Why are there so many “vortices” in the weather forecast?

In weather forecasts, we often hear the word "vortex". What exactly is "vortex"? Vortex, composed of "水" and "呙", originally means a pot-shaped vortex of water. There is an explanation in the dictionary that the low-lying area in the middle formed by the rotation of water flow is called a vortex. In addition, there are also interpretations of small round pits and wine vortices. So, what exactly is the "vortex" mentioned in meteorology?

In weather forecasts and weather news, the most common terms are "low vortex" and "cold vortex". Let's talk about low vortex first. Low vortex is a counterclockwise vortex in the Northern Hemisphere, usually existing at an altitude of 1,500 to 3,000 meters above the ground/ocean surface. Depending on the source of its generation and the area of ​​influence, low vortexes include southwest low vortex, plateau low vortex, East Asian low vortex, northwest low vortex, North China low vortex, etc. Among them, the most representative one should be the southwest low vortex, referred to as the southwest vortex.

Studies have shown that the Southwest Vortex is one of the main causes of heavy rain in the second half of the year. In my country's weather forecast, in terms of the intensity, frequency and range of the heavy rain it causes, it can be said that it is the second largest heavy rain weather system in my country after typhoons! Many rare and severe flood disasters in my country's history are closely related to the activities of the Southwest Vortex. What kind of a tough guy is the Southwest Vortex? Why is it so powerful?

The Southwest Vortex is a mesoscale closed low-pressure system with cyclonic (counterclockwise) circulation characteristics, formed on the 700 hPa isobaric surface in southwest my country due to the interaction between the special topography of the Qinghai-Tibet Plateau and the atmospheric circulation. It is not large, with a diameter of 300-400 kilometers.

Southwest vortices appear throughout the year, with the most in summer, followed by spring and autumn, and the least in winter. After the low vortex is formed, most of them will weaken and die out at the source. About 20% of the southwest vortices mainly move eastward along the Yangtze River basin into the sea; others move along the northeast and southeast paths.

Although the southwest vortex is generated in the southwest region, its influence range far exceeds its source. Studies have shown that the basin-wide flood in the Yangtze River Basin in the summer of 1998 was closely related to the abnormal activity of the southwest vortex. The southwest vortex process that occurred from June to August 1998 was more than usual (21 cases), and the proportion of developing and eastward-moving low vortices was significantly higher (more than 70%); among these 21 southwest vortex processes, 19 cases caused regional heavy rains in the upper reaches of the Yangtze River.

In addition, in early July 1935, the eastward movement of the southwest vortex caused heavy rainstorms in the Yangtze River Basin. In early August 1963, the heavy rainstorms in Hebei were the result of the interaction between cold air and the northward southwest vortex. In August 1981, under the influence of the southwest vortex, large-scale heavy rainfall occurred in the eastern part of the northwest region, causing serious losses.

Cold vortex

Cold vortex, as the name implies, is a cold low vortex, which is a cold closed low-pressure circulation system that mainly exists in the middle and upper layers of the troposphere (about 5,000 meters) in mid- and high-latitude regions. The temperature in the center is significantly lower than the surrounding area. "Vortex" in the northern hemisphere means that the high-altitude air flow rotates counterclockwise; "cold" means that it is a vortex carrying cold air.

Similarly, the cold vortices that affect our country also vary depending on their places of generation. There are Northeast cold vortex, North China cold vortex, Mongolian cold vortex... Among them, the Northeast cold vortex is the most common.

The Northeast Cold Vortex can be seen all year round and is most active in summer. Its life cycle lasts an average of 3-4 days and can be as long as 19 days.

Generally speaking, the high-altitude temperature of the Northeast Cold Vortex system is relatively low. The cold vortex can guide the dry and cold air in the middle and high latitudes to move southward. When there is warming in the lower layer (warm advection, solar radiation, etc.), unstable stratification of "dry on top and wet on bottom" and "cold on top and warm on bottom" is often formed, which is prone to produce severe convective weather such as heavy rain, hail, thunderstorms and even tornadoes. In many cases, the severe convective weather caused by this unstable stratification is not large in scale, but the energy is intense. Just like a pot of boiling water, it is difficult to judge which bubble will "burst" first.

Statistics show that in the Northeast, the most thunderstorms are caused by the Northeast Cold Vortex. In addition, 64% of squall lines are related to the cold vortex process. The cold vortex-induced hail accounts for nearly half of the total hail days in the Northeast.

In June this year, the unusually frequent Northeast Cold Vortex brought rainfall to Northeast China and other places, while also keeping the temperature in Northeast China low. The heavy rainfall and flood disasters in Heilongjiang Province were the result of the long-term maintenance of the Northeast Cold Vortex. This caused losses to agricultural production in many areas along the mainstream of the Heilongjiang River.

Like the southwest vortex, the northeast cold vortex will not only affect the northeast region, but can also combine with other weather systems to affect peripheral areas. The peripheral forces of the northeast cold vortex can reach North China, Central China, East China and other places.

On July 2 this year, a tornado hit Keyouzhong Banner, Xing'an League, Inner Mongolia, injuring 30 people and destroying more than 100 tents; on June 17, a tornado hit Shangyi County, Zhangjiakou City, Hebei Province, breaking off some trees and electric poles, and damaging some houses and walls... These are all related to the cold air "thrown out" by the northeastern cold vortex system.

In North China, the multiple strong winds in the first half of this year and the rainfall in June and July were also the result of the northeastern cold vortex making its presence felt.