In the early morning of July 25, thunderstorms began to rain in Fujian Province, which continued to be sunny for several days. The two-hour rainfall in Fujian and Qing reached 180 mm, reaching the standard of heavy rain. Under the thunderstorm attack, the extreme heat in many places in Fujian was obviously reduced. The temperature in Fuzhou at 12 o’clock on the day was only about 25 ℃, which was very comfortable. < p > < p > this rain is not Meiyu, but the “brother” of the vortex, which is brought by the cold vortex. It is the small cold air thrown down by the Yellow Sea cold vortex that induces the thunderstorm in Fujian. Cold vortex, the abbreviation of “cold vortex”, is a kind of high-altitude cyclonic vortex with deep cold air from middle and high latitudes. It has two characteristics: one is cold air mass; the other is counter clockwise rotation. In the west, there is cold air continuously replenishing and going southward, which converges with the warm and humid air flow in the south, which is easy to form thunderstorm weather. Generally, cold vortex thunderstorms mostly occur in Northeast China and North China, and occasionally reach Jiangsu Province. It is very rare that the cold vortex system affected Fujian. According to different geographical locations, cold eddies have different names, such as Mongolia cold vortex, North China cold vortex and northeast cold vortex, among which the northeast cold vortex is the most famous. In Northeast China, low temperature in winter and Thunderstorm in summer are mostly related to it. In addition to short-term heavy rainfall, more than half of the severe convective weather in Beijing Tianjin Hebei region is related to cold vortex. The thunderstorm wind related to cold vortex accounts for nearly 70% of the total thunderstorm gale weather in Beijing. Moreover, in the early stage of cold vortex generation, it may induce extratropical cyclone and produce regional precipitation, which is an important regional weather system. < p > < p > on July 26, a picture of Yaomei peak, a snow mountain 100 kilometers away from Chengdu, was on fire. Meanwhile, the highest temperature at Wenjiang meteorological station was 37.3 ℃, breaking the record. On July 27, the high temperature in Chengdu Plain was upgraded, and the old urban stations reached 39.7 ℃ and some exceeded 40 ℃. In those days, apart from Turpan, Chengdu Plain and its surrounding areas became the hottest place in China. The background of the high temperature is not the subtropical high, but the Qinghai Tibet high. The Qinghai Tibet high is a high-pressure anticyclone with its center over the Tibetan Plateau during midsummer. The circulation is strong, the scale is large and the position is stable. It is the main circulation in the upper part of subtropical troposphere in summer. Over the Sichuan Basin, the Qinghai Tibet high and the subtropical high always fight. The former is dry, with low humidity, and comes from the cool Qinghai Tibet Plateau. If it “wins the main field”, it will crowd out the low vortex and shear line that may bring precipitation, and bring pure blue sky and rapidly rising temperature; the latter comes from the sea with a lot of hot and humid air, and once it takes the lead, there will be hot and stuffy sauna days. Therefore, this time, under the control of the Qinghai Tibet high pressure, Chengdu has excellent air visibility, and of course, the temperature has also risen rapidly. From the night of June 26 to June 27, heavy rain occurred in Sichuan Basin. There were landslides in the section of Chongqing National Highway (No. 318) between August and August. After that, the main position of rainfall moved eastward and successively moved to Jianghan, Jianghuai, Huanghuai and other places. < p > < p > behind this rainfall process is the famous “southwest vortex”, also known as “southwest vortex”. The vortex is an air vortex with low pressure in the center of the lower atmosphere. When there is a strong convergence updraft in the vortex (responsible for lifting the water vapor to the high altitude and condensing into small water droplets or ice crystals), the weather of cloud and rain can be produced. In general, the closed low pressure system with cyclonic circulation at 700 HPA in Western Sichuan is called southwest vortex. The complex topography between the Hengduan Mountains and the Sichuan Basin in the eastern Qinghai Tibet Plateau and the atmospheric circulation on the plateau and the southeast side of the plateau jointly promote the formation of the southwest vortex. The whole system is 1500 ~ 3000 meters away from the ground, and gradually moves to the East, where the rain falls. It takes less than 48 hours for the southwest vortex to appear and die out, but the longest life history can reach 5-7 days after the development and eastward movement. During this process, the southwest vortex system was formed in the night of June 26 due to the joint influence of cold and warm air flow in Sichuan Basin. With the enhancement of warm and humid air flow to the South and abundant water vapor transportation, the southwest vortex moved eastward from June 27 to 28 with the cooperation of the eastward moving high trough, which successively affected Jianghan, Jianghuai, Huanghuai and other places. From July 6 to 9, continuous heavy rainfall occurred from the east of Southwest China to the middle and lower reaches of the Yangtze River. Rainstorms occurred in Chongqing, Guizhou, Jianghan, Jianghuai and Northern Jiangnan, and the rainfall in some places reached the level of heavy rainstorm. The water level of the main stream of the middle and lower reaches of the Yangtze River continued to rise, and the whole river section below Jianli exceeded the police. < / P > < p > the weather system called “low level shear line” dominates the rainfall. The shear line is the boundary line between two horizontal air flows with different directions, which changes in a cyclonic (counter clockwise direction in the northern hemisphere). In fact, it is also the boundary between the two opposing flows. For example, there is an opposite line between the southwest wind and the northeast wind, which is the shear line. On the shear line, there are often horizontal convergence and upward movement of airflow, which is easy to produce cloudy and rainy weather. Shear line can appear all the year round, but it is most frequent in late spring and early summer. Most of the heavy rainfall in June and July is related to low-level shear line and low-level jet stream. The shear line in spring is mainly active in South China, which is called South China shear line; at the turn of spring and summer, it is mostly located in the Yangtze Huaihe River Basin, known as the Jianghuai shear line. The month mainly occurs in North China, known as the North China shear line. It can be seen that its active area is related to the North-South oscillation of the ridge line of subtropical high. The distribution of rainfall on the shear line is very uneven, and rainstorms often occur in areas with strong convergence and abundant water vapor supply. From July 6 to July 9, the continuous heavy rainfall occurred in the eastern part of Southwest China to the middle and lower reaches of the Yangtze River. If it was not for the “great help” of the low-level jet stream, the low-level shear line alone would not have caused such a heavy rain. Generally speaking, the emergence of precipitation requires water vapor and dynamic uplift conditions, and the latter is to make water vapor cool and condense to form precipitation. In order to form rainstorm, the above two conditions need to last for a long time. It can be said that water vapor is a necessary condition for all rainfall. Low level jet is a “express brother” who transports water vapor, heat and momentum to the low altitude atmosphere. < p > < p > it transports warm and humid air from the tropical ocean to the lower part of the drier and colder air to the north, thus forming an unstable atmospheric structure and producing a strong upward movement, thus bringing convective precipitation such as rainstorm and hail. When the wind is strong, it can reach level 8 (20 m / s), which is equivalent to the wind near the center of a tropical storm, and sometimes even tornadoes. In South China, the low-level jet is generally southwest or southerly, and mostly occurs in the West or north edge of the subtropical high. From April to June every year, before the first northward jump of the subtropical high, there is a strong warm advection in the west side of the subtropical high. Under certain weather conditions, the weather system similar to vortex is easy to appear in the low altitude. If it is combined with the southern branch channel, it will often cause rainstorm or even severe rainstorm in South China. The intensity of the low-level jet has obvious diurnal variation, which is obviously enhanced at night and early morning, but relatively weak in day and evening. < p > < p > from July 21 to July 22, the 24-hour rainfall in many places in southern Shandong Province generally exceeded 100 mm, reaching the level of heavy rainstorm, and some places even approached the extreme rainstorm. From the night of July 22 to July 23, the rain was fierce in many places from Bohai Bay through Shandong Province to Anhui Province and Jiangsu Province, accompanied by severe weather such as thunderstorms and gales. In the Bohai Strait and the northern and central Yellow Sea, strong winds of magnitude 9-10 and gust force 11 also appeared, and the passenger ships stopped sailing. < / P > < p > if you have a chance to see the weather radar mosaic on the night of the 22nd, you will find that large areas of strong convective echoes in the above areas are very similar to the appearance of typhoons. In fact, the “behind the scenes” of the heavy rainfall and strong wind is the extratropical cyclone, which belongs to the same cyclone as the tropical cyclone. The extratropical cyclone, also known as “extratropical low pressure” or “frontal cyclone”, is an approximately elliptical baroclinic cyclone active in the middle and high latitudes of temperate zone. From the structure, it is a cold core system, the central pressure is lower than the surrounding, and has the property of cold center. They can be found all year round and can be produced on both land and sea. According to the different origins, it can be divided into Mongolia cyclone, Jianghuai cyclone and East China Sea cyclone. With the help of the westerly belt, extratropical cyclones move from west to East. During the movement, it often brings windy and rainy weather, accompanied by heavy rain, heavy snow or other severe convective weather. Sometimes, the maximum wind force near the ground can reach more than 10.