Typhoon Gemi lands strongly! How should different regions focus on prevention in the "seven down and eight up" period of "catalysis"?

Recently, we have entered the most critical period of flood prevention, which is from the end of July to the beginning of August. This period is also the period of active typhoons. The successive arrival of this year's third typhoon "Gemi" and the fourth typhoon "Pabion" has brought the risk of heavy rain.

There is an idiom called "七上八下", which means a feeling of panic and confusion, and is often used to describe a scattered or chaotic state. Why did meteorologists "create" another "七下八上"?

01 What is “Seven down and eight up”?

In meteorology, "the end of July and the beginning of August" refers to the period from July 16 to August 15 of the Gregorian calendar every year. It is the period when precipitation is most concentrated in northern China, and is also known as the rainy season or peak flood season in these areas. Especially in North China and Northeast China , precipitation is most concentrated from the second half of July to the first half of August of the Gregorian calendar every year . According to the flood control standards for the rainy season, this period can sometimes be extended to the second half of mid-July to the second half of mid-August , about one month. Because of this, "the end of July and the beginning of August" has become the most critical period of China's flood season.

Stormy sky (copyright image from the gallery, reprinting may cause copyright disputes)

02Seven characteristics of precipitation in the period from July to August

First, the precipitation is concentrated . During the period from the end of July to the beginning of August, the precipitation in northern China increased significantly, and the precipitation accounted for a large proportion of the annual precipitation, especially in North China and Northeast China. The contribution of precipitation in this period to the annual precipitation was particularly significant.

Second, the intensity is high . Heavy rains occur frequently with high precipitation intensity. Short-term heavy precipitation may cause rapid rise in water levels, saturation of reservoirs, and surge in river flows .

Third, it is highly localized . Precipitation is unevenly distributed, and localized heavy precipitation often occurs, that is, the precipitation in some areas far exceeds that in other areas.

Fourth, there are large inter-annual variations . The precipitation in the "late July and early August" period varies significantly between different years, with some years having more precipitation and some years having less.

Fifth, it is accompanied by severe convective weather . During the precipitation process, it is often accompanied by severe convective weather such as lightning, strong winds, and hail . These weather phenomena further increase the disaster risk during the period of "the end of July and the beginning of August".

Sixth, there is more rainfall in the north and less in the south . From a national perspective, the precipitation during the period from the end of July to the beginning of August generally showed a spatial distribution characteristic of "more rainfall in the north and less rainfall in the south". The northern regions, especially North China, Huanghuai, and Northeast China , had more rainfall, while the southern regions had relatively less rainfall.

Seventh, the location of the main rain belt will change periodically . In mid-to-late July, the main rain belt is often located between the Yangtze River and the Yellow River ; from late July to early August, the main rain belt will move northward to the Huanghuai area, North China, and the central and southern parts of Northeast China .

The Central Meteorological Observatory released a medium-term forecast on July 25, saying that precipitation will mainly be concentrated in North China, Northeast China, South China, and East China, and the heavy rain warning level in these areas is high (Source: Central Meteorological Observatory)

03 Why is it rainy in the lower part of July and the upper part of August?

The rainy climate characteristic of "July-August" is formed by the joint action, interweaving and mutual influence of multiple factors.

Around mid-July each year, the subtropical high pressure in the western Pacific Ocean begins to move northward, reaching its northernmost position in August. During this process, a warm and humid air current surrounds the western periphery of the subtropical high pressure, which continuously transports water vapor from the ocean surface to the land, providing good water vapor conditions for heavy rainfall in North China and Northeast China. When this warm and humid air current meets the cold air moving eastward and southward, the water vapor condenses and releases latent heat, forming heavy and continuous rainfall.

Around late July, the northern edge of the summer monsoon advanced to North China, further intensifying precipitation in the region.

Topography also affects precipitation. For example, the Taihang Mountains and other terrains force water vapor and warm and humid air to rise, which can also increase the intensity of precipitation. These mountains act as a "barrier" to water vapor in summer, causing water vapor to condense into clouds and cause rain during the lifting process.

The "July-August" period is also a period of active typhoons in the northwest Pacific , and there are precedents in history that typhoons "visit" North China during this period. Typhoons carry a lot of water vapor, and once they interact with the weather system in North China, they often bring significant increases in precipitation.

The temperature of the ground and the lower atmosphere is high, while the upper atmosphere is relatively cold. This cold upper and warm lower atmospheric stratification makes the atmosphere unstable. Especially in the afternoon and evening , local thermal convection develops vigorously, and short-term thunderstorms, gale and hail are prone to occur.

In recent years, the location of my country's main summer rain belt has experienced interdecadal changes. In some periods, the main rain belt will advance northward to North China, resulting in more precipitation in the region. This interdecadal climate change background also provides favorable conditions for more rain in the "last seven days and the first eight days".

The Central Meteorological Observatory issued a high temperature forecast on July 25. Except for the Qinghai-Tibet Plateau, the temperature in areas without precipitation is relatively high (Source: Central Meteorological Observatory)

04 "July down and August up" prevention focus

Due to the concentration and intensity of precipitation, floods and secondary disasters are prone to occur during this period, such as urban waterlogging, mountain torrents, landslides, mud-rock flows , etc., posing a threat to people's lives and property. Since China has a vast territory and the precipitation in the period from the end of July to the beginning of August is mainly concentrated in the northern region, the methods and priorities for disaster prevention in different regions during the period from the end of July to the beginning of August are also different.

(1) Prevention focus in inland areas : Due to the concentration and large amount of precipitation, it is easy to cause the water level of rivers, lakes and other water bodies to rise, even exceeding the warning line, forming flood disasters, which will submerge farmland, houses, roads, etc. Severe convective weather such as lightning, strong winds, and hail are characterized by strong suddenness and destructive power, and are likely to cause disasters such as electric shock, house collapse, and crop damage. Secondary disasters such as urban waterlogging, mountain torrents, landslides, collapses, and mud-rock flows caused by heavy rainfall are often more harmful than direct disasters, because they often occur near densely populated areas or major traffic routes, posing a serious threat to the safety of people's lives and property.

The meteorological warning map for flash flood disasters jointly issued by the Ministry of Water Resources and the China Meteorological Administration on July 24 (Source: China Meteorological Administration)

(2) Coastal areas should focus on prevention: This period is the most active period for typhoons in the Northwest Pacific and South China Sea. The strong winds and heavy rains brought by typhoons will pose a major threat to the safety of life and property in coastal areas. Large amounts of precipitation in a short period of time can easily paralyze urban drainage systems and cause waterlogging . Storm surges are abnormal rises and falls of sea water caused by typhoons or strong winds, which can flood low-lying coastal areas and pose a threat to the safety of life and property of coastal residents.

The Central Meteorological Observatory released a precipitation forecast on July 25, with precipitation mainly concentrated in North China, Northeast China and the southeastern coastal areas affected by the typhoon (Source: Central Meteorological Observatory)

♦Key points for tourists to take precautions : The end of July and the beginning of August are the summer vacation, and many people choose to travel. For them:

(1) Pay attention to weather forecasts and avoid traveling during heavy rain, especially avoid traveling to mountainous areas, river valleys , and other areas prone to disasters such as flash floods, mud and rock flows, and landslides .

(2) Avoid staying in open areas, under large trees, next to tall towers, and other places prone to lightning strikes, and promptly go to a safe room to avoid rain and lightning.

(3) Take precautions against the impact of strong winds on tourism activities, such as avoiding outdoor activities at the seaside, on mountaintops , and other places with strong winds.

(4) Avoid swimming or other water activities in waters where the water conditions are unknown to prevent drowning accidents. Do not risk wading through flooded roads or bridges to ensure your own safety.

(5) Pay attention to driving safety and avoid driving on roads with deep water to prevent the vehicle from stalling or other traffic accidents. When driving on mountain roads, pay attention to prevent disasters such as landslides and falling rocks to ensure driving safety.

The geological disaster warning map jointly released by the Ministry of Natural Resources and the China Meteorological Administration on July 24 (Source: China Meteorological Administration)

The period from the end of July to the beginning of August is a critical period for flood prevention, and the most important thing is to prevent all kinds of disasters. Taking scientific and reasonable preventive measures can ensure the safety of our lives and property.

Author: Wang Yuanhong, Senior Weather and Climate Engineer

Produced by: Science Popularization China