Heavy rain and drought in the north in July and August, and the causes of plum rain in June

Causes of heavy rains:

The process of forming heavy rains is quite complicated. Generally speaking, from the perspective of macroscopic physical conditions, the main physical conditions that produce heavy rains are sufficient and continuous water vapor, strong and Persistent upward air motion and structural instability of the atmosphere. Favorable combinations of weather systems of various sizes, large, medium and small, and underlying surfaces, especially terrain, can produce heavy rainstorms. Weather systems causing widespread heavy rains in China mainly include fronts, cyclones, shear lines, vortices, troughs, typhoons, easterly waves, and the Intertropical Convergence Zone. In addition, thermal thundershowers can also cause short-duration, small-area heavy rainstorms in arid and semi-arid local areas. Heavy rain often falls from cumulonimbus clouds. The conditions for the formation of cumulonimbus clouds are that the atmosphere must contain sufficient water vapor and have a strong upward movement to transport the water vapor upward quickly. The water droplets in the cloud will continue to grow under the influence of the upward movement until the updraft cannot support it. Falling sharply to the ground.

Causes of drought:

Roughly from mid-July to mid-August. At this time, the Meiyu stationary front has moved to the middle and lower reaches of the Yellow River and the Northeast in early July. The middle and lower reaches of the Yangtze River are controlled by the "subtropical high", forming anticyclonic weather, dominated by downdrafts, with long days, strong solar radiation, and low temperatures. High, evaporation is strong. Crops also grow quickly and farmland requires a lot of water. However, due to the single air mass, except for thunderstorms in local areas, there are no large rainy areas. Dry and hot weather generally occurs, so it is called "Fu Drought". During this season, afternoon temperatures in the middle and lower reaches of the Yangtze River generally reach 33 to 35°C, and high temperatures as high as 43 to 45°C are recorded in some places. It mainly occurs in the Yangtze River Basin and Jiangnan region of China, especially in Hubei, Hunan, Jiangxi, Jiangsu, Anhui and other provinces. When the subtropical high in the western Pacific is controlled and there is little typhoon activity, severe drought is likely to occur and precipitation is significantly less than the multi-year average. Generally from mid-July to mid-August. (Futian begins on the third Geng Day after the summer solstice, approximately 10 days after July 12 every year.)

Causes of Meiyu:

①In high latitudes in Asia There is blocking high pressure or stable high pressure ridge in the middle of the troposphere, and the atmospheric circulation is relatively stable and less changeable;

② The westerly circulation in the mid-latitudes is straight, and frequent shortwave activities provide cold air conditions for the Jianghuai region;

< p>③The western Pacific subtropical high has an obvious westward and northward jump process. The 500hPa subtropical high ridge is stable between 20 degrees and 25 degrees north latitude, and warm and moist air flows from the edge of the subtropical high to the Jianghuai basin. Under such circulation conditions, the Meiyu front lingers in the Jianghuai basin, often accompanied by southwest vortices and shear lines, and mesoscale systems are active above the Meiyu front. It not only maintains continuous rainfall during the Meiyu period, but also provides abundant water vapor for heavy rains.