What are the causes and areas of continuous rainy weather?

The cold weather in early spring or late autumn in China lasts for several days or even months, with continuous rain and few sunshine, also known as low temperature and continuous rain. From February to April all the year round, the early rice sowing and seedling raising season has entered from South China to the middle and lower reaches of the Yangtze River. At this time, winter fashion has a certain intensity, and cold air activities are more frequent. When the cold air from the south reaches Jiangnan and South China, especially when the main cold air from the east of China meets the warm and humid air from the tropical ocean, it often forms low temperature and rainy weather at the intersection of the two, causing early rice seedlings to rot and die, which is harmful to agricultural production such as spring sowing. The continuous low-temperature rain is different from the precipitation in the late spring and the first flood season in South China, and it is also different from the plum rain in the Jianghuai basin in early summer. Although the latter two phenomena can also be called continuous rain, they have high temperature, high humidity and heavy rainfall; The former is characterized by low temperature, less sunshine and less rainfall. Even rainy days in autumn, if they appear earlier, will also affect the harvest of crops such as late rice.

In spring, the warm and humid airflow in the south began to be active, and the cold air in the north began to decay, but it still had a certain intensity and was active frequently. The intersection of cold and warm air (that is, the front) often stagnates or lingers between the Yangtze River and South China. A quasi-static front appears on the ground weather map, and an east-west shear line appears on the 700 HPA isobaric map, which is located on the north side of the ground quasi-static front. There is continuous rainy weather between the ground front and the shear line on the 700 HPA isobaric surface. When the front and shear line are located in the south, there is continuous rain in South China. When going north, it appeared in the Jiangnan area between the Yangtze River and Nanling Mountains. Continuous rain in autumn occurs when the cold air in the north becomes active and the warm and humid air in the south begins to decay, but it still has a certain intensity. The process is similar to that in spring, but the area where cold and warm air meet is different, so even the rainy area is different from that in spring.