The southwest vortex is also called the southwest vortex.Under the combined action of the special topography of the Tibet Plateau and the southwest region and certain circulation, a shallow vortex is generated at the low altitude in southwest China.[3]It is a mesoscale system with a scale of about 300-500km, belonging tomesoscale eddy 。
The southwest vortex occurs on the 700 or 750 hPa isobaric surface in southwest China under the special topography of the Qinghai Tibet Plateau and certain circulation conditionsCyclonic circulationThe vortex with closed contour lines is a mesoscale system with a scale of about 300-500km, belonging tomesoscale eddy 。The southwest vortex is one of the important weather systems that affect the precipitation in China.The initial research on the southwest vortex can be traced back to the 1940s.
The southwest vortex is nearly circular and asymmetric, and its structure is different in different development stages.The southwest vortex in the initial stage is a warm shallow weather system;In the mature stage, the positive vorticity of the strongly developed southwest vortex can extend to more than 100 hPa, which is a deep warm humid low-pressure system. The momentum, stratification, vertical movement, etc. in the vortex area are asymmetric;The southwest vortex in the weakening stage is a baroclinic shallow system, and the low tropospheric vortex is a cold structure.[1]
Main features
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(1) The occurrence of the southwest vortex has obvious annual, seasonal, monthly and daily changes, with the most occurrence in summer, most in April September, followed by spring and autumn, and the least in winter;It appears most in July and least in February;The probability of vortex formation at night is more than twice that during the day.
(2) The southwest vortex mainly appears in Jiulong, Batang, Deqin, Kangding and Changdu, followed by Sichuan Basin.
(3) More than half of the southwest vortices have a life cycle of less than 36 hours, but individual vortices can last for 6-7 days in summer.The long-lived vortex mainly occurs in spring and summer, which may be related to atmospheric stratification and water vapor supply.
(4) After the formation of the southwest vortex, only about one-third (38%) of the potential height of the vortex decreases, with an average decrease of - 3.1 g pdm in 12 hours;Most of the vortex weakened and disappeared at the source.
(5) The moving southwest vortex only accounts for 20.5% of the total vortex, and mainly takes the east path (70.7%), moving eastward along the Yangtze River basin into the sea;The northeast route (21.2%) and the southeast route (8.1%) are less than one third.
(6) Some of the vortex weakened and disappeared in the source area, and some could move out of the source area. The southwest vortex moving out of the source area has the greatest probability from May to August.
(7) Most of the vortices moving out of the vortex source are eastward, that is, they move eastward from the source along the Yangtze River basin into the sea.
(8) The mesoscale system with a scale of about 300-500km belongs tomesoscale eddy 。
(9) The southwest vortex is nearly circular and asymmetric, and its structure is different in different development stages.[2]
formation mechanism
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Large scale circulation background of southwest vortex formation
There are two important conditions for the formation and development of the vortex. One is that there should be a strong southerly flow on the eastern side of the plateau, so that the height of the isobaric surface will continue to decrease due to temperature rise and pressure drop. The other is that there should be a trigger condition for the formation of the vortex. For example, the trough moves rapidly from the northwest to the east, so that the south-north flow meets and converges strongly.At the same time, the friction of the plateau side boundary is also a very favorable condition. This is because the southerly flow in the southeast of the plateau will generate cyclonic shear due to the friction of the plateau side boundary, and the northerly flow in the northeast of the plateau will generate cyclonic shearAnticyclonic shear, vortex generated by convergence.
The formation of the southwest vortex has a great relationship with the transport of the southern branch air flow. The Indian monsoon trough is located to the north, which is conducive to the formation of the southwest vortex. This is not only the Indian monsoon trough transports positive vorticity, but also warm and wet air flow.Based on the angular momentum tendency equation, the analysis shows that the angular momentum advection is closely related to the frequency of the vortex, and the change of the angular momentum advection is one of the reasons for the monthly frequency change of the vortex.The interaction between stratified flow and topography also explains the fact that the vortex activity is active in early summer and weakened in midsummer.The boundary layer friction on the eastern side of the plateau generally explains why the southwest vortex often appears in the southwest area on the eastern side of the plateauatmospheric boundary layerInside.
Occurrence and development mechanism
The interaction between atmospheric circulation and topography has an important influence not only on the formation of vortex, but also on its maintenance.The release of condensation latent heat and the formation and maintenance of low-level jet are beneficial to the development of vortex.The occurrence and development of the southwest vortex are well explained by the disturbance mechanism of the upper potential vorticity, the development mechanism of the inclined vorticity and the non-equilibrium forcing mechanism of the atmosphere.
The southwest vortex is a mesoscale system generated in a specific area. Many studies have been made on the influence of topography and other factors on the formation of the southwest vortex.The release of condensation latent heat is closely related to the atmospheric convection and the precipitation.At present, the focus may be on the research of the development mechanism of the southwest vortex before the precipitation, and the research of how much the release of condensation latent heat can strengthen the southwest vortex and how long it will last after the precipitation, which will help to predict the southwest vortex and the precipitation it will bring.[1]
Research History
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The initial research on the southwest vortex can be traced back to the 1940s.Chinese meteorological science and technology workers have carried out a series of studies on the activities and structural characteristics of the southwest vortex, its formation and maintenance mechanism, and its development and eastward movement mechanism. Their research results have important guiding significance for the correct prediction of heavy precipitation and the decision-making of disaster prevention and mitigation in the southwest and eastern regions of China.
Ma Zhenfeng, Chen Zhongming, Liu Hongwu and others have summarized some research achievements in different periods and aspects of concern on the southwest vortex research at the end of last century and the beginning of this century respectively. For example, Chen Zhongming and others summarized the research progress of the southwest vortex before and after the second atmospheric science experiment, and Liu Hongwu and others summarized the research achievements of the southwest vortex from the research methods.
From the statistical analysis of the activities of the southwest vortex in the past 50 years, great progress has been made in the following six aspects: the research on the structure of the southwest vortex, the large-scale circulation background and climate causes of the formation of the southwest vortex, the research on the generation and development mechanism of the southwest vortex, the research on the movement mechanism of the southwest vortex, and the research on the interaction mechanism of the southwest vortex with the plateau vortex and tropical cyclones.
Statistical analysis of southwest vortex activity
The statistical analysis of the southwest vortex activity is a basic work, and also a very necessary and meaningful work.The work in this regard has been carried out earlier, and there are new time series statistical research results irregularly. Although the scope of the southwest vortex is counted, the sample length is different, and the standard of the vortex is slightly different, the conclusions obtained are basically consistent. For example, the activity of the south vortex is frequent in early summer, the vortex sources are most concentrated in Kowloon, followed by the Sichuan Basin, and most of the vortices move out of the vortex source by the easterly path,That is, from the source area to the sea along the Yangtze River basin.
On the basis of statistical analysis, the addition of observation stations on the source and main moving paths of the southwest vortex will help to monitor the activities of the southwest vortex.Unifying the criteria for the activity characteristics of the southwest vortex will enable us to better understand the climate characteristics of the southwest vortex.At the same time, we can also see from the statistical analysis and research work that there is no evaluation criteria for the disaster caused by the Southwest Vortex and the corresponding database of the disaster caused by the Southwest Vortex.Therefore, it is necessary to carry out a long-term series analysis of the activities of the southwest vortex and related disastrous weather, put forward the evaluation criteria for the strength of the southwest vortex and its disasters, and establish a database of the southwest vortex.
Study on the structure of southwest vortex
The research on the structure of the southwest vortex shows that the southwest vortex is nearly circular and asymmetric, and its structure is different in different development stages.
For a typical southwest vortex, the southwest vortex in its infancy is a warm shallow weather system;In the mature stage, the positive vorticity of the strongly developed southwest vortex can extend to more than 100 hPa, which is a deep warm humid low-pressure system. The momentum, stratification, vertical movement, etc. in the vortex area are asymmetric;The southwest vortex in the weakening stage is a baroclinic shallow system, and the low tropospheric vortex is a cold structure.The boundary layer structure and its evolution during the activity period of the southwest vortex are very complex. In addition, the characteristics of temperature, humidity and dynamic fields of the southwest vortex at different stages need to be more comprehensively described.
Therefore, the use of more unconventional observation data, such as radar and satellite data, atmospheric science experiment and intensive observation data, and the use of numerical prediction simulation technology and data assimilation technology will help reveal the structure and evolution of the southwest vortex in a more detailed and objective way.[1]
Mobile mechanism
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The invasion of cold air, the inhomogeneity of wind field distribution and the inhomogeneity of physical quantity field distribution make the vortex move easily towards the positive vorticity center, convergence center and cold and warm advection center (depending on the strength of cold and warm air forces).The Southwest Vortex is a mesoscale system, which can further strengthen the calculation and analysis of physical quantities, and explore the relationship between physical quantities and the movement and development of the Southwest Vortex in combination with numerical prediction products. The development mechanism and predictability of the movement of the Southwest Vortex based on the changes in physical quantities can be further strengthened.
Moving Features
① The annual average frequency of moving out of the southwest vortex accounts for 41% of the total;
② The moving direction of the southwest vortex is basically the same as that of the air flow on the corresponding 500 hPa surface, but slightly to the south;
③ The moving speed is 50% - 70% of the wind speed at 500 hPa;
④ AtShear lineThe upper southwest vortex often moves eastward along the shear line;
⑤ The development of the low trough on the Qinghai Tibet Plateau at 500 hpa moves eastward, which is conducive to the eastward movement and development of the southwest vortex.
Move Path
① Moving southeastward to the sea via Guizhou, Hunan, Jiangxi and Fujian will sometimes affect Guangxi and Guangdong;
② Move eastward along the Yangtze River into the sea;
③ It moves to the northeast, goes to sea through Shaanxi and North China, and sometimes even enters the northeast.
interrelation
① The large trough along the coast of East Asia has developed significantly, the Pacific high is located to the south, and the vortex moves to the southeast;
② There is no large trough in the east, the Pacific high is strong, and the vortex mostly moves to the northeast;
③ The Pacific high is weak or normal, and the vortex moves due east.
Mechanism research
It is very important to study the movement mechanism of the southwest vortex. The southwest vortex moving out of its source will affect the weather in the downstream area.
Ye Duzheng and others pointed out that the vortex located at the bottom or front of the trough and close to the front area, small streams of cold air can invade within 12 days after the vortex is generated, and the vortex can develop and move eastward. Because the vortex is located in the warm and humid air flow in the front of the trough, it moves to the northeast under the guidance of the southwest air flow in front of the trough.The southwest vortex located in a relatively uniform warm region, corresponding to the 500 hPa wide warm ridge, is often accompanied by shear line coordination, thus leading the 700 hPa vortex to move eastward.
Chen Zhongming and others separated the environmental field from the disturbance field by using the two-layer model, and obtained the influence of the environmental field and its distribution characteristics on the movement of the southwest vortex. They believed that the movement of the southwest vortex was affected by many factors, of which the environmental flow field played a more important role,Furthermore, the barotropic model is used to discuss that the asymmetry of the vortex structure and its interaction with the environment is an important reason for the vortex movement to deviate from the guidance flow. The existence of meridional flow makes the vortex move towards a small angle to the right of the wind direction in the maximum wind speed area, which is similar to the situation where the typhoon wind moves along the direction of the wind direction in the maximum wind speed area.In the actual atmosphere, the structure of the vortex is often that the wind field circulation is nearly circular, and the wind speed distribution is asymmetric. Especially during the development of the vortex, the forecasters found that when the southeast gale appeared in the northeast quadrant of the vortex, the vortex often moved slowly to the north, sometimes even stopped moving less, resulting in continuous rainstorm in western Sichuan, while when the southwest gale appeared in the southeast quadrant of the vortex,The typical characteristics of the two kinds of vortex rainstorm may be related to the asymmetric structure of the vortex, which causes the vortex to move northward and northeast.
The case selected by Ding Zhiying and others occurred in the southwest vortex from April to July, 1981 to 1987. Through the composite diagnosis of the physical quantities and kinetic energy of the southwest vortex that developed eastward movement and did not develop eastward movement, it was found that before moving out of the vortex, it was warm advection, and behind the vortex, it was cold advection. The upper level was controlled by warm advection. The southwest low-level jet was strong. The lower level was mainly convergence, and the ascending motion area was located in the northeast of the vortex;The moving out vortex has a weak negative vorticity advection corresponding to it at 200hPa;The production of the disturbance kinetic energy of the moving vortex is greater than the consumption, and the cumulus convection after the moving vortex is the main factor to maintain the vortex.
Ding Zhiying et al. simulated a southwest vortex, and found that cumulus convection factors have a certain impact on the movement of the southwest vortex.When the cold and warm advection is weak, the strong cumulus convection can increase the southwest wind component near the vortex, and promote the negative vorticity in the east of the vortex, which is not conducive to the eastward movement of the vortex.The upper level cold and warm advection has a great influence on the movement of the vortex. When the cold and warm advection is strong near the vortex, it is conducive to the eastward movement of the vortex.
Lu Jinghua et al., through the numerical calculation and diagnostic analysis of a 6-layer baroclinic model for a case, believe that the physical quantity field at the height of 700hPa is unevenly distributed, making the southwest vortex move towards the 700hPa convergence centerPositive vorticity advectionThe direction of increase and the rotation center of momentum perpendicular to vortex flux move;The movement of the southwest vortex depends to a large extent on the role of the guiding airflow;The distribution of the atmospheric baroclinicity intensity near the 700hPa height is uneven, which will affect the moving direction of the vortex.
Pan Yang et al. statistically classified the vortex activities in the southwest Sichuan Basin during the summer of 1991 – 2004 (June – August), and analyzed the differences in the three-dimensional spatial structure of the circulation background between the moving vortex and the stationary vortex, as well as the distribution characteristics of the deviation of meteorological elements,The climatological characteristics of the three-dimensional circulation structure affecting the eastward movement of the vortex are summarized: the cold air intrusion in the middle and upper troposphere in the mid latitude region of East Asia causes the lower temperature in the middle and upper troposphere, and the geopotential height decreases. With the 20 ° 30 ° N south of the cold deviation center, a strong westerly wind appears from the tropopause to 850 hPa, and the maximum westerly deviation is 200 hPa above the lower reaches of the Yangtze River.On the one hand, the zonal gradient of the upper level wind speed difference has strengthened the upper level divergence in the middle reaches of the Yangtze River, which is conducive to precipitation andCyclonic circulationThe power lifting mechanism of development.On the other hand, the westerly bias in the lower troposphere forms an abnormal water vapor transport belt from the southern foot of the Qinghai Tibet Plateau to the south of the Yangtze River in eastern China, which strengthens the westerly wind water vapor transport in the south of the vortex and provides sufficient water vapor for the precipitation latent heat feedback in the east of the vortex.The southwest vortex is easier to move out of the basin under such favorable circulation situation and water vapor conditions.
Yang Shuai and others studied the activities of the high and low level jet and the southwest vortex in the Meiyu rainstorm, and found that the trend of the high level jet was closely related to the activities of the southwest vortex. When the eastern part of China was located in the northwest jet, the southwest vortex was stable and little moved;When it is located in the westerly jet stream, the southwest vortex moves rapidly eastward;When it is located in the southwest jet stream, the southwest vortex strengthens and its moving speed slows down.
Wang Xinmin and others believed that the invasion path of cold air was the key to the development and movement of the southwest vortex.The guiding airflow of the environmental field (high-altitude jet stream, 500hPa airflow) plays an important role in the movement of the southwest vortex. The interaction mechanism between the asymmetric structure of the vortex and the environmental flow field can reasonably explain the movement of the vortex.[1]
Weather characteristics
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(1) When the vortex is in place, rainy weather may occur;When the vortex moves out, 95.5% has precipitation, and the rain area is mainly distributed in the center area of the vortex and the right front of the vortex moving.
(2) The low vortex weather changes from day to day, and it is generally worse at night or in the morning than during the day.
(3) When the southwest vortex moves eastward, the rain area also moves eastward, and the precipitation intensity increases. At the same time, it causes the occurrence and development of surface frontal cyclones, and strong winds, low clouds, and poor visibility also appear.
thunderstorm
Thunderstorms occur in Sichuan Basin in southwest China, and most thunderstorms occur in the southeast of the southwest vortex. Due to sufficient water and gas, when the vortex moves eastward out of Sichuan, it often causes large-scale rainstorm in the middle and lower reaches of the Yangtze River, Huang Huai River basin, North China and other regions.
Features: no out of Sichuan, no dangerous weather phenomenon, thunderstorms will occur during the day and night, especially at night.
interrelation
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Many weather cases of heavy precipitation caused by the southwest vortex show that the southwest vortex is closely related to the plateau vortex and tropical cyclone.When the southwest vortex is coupled with the plateau vortex, both the southwest vortex and the plateau vortex are developed;When the tropical cyclone lands in the west and moves northward, on the one hand, it blocks the southwest vortex, and on the other hand, it transports water vapor and energy, enabling the vortex to develop.But the southwest vortex and other weather systems such as westerly trough, shear lineSubtropical highThe interaction between different scale systems, such as the South Asia High, the South Asia High and so on, will be another aspect of in-depth research on the southwest vortex.[1]
Main impacts
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The southwest vortex plays an important role in the weather system affecting precipitation in China.As far as the intensity, frequency and scope of rainstorm caused by the southwest vortex are concerned, they are second only to typhoons. Many rare catastrophic floods in China's history are closely related to the activities of the southwest vortex.[2]
Sichuan is located in the source area of the southwest vortex, and is greatly affected by the southwest vortex with high frequency. For example, the Sichuan Basin was affected by the strong development of the southwest vortex from July 11 to 15, 1981, and a once-in-a-century catastrophic flood occurred;
From July 28 to 30, 1993, a heavy rainstorm occurred in the southwest of Sichuan Basin. The 24-hour precipitation in Emeishan City reached 509.5mm, breaking the 40 year daily precipitation extreme value in Sichuan;
From July 6 to 9, 2005, a continuous heavy rain weather process occurred in Dazhou City, eastern Sichuan Province. The cumulative rainfall in Xuanhan County reached 320.9 mm, and the cumulative rainfall in its villages and towns reached about 500 mm. The long-term stability of the southwest vortex in the northeast of the Sichuan Basin is an important reason for this continuous heavy rain weather process.
The southwest vortex not only affects the weather of the source area, but also affects the area to the east of the source area when it moves outward. For example, the southwest vortex moved eastward on May 4-7, 1974, causing heavy rain in parts of Hunan and Jiangxi provinces. One of the important influence systems that produce the "63.8" heavy rain in Hebei is the southwest vortex moving eastward and northward,In 1998, a basin wide flood occurred in the Yangtze River basin since 1954, which was closely related to the frequent occurrence and eastward movement of the southwest vortex.
