Physical mechanism of sandstorm weather in Beijing

The weather situation favorable to strong winds or strong winds, favorable distribution of dust sources and favorable air instability conditions are the main reasons for the formation of sandstorms or strong sandstorms. Strong wind is the driving force of sandstorm, and the source of sandstorm is the material basis of sandstorm. Unstable thermal conditions are conducive to the increase of wind power and the development of strong convection, thus carrying more dust and winding it higher.

In addition, drought and lack of rain in the early stage, warming weather and rising temperature are the special weather and climate background for the formation of sandstorms; The convective cells before the ground cold front develop into clouds or squall lines, which is a small and medium-sized system conducive to the development and strengthening of sandstorms. The topographic condition conducive to the increase of wind speed, that is, the narrow tube effect, is one of the favorable conditions for the formation of sandstorms.

The main component of soil and yellow sand is silicate. When there is less rain and the temperature gets warmer, the silicic acid on the silicate surface loses water.

H2SiO4=SiO3 -2+H2O (gas)

In this way, the surface of silicate soil micelles and sand particles will be negatively charged and repel each other, becoming aerosols that cannot be aggregated together, thus forming sand blowing, that is, sandstorms. Sandstorms are essentially silicate aerosols with negative charges.

Physical mechanism of sandstorm formation

Under the favorable conditions of large-scale environment, high-altitude dry and cold jet, strong vertical wind speed, wind direction shear and thermal instability stratification, the generation and development of small and medium-scale systems near the front area intensified the pressure and temperature gradient before and after the front area, forming a huge pressure and temperature gradient before and after the front area. Under the joint action of momentum downward transfer and gradient deviation wind, the wind speed in the near-surface layer rises sharply, causing dust on the surface and forming sandstorm or strong sandstorm weather.

Main failure modes of sandstorm

⑴ Strong wind: Strong wind accompanied by dust damages buildings and public facilities, causing human and animal casualties.

⑵ Sand burial: Farmland, channels, cottages, railways, grasslands, etc. are buried by a large number of quicksand in the form of quicksand, especially posing a serious threat to transportation.

⑶ Soil wind erosion: The dust source and affected area of each sandstorm will be harmed by wind erosion in different degrees, and the depth of wind erosion can reach 1 ~ 10 cm. It is estimated that the annual loss of soil fine matter caused by sandstorms in China is as high as 106 ~ 107 tons, most of which are below 10 micron, which seriously damages the land productivity of farmland and grassland in the source area.

⑷ Air pollution: In the source area and influence area of sandstorm, the inhalable particulate matter (TSP) in the atmosphere increases, and air pollution intensifies. Taking 1993 "5.5" sandstorm as an example, the outdoor air TSP concentration in Jinchang, Gansu Province reached 10 16 mg/m3, and the indoor air TSP concentration was 80 mg/m3, exceeding the standard by 40 times. From March to April, 2000, Beijing was affected by sandstorms, and the number of days when the air pollution index reached Grade 4 or above was 10, which also affected many cities in eastern China. From March 24th to 30th, the daily pollution index of Nanjing, Hangzhou and other cities 18 exceeded Grade IV.

The harm of black wind

The harm of black wind mainly has two words, one is wind and the other is sand.

There are two hazards of strong winds: one is wind damage, and the other is scraping the ground.

Let's talk about the typhoon first. Strong winds destroy buildings, blow down or pull up tree stems, tear up farmers' plastic greenhouses and farmland plastic films, and so on. In addition, April and May in northwest China are the time when cash crops such as melons, fruits, vegetables, beets and cotton emerge, cotyledons or true leaves grow, and fruit trees bloom, at this time, they are least resistant to sandstorms. Light leaves are covered with dust, which weakens photosynthesis, affects respiration and reduces crop yield; In the worst case, the seedlings will die, not to mention the mature results. For example,1May 5, 1993, black wind in northwest China knocked out 85,000 stamens of fruit trees, and shelter forests and timber forests were broken or uprooted 109400. In addition, the strong wind blew down the telephone poles, causing water and electricity cuts and affecting industrial and agricultural production. 1On May 5, 993, the black wind caused power failure and water cut, and Jinchang Jinchuan Company alone caused economic losses of 83 million yuan.

When strong wind acts on loose soil in arid areas, it will scrape off a layer of topsoil, which is called wind erosion. For example,1May 5, 1993, the average wind erosion depth of black wind 10 cm (up to 50 cm), that is, an average of 60 to 70 cubic meters of fertile topsoil per mu was blown away by the wind. In fact, strong winds not only blow away the fine clay and organic matter in the soil, but also pile up the sand in the soil, which greatly reduces the fertility of the soil. In addition, the strong wind with sand will wear off a layer of buildings and crops, which is called abrasion and also a disaster.

The harm of sand is mainly sand burial. As mentioned above, in narrow pipelines, windward and uplifted terrain, due to high wind speed, the wind-blown sand hazard is mainly wind erosion, while in leeward depressions and other terrain with low wind speed, the wind-blown sand hazard is mainly sand burial. Such as1where sand burial occurred in the black wind on May 5, 993, the average thickness of sand burial was 20 cm, and the thickest part reached1.2 m.

More importantly, the loss of human life. For example,1May 5, 993, 85 people were killed, 264 people were injured and 3 1 person was missing. In addition, the death loss of large livestock was 1.2 million, the crops were affected by 5.6 million mu, the total length of the lifeline canal buried by sand in arid areas was more than 2,000 kilometers, and the Lanzhou-Xinjiang railway was stopped for 3 1 hour. The total economic loss exceeded 540 million yuan.