection behind the westerly trough in the front, CISK often appears due to the involvement of dry and cold airCISK mechanismUnable to continue to operate effectively, and baroclinic potential energy gradually appears in the system, which is actually divided into several stages:
(1) The divergence strengthening period in front of the trough: TC was affected by the southwest jet stream in front of the high trough, and the divergence was strengthened and even erupted, which was reflected in almost all the development processes of turning TC in the northwest Pacific, specifically the instantaneous eruption of systematic convection, and the movement gradually began to show the northeast component.
(2) The cold flow in the trough enters into the SubTC like stage: when the TC further penetrates into the westerly trough, the system is gradually affected by the cold advection in the trough and baroclinic energy appears.At this time, the cold air mass at the bottom layer slowly permeates, and at the same time, it greatly lifts the convective cloud cluster that was originally rich in tropical water vapor in TC, so as to quickly condense precipitation and reduce the temperature of the convective cloud top.In this process, the whole system presents a stable convection state of cold at the bottom and warm at the top, so there will be few new large convective eruptions. At the same time, a ground cold front starts to appear from the southwest side of the systemCyclonic curvatureThe center is generally eccentric at the side where the cold advection enters.In this state, TC generally reaches the maximum precipitation intensity in the transition period, but the overall wind speed weakens significantly, while the central pressure rises, and TC maintains about 12-36 hours in this state.
(3) The baroclinic potential can replace the condensation latent heat, and the system completes the conversion: when a warm front appears in the system, it can be considered that the replacement of the condensation latent heat has been completed, so the system has been completely transformed into an extratropical cyclone system (generally, the organization starts to stop at this time), and there are two typical fronts, one cold front and one warm front, but at the same time, the high-level still maintains a small amount of warm core structure,The precipitation in this stage is mainly intermittent.[1]
classification
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Temperate cyclones are mainly divided into three categories according to their causes: westerly, frigid and tropical:
The westerly extratropical cyclone refers to a situation where the height field trough line fluctuates from the upper level of the westerly belt (500 Hpa or above), and then gradually induces the extratropical cyclone at the bottom. This type of extratropical cyclone generally has a relatively short duration, a fast speed of movement, and a moderate degree of development.
The cold zone extratropical cyclone refers to the low-level extratropical cyclone formed by the impact of the polar cold air mass southward on the westerlies in the middle and high latitudes. This type of extratropical cyclone generally appears in the bottom structure first and then develops into the upper level, which is relatively common among the three types.Generally, the cold temperate cyclones maintain for a long time, with strong intensity and strong cold advection, and often bring strong wind to cool down.
Tropical extratropical cyclone refers to an extratropical cyclone that gradually loses its tropical transformation after the tropical system is impacted by baroclinic energy. As this type of extratropical cyclone is developed and transformed from the tropical system, its bottom and upper structures will be slightly different from general extratropical cyclones, and the precipitation is generally strong, but because it is often accompanied by trough lines, its moving speed is also very fast,In particular, it can reach a moving speed of 60-80km/h after entering the area guided by the high-level westerly jet.[2]
Evolution process
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The evolution process of extratropical cyclones can be roughly divided into initial stage, development stage, mature stage (imprisonment stage) and extinction stage.
Nascent stage
There used to be a quiet front on the ground. To the north of the front is cold air, and to the south is warm air. Cold air moves from east to west, and warm air moves from west to east. When cold air moves southward under the front, warm air rises northward, and there are 1~2 closed isobars.
Nascent stage
During this period, the extratropical cyclone can be shown on the visible satellite image as the frontal cloud band becomes wider, bulges toward the cold zone, turns white, and has more moderate and high clouds.
Development period
With the development of fluctuations, the pressure further decreases, the closed isobars increase, and the cold air further moves southward. Showers or snow showers appear near the cold front, and precipitation also occurs in front of the warm front, and the precipitation area expands.With the development of cyclones, the disturbance in the lower level gradually develops to the upper level, the air flow rises in a spiral manner, and the low trough in the upper level also gradually deepens.
Development period
During this period, the extratropical cyclone shows that the frontal cloud belt bulge is more obvious on the satellite cloud image, and the rear boundary of the mid high cloud begins to concave inward toward the cloud.With the establishment of the bottom closed isobaric ring, the main signs of the development period of extratropical cyclones are the decrease of air pressure and the increase of central wind speed.
On the west side of the system center, the cold advection continues to move southward, and cyclonic rotation occurs, leading to the emergence of a typical cold front cloud system on the south side of the system center. Generally, the length of this cloud system is long, and some can even reach thousands of kilometers.The supplement and enhancement of cold flow also led to the strengthening of southwest air flow in front of the front, so warm advection appeared in the east of the system, and then a warm front was formed. A large number of precipitation cloud systems developed in front of the warm front in the northwest of the system, whose length was shorter and more irregular than the cold front cloud, but its width was larger, and the total precipitation intensity was larger.
The cold and warm air mass produces convergence and uplift at the center of the extratropical cyclone, and then there are moderate cumulonimbus clouds and rotating updrafts, thus driving the development of the high-level trough line. When the system connects the upper and lower structures and the cold flow after the trough no longer continues to strengthen, it is considered to have reached its maturity period (confinement period).In this stage, the cloud picture shows that the middle high cloud system appears in the center of the system, and the cold front cloud system is lengthened and narrowed,Warm front cloud systemThickened and widened, and gradually changed to a comma shape as a whole. A large clear sky area can be seen behind the cold front cloud.
mature period
Maturity period (confinement period)
During the mature period (confinement period), the cyclone reached its peak, and it has become a circular closed circulation from the ground to the height of 500 mbar.The cold front on the ground gradually catches up with the warm front and lifts the warm air on the ground, and the cyclone begins to trap.At this time, the range of cloud and rain is the largest, the intensity is strengthened, the wind force is increased, and the weather changes most violently.But since the ground has been occupied by cold air and becomes a cold vortex, the cyclone begins to weaken.During this period, extratropical cyclones show obvious dry tongue and spiral structure in the rear of the cloud system on satellite cloud images.The cloud belt extends to the vortex center.At this time, the cold advection is no longer strengthened, and the far end of the cold front cantilever often throws out a large number of positive vortex small cyclones, which can develop into a tropical system or even a new extratropical cyclone if conditions are suitable.The warm front maintains the original state of heavy precipitation, while the cloud layer is further thickened.
A trapped front appears in the center of the system, which is generally a cold trapped front in the Northern Hemisphere. At this time, if the latitude is high or the previous base temperature is low, it is very likely that there will be a blizzard or persistent heavy snowfall, which is also the stage where the extratropical cyclone has the greatest impact on the ground.From the upper level, the upper level of the extratropical cyclone in this stage is mainly dominated by the westerly trough, which has generally deepened to more than 2000km, and a few of them may have the existence of cut-off vortex, then the intensity of the underlying warm air is stronger, the cold advection is stronger, and the central convergence and uplift are more obvious.From the cloud map, the extratropical cyclone at this time is the same as that at the later stage of the development period, except that 3/4 circles are involved near the center, and the gap is generally located in the southwest quadrant (some have also had this number of circles, usually caused by high-altitude cold vortex). From the water vapor map, it can be clearly seen that the front of the cold front is controlled by the warm and humid area, and the front of the warm front reaches the peak,But behind the cold front is a large dry area, and the level inside the cold fronttemperature gradientandBarometric gradientVery large, the humidity difference is usually more than 40.
Extinction period
Extinction period
In the final stage of cyclone development, warm air only remained in the southeast corner of the ground, and the convergence of the whole cyclone center in the lower level was strengthened. The ground pressure has become a cold vortex, and the low-pressure center began to be blocked.The closed circulation from the ground to about 500 mbar has weakened, the upward movement has disappeared, and the cyclones have weakened, or even disappeared.During this period, the extratropical cyclone in the satellite cloud image shows that the dry tongue extends to the center of the cyclone, the spiral cloud belt rotates more than one circle around the center, and the high-low circulation center coincides with the center of the cloud vortex.As the extratropical cyclone enters the confinement stage, the cold advection behind the trough is no longer strengthened, the baroclinic energy of the system is gradually consumed, and the warm front also begins to enter the front vanishing stage.
With the weakening and disappearance of the warm front, the uplift column of the system began to weaken, and other areas except the southeast side were gradually controlled by the cold air mass. At the same time, because the center still maintained the low pressure convergence inertia, the system as a whole turned into a cold center convergence structure, and the central pressure began to rise rapidly, further strengthening the weakening of the warm advection, and gradually entering a vicious circle.When the dry tongue control center is shown on the water vapor chart, it can be considered that the system has turned into a cold center structure, so that the system is no longer classified as an extratropical cyclone. After that, if the weak cold advection weakens, the system will return to the frontal nature. If the cold advection remains, the system will become a cold air mass, gradually transforming into an atypical cold high pressure situation (high air trough) on the ground.
These stages are the life history of a single cyclone.The development process of frontal cyclones in East Asia and China is generally about 3 days, the short one is about 1 day, and the long one is about 4-5 days.[2-3]
features
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Extratropical cyclones are mainly operated and strengthened by the baroclinic pressure provided by the westerlies. They can appear all year round and can be generated on land and sea.The whole life history of extratropical cyclones from generation, development to extinction is generally 2 to 6 days, which can be roughly divided into four stages: birth, development, imprisonment and extinction.Among them, the extratropical cyclone developed to its peak in the confinement stage.At this stage, the range of cloud and rain is the largest, the wind force increases, and the weather changes most violently.The extratropical cyclone is one of the important weather systems that cause large-scale weather changes, and has an important impact on the weather changes in the middle and high latitudes.Temperate cyclones often bring windy and rainy weather, accompanied by rainstorm, snowstorm or other severe convection weather, and sometimes the maximum wind near the ground can reach more than 10.[15]
An extratropical cyclone, as its name implies, has both extratropical and cyclonic characteristics, which are manifested in its structure as follows:
1. Temperate: cold front and warm front coexist, upper westerly belt trough line;
2. Cyclonic: cold and warm advection cyclonic involvement, with convergence updraft in the center.
Induced system
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1. Mesoscale convergence system MCC:
As one of the most common inducing systems of extratropical cyclones,MCC often appears at the end of the cold front cantilever, especially in the east-west area of the long cantilever. The cold flow in this area often moves slowly because it is too far away from the horizontal dynamic center, and cannot effectively support the continuation of the front, resulting in the collapse of the cold flow, which makes the warm and humid air flow break through north quickly here, and blocks the remaining cold front cantilever to form a closed low-pressure circulation,This is the prototype of MCC system.
When the system appears, with the weakening of cold flow and the consumption of dynamic energy, the cold core air mass inside the circulation is gradually heated and lifted until its physical properties as a cold air mass disappear. At this time, we think that the MCC system is formed. For such systems, generally it will continue to develop and move along the residual front towards the center of temperate cyclones,It causes irregular precipitation center on the cold front.
When the system gradually lags behind the cold front cantilever and enters the cold advection control area in the process of gradually moving northward, it is possible that a new small extratropical cyclone may appear in a state similar to the tropical cyclone's transition period. The initial change of this type of cyclone is affected by the strong cold advection and merges to the front, and finally becomes part of the front.In particular, when MCC systems appear in large numbers at the end of the cold front cantilever, we can think that there is a "cyclone family" in this area, but pay attention to treat the "cyclone family" differently from the mother temperate cyclone.
2. Cold front squall line:
As we all know, the cold front is the strongest frontal area in an extratropical cyclone. On average, when the extratropical cyclone develops
In the mature period, a squall line system often appears in this area, and the squall line is characterized by multiple waves and high intensity. The gust front wind speed of some strongly developed squall lines can reach or even exceed Grade 12. This area is the place where the wind speed of extratropical cyclones is the highest except the center.
3. Cold surge and induced disturbance:
Cold surge, as readers familiar with the tropical system know, refers to a situation where the denatured cold air mass in the mid latitude region moves southward and the uplifted warm current ignites convection around the tropical system, while the disturbance induced by cold surge is a rare situation, which generally only occurs in the mid and low latitudes of the northwest Pacific in midsummer.
When the cold front cantilever of an extratropical cyclone breaks out of a MCC system and goes south into the tropical ocean, and then when the reappearing front breaks again due to a particularly strong supplementary cold current, it is generally difficult to form a second MCC system. At this time, the modified cold advection will go south into the previous MCC, stimulate a lot of convection and gradually form a convergence center and disturbance system,At this time, if the surrounding environment is suitable and the water temperature is sufficient, it is likely to form a tropical disturbance, or even develop into a tropical cyclone.[2]
Development belt
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Generally, it will only develop and affect the coastal land in the temperate sea area of 30 to 60 degrees north latitude and south latitude, but it is also very rare in low latitude areas (such as China
The tropical cyclone from South China to East China coast moved inland and was affected by cold air and transformed into an extratropical cyclone (for example, Taipei raccoon and tropical storm Haigauss).From the global average over the years, there is an obvious extratropical cyclone generating area in the southern hemisphere slightly north of the westerly circulation, and it runs through the whole southern hemisphere.In the Northern Hemisphere, extratropical cyclones are mainly generated in the following areas:
1. Siberia:
The extratropical cyclones in this area mostly move southward from the Arctic Ocean polar vortex, or induce westerly long wave trough to the southLake BaikalThe belt formed the embryonic form of temperature and air, and then moved eastward into the Sea of Japan and the Sea of Hubei, where it developed strongly and reached a peak state, and finally merged into the Aleutian Vortex and landed in Alaska, North America, where it weakened and dissipated.At the same time, there are also frontal extratropical cyclones that began to form in the Altay region and reached the strongest in the eastern part of China, and obviously affected China.
2. Northeast North America:
This area is mainly caused by the joint influence of the Aleutian split outflow and the southward movement of the North American polar vortex, which forms a cross trough at the upper level. At the same time, the water vapor in the bottom layer of the Gulf of Mexico moves northward along the Central American Great Plain, forming convergence in the leeward slope low-pressure area formed on the east side of the Cordillera Mountains, and finally leading to the formation of a vast low-pressure area at the Canadian Shield in the north of the North American Great Plain,After moving eastward, the system strengthened its entry into the North Atlantic Ocean under the influence of the perennial North American Trough, then reached the strongest near Iceland and moved eastward to land on the coast of Western Europe.The extratropical cyclones active in China mainly have two relatively concentrated zones, one is 25-35 ° north latitude, such as Jianghuai cyclone, East China Sea cyclone and Taiwan cyclone;One is at 45-55 ° north latitude, such as Mongolia cyclone, Northeast cyclone, Yellow River cyclone and Yellow Sea cyclone.[2]
influence
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The extratropical cyclone is one of the important weather systems that affect the large-scale weather changes in the middle and high latitudes. The extratropical cyclone and its frontal system, driven by baroclinic instability, can cause obvious or intense weather phenomena, such as extreme temperature, extreme heavy precipitation, strong storms and storm surges[16]。
The average radius of extratropical cyclone is 1500km, and its range is generally about 500~3000km.The impact of extratropical cyclones is not limited to temperate regions, and some large extratropical cyclones can also affect subtropical and tropical regions.The extratropical cyclones are mainly concentrated in the rising area of warm moist air flow, that is, the northwest northwest northeast side of the central and western regions and the vicinity of the warm front. Of course, there will also be cold front shear cloud systems on the cold front, which generally have the strongest precipitation rate of the whole storm, but the width of the rain belt is almost the smallest of all cyclones.However, the writing area in front of the cold front and behind the warm front is known as the cyclonic warm area. The southwest wind prevails in the area, and there are often inverted troughs. It is a good place for the development of cumulus clouds, and there are also strong small and medium-sized systems such as squall lines.
The extratropical cyclone has a greater impact on the westerly fluctuation, and the stronger extratropical cyclone can even affect the activities of the tropical system in the middle and low latitudes (cold front cold surge, upper cold vortex induced disturbance).In a word, extratropical cyclones, as the most important weather system affecting northern China in spring and autumn, are one of the most important mesoscale systems in the process of mid latitude bottom weather changes. Their intensity changes and movements often have an immeasurable impression on the near ground. Therefore, we carefully analyze the state of extratropical cyclones and the changes in the surrounding circulation,We can roughly predict various changes in the weather around us in the next stepMeteorological forecastIt is very important.[2]
Maritime extratropical cyclone
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An extratropical cyclone on the sea is an atmospheric vortex formed or moving on the extratropical sea, with the central pressure lower than the surrounding pressure.The air within the cyclone range of the northern (southern) hemisphere rotates counterclockwise (clockwise), and its intensity can be expressed by the central pressure or the maximum wind speed.The central pressure of extratropical cyclones on the sea is generally 970~1010 mbar, and the maximum wind speed on the sea surface can exceed 30 m/s.The extratropical cyclone has a large range and a long wind area, which can form dangerous weather and huge waves such as strong wind, precipitation and thunderstorm on the sea. In winter, it can sometimes guide the cold air behind the low pressure to the south, forming cold wave weather, which seriously threatens maritime navigation and fishery production. This is one of the important disastrous weather systems on the extratropical sea.
The research on the nature, structure and cause of formation of extratropical cyclones began in the early 19th century, but it was not until the early 20th century that the theoretical basis for the occurrence and development of cyclones was established after Pierkenis and his son founded the theory of extreme front.Cyclones can be divided into frontal and non frontal ones.Most of the extratropical cyclones on the sea belong to those with fronts, and their temperature distribution is asymmetric.In the Northern Hemisphere, the central axis of frontal cyclones usually inclines to the northwest, and the intensity gradually weakens from the ground surface.The development of frontal cyclones generally goes through three stages: birth, development and extinction.Because the friction of the sea surface is small and the air gathers less towards the cyclone center, the sea is more conducive to the development of cyclones. Cyclones moving from the mainland to the sea often strengthen again, especially over the ocean front area. Because of the strong baroclinicity (see baroclinic atmosphere), it is easier to generate cyclones or strengthen cyclones.
The coastal waters of Asia and the east of the Americas are the two most active areas of extratropical cyclones in the northern hemisphere.As far as offshore China and the Northwest Pacific Ocean are concerned, the sea area north of 20 ° N can be affected by extratropical cyclones throughout the year.The statistical results of cyclones off the coast of China show that the activities of cyclones from March to May in the Gregorian calendar are the most, which is the peak season of the year;January and December are the months with the least cyclone activity throughout the year.
Throughout the year, extratropical cyclones affecting China's offshore mainly occur in two regions: ① from the middle and lower reaches of the Yangtze River in China to the southern sea of Japan; ②From central Mongolia to the east side of the Great Khingan Mountains in northeast China.The generating areas of these two extratropical cyclones are just the same as the north and south main front belts appearing in East Asia and the northwest Pacific.After cyclones are generated, their moving direction is basically the same as that of the air flow in the middle troposphere, and the symmetric velocity is proportional to the air flow velocity in the middle troposphere.There are mainly three paths of extratropical cyclones in the northwest Pacific: ① from west to east; ②From southwest to northeast; ③First from northwest to southeast, then turn to northeast.However, no matter which direction it moves, if the frontal cyclone does not disappear in the movement, it will eventually move to the Aleutian Islands and the ocean to the east, where the frontal cyclone will be trapped.
extratropical anticyclone
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extratropical anticyclone It refers to the high pressure system generated and active in middle and high latitudes and temperate regions.Seen from the pressure field, the central pressure is higher than the surrounding pressure, and there is a high pressure system with closed isobars.From the wind field, the wind rotates clockwise around the center of the high pressure in the high pressure area of the Northern Hemisphere.Therefore, it is also calledAnticyclone。Extratropical anticyclones are generally generated in high latitudes and composed of cold air masses, which move southward or southeast under the guidance of appropriate atmospheric circulation.It affects the middle and low latitudes and becomes a cold air activity.Sometimes the intensity of cold wave can be reached.Therefore, it is also calledcold anticyclone 。
The horizontal range of extratropical anticyclones generally reaches several thousand kilometers, sometimes occupying most of China.Its life history is generally divided into:
Birth stage, development stage and extinction stage.When an extratropical anticyclone moves from high latitude to southeast, its front often forms a cold front because it intersects with the warm air mass.Therefore, there are often cloud systems or wind and rain weather.However, when the cold front passes through and is controlled by the extratropical anticyclone, especially near the center of the anticyclone, it is mainly sunny weather.It often forms in winterfrost。[2][4]
research method
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The research methods of extratropical cyclones can be roughly summarized into two categories: first, according to the synoptic definition of cyclones, use the objective identification and tracking algorithm of cyclones to determine the tracking cyclones, and then study the change characteristics of their frequency, path and intensity;The second is to use the analysis method of extreme events such as percentile to explore the characteristics of cyclone activity based on the long series of hourly pressure change data.NCEP/NCAR reanalysis sea level pressure field data, ERA-40 reanalysis sea level pressure field data and station pressure observation data are often used in the research.[5-7]
The automatic identification and tracking algorithm of cyclones is mainly based on the objective definition of extratropical cyclones, that is, the objective identification of the low-pressure center of the reanalysis sea level pressure field.By comparing the value of the central grid point of the cyclone with that of the surrounding grid points, the low-pressure center can be identified;For different research areas, the average difference between the grid point value of the cyclone center and the grid point value around it meets a certain threshold, so that the low-pressure center reaches a certain intensity.In addition, the cyclone center shall be stored for at least 12 hours or 24 hours continuously.The cyclone path tracking program can be used to judge whether a cyclone is new or moved from the last time.[8-10]
Change trend
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Research shows that one potential impact of global warming is the change of frequency or intensity of extratropical cyclones.[11]In recent years, a large number of studies have shown that in the second half of the 20th century, the frequency of winter cyclones in the Northern Hemisphere increased at high latitudes (60 °~90 ° N), but decreased significantly at mid latitudes (30 °~60 ° N), which means that the storm track has a trend of northward migration, and the intensity of cyclone activity is increasing at both mid and high latitudes.[11-12]
In the two cyclone prone regions of the North Atlantic and the North Pacific, strong cyclone events (center pressure ≤ 970 hPa) in winter have increased significantly since the 1970s,[13-14]Especially in the North Atlantic and Western Pacific regions, the frequency, intensity and deepening rate of winter cyclones are increasing.[10][12]It is worth noting that the East Pacific Ocean and the North American continent show many opposite characteristics of change, and the interdecadal changes of cyclone activities in the Atlantic and Pacific Ocean also show a dipole pattern in the north and south.[12]
Cause analysis
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Impact of global warming
Interannual variation of mongolian cyclone sandstorm days in spring 1979-2001[8]
In the process of global warming, the winter temperature in the Northern Hemisphere has the most intense upward trend, especially in the middle and high latitudes.McCabe et al suggested that there was a good correlation between the change of winter cyclone frequency and the change of winter temperature in the Northern Hemisphere. Specifically, the winter temperature in the Northern Hemisphere was negatively correlated with the frequency of cyclones in mid latitude (30 °~60 ° N), and positively correlated with the frequency of cyclones in high latitude (60 °~90 ° N). Both correlations were significant and the correlation between the former was stronger.It can be seen that global warming may increase the activity of cyclones at high latitudes and decrease the activity of cyclones at middle latitudes, which supports the hypothesis that global warming may cause the northward migration of cyclone paths in the Northern Hemisphere.[11]
Relationship with mid latitude baroclinic waves
Geng et al. obtained from the principal component analysis that, in 1958-1998, the activity of cyclones in the North Atlantic region was closely related to large-scale baroclinicity in the lower troposphere.At the same time, by studying the maximum growth rate of eddy (a parameter representing baroclinicity) and the zonal wind of 200 hPa, they pointed out that the large-scale baroclinicity and jet flow in the lower troposphere in the north of the North Atlantic are increasing, and their changes correspond to the cyclonic activity in the North Atlantic, that is, the increase of baroclinicity and jet flow and the maximum increase of the frequency of cyclones in the north of the North Atlantic, the cyclone deepening rateThe moving speed is consistent with the increase of central air pressure;Graham et al.'s research shows that the activity of baroclinic waves in the mid latitude of the Northern Hemisphere has increased in the past 40 years, and the enhancement of cyclone activity is obviously due to the increase of upper tropospheric wind and vertical wind shear in the central North Pacific.[10][14]
850hPa annual average temperature change[8]
Relationship with the North Atlantic Oscillation
Geng et al. applied the principal component analysis to obtain the relationship between the cyclone activity in the North Atlantic region and the North Atlantic Oscillation (NAO) in 1958-1998. The increase of the frequency of cyclones in the north of the North Atlantic, the increase of the cyclone deepening rate, moving speed and central pressure are also consistent with the change of the North Atlantic Oscillation (NAO) index, that is, the frequency of cyclones in the Atlantic Ocean is significantly correlated with NAO.This is consistent with the research results of Graham et al. and Gulev et al. It is worth noting that the transformation of NAO in the mid-1970s led to considerable changes in the associated European storm track.[10][12][14]
Relation with sea surface temperature
Lambert believes that the change of strong cyclone events in the Pacific Ocean is related to the equatorial and mid latitude SST gradients, which is not obvious in the Atlantic Ocean.Graham et al. proposed that the increase of 25 ° - 40 ° N strong cyclones in the Pacific may be related to the increase of SST in the tropical area of the northwest Pacific.[13-14]
In addition to the above factors, Gulev et al. research shows that the Pacific North American type (PNA) system is mainly related to the frequency of cyclones in the East Pacific, and controls the activity of cyclones in the Gulf area and the coast of North America.[12]
In the second half of the 20th century, the frequency of winter extratropical cyclones in the Northern Hemisphere increased at high latitudes, but decreased significantly at mid latitudes, that is, the path of cyclones tended to shift to the Arctic, while the intensity of cyclone activity increased at both mid and high latitudes.In East Asia, the activity of extratropical cyclones is closely related to the change of dust storms. In the past 50 years, the activity of cyclones in East Asia has also shown the characteristics of increasing in mid high latitudes and decreasing in mid low latitudes. In particular, the weakening trend of strong cyclones in most parts of China corresponds to the decreasing trend of dust storms in China over the past half century.
Global warming caused by human activities may be the main reason for the northward migration of the cyclone path in the Northern Hemisphere.In addition, large-scale baroclinity, jet flow andNorth Atlantic Oscillation(NAO) is also an important factor affecting cyclone activities in the North Atlantic region.
