Causes of Summer Rainstorm in Guanzhong Plain

Analysis of rainstorm in Guanzhong plain

Expert explanation

I. General information of Guanzhong Plain:

Guanzhong refers to the Weihe River alluvial plain at the northern foot of the Qinling Mountains in Shaanxi Province (around the Weihe River Basin), with an average elevation of about 500 meters, so it is called Weihe Plain and Guanzhong Plain. Its northern part is the Loess Plateau in northern Shaanxi, and its southern part is the southern Shaanxi basin and Qinba Mountain area. Shaanxi is an area with developed industry and agriculture, dense population and rich land, so it is known as "Qinchuan in 800 Li".

Central Plain of Shaanxi Province, China. Also known as Weihe Plain and Guanzhong Basin. Baoji in the west, Tongguan in the east, Qinling in the south and Beishan in the Weihe River in the north. It is narrow in the west and wide in the east, about 300 kilometers long from east to west and about 400 meters above sea level. The terrain is high in the west and low in the east, with a drop of 340 meters at the riverbed of the Yellow River. It is formed by fault depression, commonly known as Weihe Graben, and alluvial into plain through Weihe River, Jinghe River and Luohe River. With flat terrain, fertile soil and warm climate, irrigated agriculture has been famous since ancient times and is known as "Qinchuan in 800 Li". Rich in wheat and cotton. There are large and medium-sized cities such as Xi, Xianyang and Baoji. There are frequent earthquakes in Guanzhong in history, and the earthquake in Huaxian County reached 8 on the Richter scale in 1556.

Guanzhong basin is bordered by Qinling Mountain in the south and Beishan Mountain in the north. Between the northern Shaanxi Plateau and the Qinling Mountains. From Baoji Gorge in the west to Tongguan Port in the east, it is about 360 kilometers long from east to west, narrow in the west and wide in the east. The total area is 39,064.5 square kilometers. Guanzhong basin is formed by river alluvial and loess accumulation, with flat terrain, fertile soil, rich water resources and good mechanical farming and irrigation conditions. It is the area with the best natural conditions in Shaanxi, known as "Qinchuan in 800 Li". The basic landform types are river terraces and loess tablelands. The Weihe River passes through the basin and flows into the Yellow River. The waterway is low and flat, with an altitude of 326-600 meters. From the Weihe River to the south, north and south, the terrain is asymmetrical and stepped, from the alluvial terrace of the first and second rivers to the first or second loess plateau 200 ~ 500 meters higher than the Weihe River. Terraces are distributed continuously on the north bank and incomplete on the south bank. The main tributaries of Weihe River also have corresponding multilevel terraces. The broad terrace plain is the most fertile area in Guanzhong. Between the second terrace on the north bank of Weihe River and the northern Shaanxi Plateau, there is a loess tableland in Weibei extending from east to west. The plateau is wide, with an altitude of 460 ~ 800 meters, and it is the main grain-producing area in Guanzhong. The loess tableland on the south side of Weihe River is intermittently distributed, about 250 ~ 400 meters higher than Weihe River, and it is stepped or inclined shield-shaped. It slopes gently from the northern foot of Qinling Mountains to Weihe Plain, such as Wuzhangyuan in Qishan, Shenheyuan, Shaolingyuan and Bailuyuan in the south of Xi, Yangguoyuan in Weinan, Gao Yuanyuan in Hua County and Yuyuan Garden in Huayin. At present, they have developed into a comprehensive agricultural area dominated by forests and gardens. Also known as Weihe Plain or Weihe Basin. It is a graben structural plain. Located in the middle of Shaanxi Province, between Qinling Mountain and Weibei Mountain (Laolongshan Mountain, Guishan Mountain, Wangyaoshan Mountain, etc.). ). It starts from Baoji in the west and reaches Tongguan in the east, with an altitude of about 325-800 meters and a length of about 300 kilometers. The width varies from north to south, with the widest in the east reaching100km, about 75km near Xi 'an and only 20km around Meixian. It gradually shrinks into a canyon in Baoji, which looks like a "new moon". It covers an area of about 34,000 square kilometers. It was called "Guanzhong" in ancient times because it was between Hangu Pass and Big Three Pass (one said it was between Hangu Pass, Big Three Pass, Wuguan Pass and Xiaoguan Pass). During the Spring and Autumn Period and the Warring States Period, it was the hometown of Qin State, known as "Qinchuan in 800 Li". The Guanzhong basin, sandwiched between the northern Shaanxi Plateau and the Qinling Mountains, is a giant rift zone formed during the Himalayan movement. There are high-angle normal faults on both sides of the basin. There are a series of springs and hot springs exposed along the fault line. The mountains on the north and south sides rise along the fault line, and the basin slowly descends, forming a graben-type structural plain. Central Shaanxi plain

After the formation of Guanzhong Plain, not only loess accumulated in it, but more importantly, Weihe River and its tributaries on both sides carried a lot of sediment to fill and deposit it, and it was loosely deposited in Quaternary, with a maximum thickness of more than 7000 meters. Due to intermittent crustal changes and river undercutting, terraces with different heights have been formed.

Second, the cause of heavy rain in Guanzhong Plain:

Weihe Plain has a continental climate, with an average annual temperature of 6 ~ 13℃ and an annual precipitation of 500 ~ 800 mm, of which 60% is from June to September, with short-term rainstorm, little precipitation in winter and spring, and frequent droughts in spring and summer.

Rainstorm: a heavy precipitation weather with 24-hour rainfall greater than or equal to 50 mm, 6-hour rainfall greater than or equal to 30 mm and 1 hour rainfall greater than or equal to 16 mm. ..

1. If the warm and humid airflow and cold air outside the subtropical high meet violently in July, August and September in central Shaanxi in summer and autumn, a large-scale disastrous rainstorm will be formed;

This kind of rainstorm weather may happen every year in Guanzhong Plain.

2. Sometimes it appears in convective cloud system or single convective cloud block, and belongs to mesoscale weather system-strong convective weather in meteorology. The horizontal scale of this weather is generally within 200 kilometers, and some are only a few kilometers. It is a disastrous weather with short duration, bad weather and strong destructiveness in meteorological disasters.

Strong convective weather is the accumulation of more water vapor on the ground. Because the specific gravity of water vapor is lighter than that of air, the specific gravity of hot and humid air mixed with air will be lighter than that of air, and the hot and humid air will float up to form a rising hot and humid airflow. When it rises to a certain height, due to the temperature drop, water vapor will condense into small water droplets to form clouds. At this time, its volume will soon be reduced to11244 of the original volume, forming a low pressure area. At this time, the area will attract air from all directions to supplement. Due to constant attraction, the hot and humid air on the ground will rotate and rise. Due to the centripetal force of rotating air, it will more strongly attract the cold air above and the hot and humid air below to meet, thus forming.

The Guanzhong Plain is prone to strong convective weather, but the scope is small and the impact is not too heavy.

3. The rainstorm brought by typhoon.

Typhoons rarely appear in Guanzhong Plain, and only typhoon "Dai Wen" will bring heavy rain when 1956 passes through Shaanxi.

This kind of rainstorm is rare in Guanzhong Plain for decades.

Three. History of rainstorm in Shaanxi;

Areas with frequent rainstorms every year in Shaanxi: Qinling Mountains in the south-central part of the province, Hanzhong, Ankang and Baoji, and mountainous areas in the south of Shangluo. Among them, southern Shaanxi is a rare rainstorm-prone area in China.

In Shaanxi, the rainstorm usually occurs in 6.7.8.9 month in southern Shaanxi, July, August and September in Guanzhong Plain and September in northern Shaanxi, 10. And it has the characteristics of "seven ups and eight downs".

Especially in July of 20 10, the intensity, scope, duration, casualties, property losses and geological disasters of heavy rain in Shaanxi are extremely rare for decades.

Data: Based on the collection, statistics and analysis of the historical data of Guanzhong Plain in recent 200 years, the grade sequence, annual change, seasonal change, spatial change and the causes of flood disasters in Guanzhong Plain during this period were studied. The results show that there were 8/kloc-0 floods in Guanzhong Plain during this period, with an average of/kloc-0 floods every 2.53 years. Since the middle of the Republic of China, the frequency of floods in this area has obviously increased. The flood disasters in this area are mainly secondary (moderate waterlogging), with 47 occurrences, accounting for 58% of the total flood disasters; Three-level (catastrophic flood) occurred 24 times, accounting for 29.6% of the total flood disaster; Grade I (mild local waterlogging) and Grade IV (extreme waterlogging) occurred 7 times and 3 times respectively, accounting for 8.6% and 3.8% of the total flood disasters respectively. In recent 200 years, the occurrence of flood disasters in Guanzhong Plain has obvious temporal and spatial differences. The flood disasters are mainly concentrated in Xi 'an, Lantian, Zhouzhi, Weinan, hua county, Dali and Huayin in the middle and lower reaches of Weihe River, Tongchuan in the lower reaches of Luohe River and Baoji in the lower reaches of He Qian. In recent 200 years, the occurrence of flood disasters in Guanzhong Plain has obvious seasonal differences, mostly concentrated in July to September in summer and autumn. In addition, the Guanzhong Plain is flat, the soil is loose, and the surface vegetation is scarce. The same precipitation is more likely to cause floods in this plain.

Detailed explanation (end)

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