Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: Earth knowledge Bureau (ID:diqiuzhishiju), author: Feng Yun Meng Yuan, revised manuscript: gu Hanying / Editor: Fruit Chestnut

Since the beginning of this year, there have been 9 sand and dust weather processes in China, which is 3 times more than that in the same period of the usual year (6), but also the most in the past 10 years. since the beginning of this century, it is only less than 2001 (12), 2002 (10) and 2004 (10).

The current number of sand and dust weather this year

It's already more than in some years, ▼.

Among them, the first sand and dust weather process this year appeared on January 12, which was 32 days earlier than that of the usual year. Since March, there have been six sand and dust weather processes in China, one more than that in the same period of the year (5).

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Why is the dust weather so active this year?

The formation condition of sand-dust weather sandstorm is a kind of strong sand-dust weather, that is, strong winds bring sand particles and dust mats into the air, resulting in a significant decrease in air turbidity and visibility.

Generally speaking, when the horizontal visibility caused by a large amount of sand dust is less than 1 km and the wind speed reaches force 6 or above, it can be called a sandstorm; while the strong wind and dust weather with relatively weak horizontal visibility of more than 1 km is called sand blowing; the dust weather with weak wind speed (less than 3 m / s) and suspended dust is called floating dust. These three types of weather phenomena can be collectively referred to as dust weather.

Sandstorm grade ▼

In order to form a sandstorm, three conditions need to be met: the dry and loose sandy material as the sand source, the sand lifting condition to carry the dust away from the earth, and the unstable atmospheric structure (or ascending condition) that can make the dust soar.

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Dry and loose sandy material is the material basis of sandstorm. The sand sources that affect the sand and dust weather in China come from a large area of deserts in China and neighboring countries, including the Taklimakan Desert and the Badain Jaran Desert in China, as well as the Gobi Desert in Mongolia and the wastelands of Central Asia.

The "ammunition" of the sandstorm can be said to be very abundant, ▼.

These huge desert areas have a lot of dry dust, just waiting for the follow-up dynamic conditions to soar up.

The condition of sand lifting requires a strong enough wind. The sand source affecting the sand and dust weather in China is located in the inland region of the middle latitudes of Asia, near the cold air source of Siberia (with the corresponding Siberian high). In autumn, winter and spring, there is often a strong cold high pressure carrying cold air southward; at this time, the strong wind area outside the cold high pressure becomes the key to the yellow sand flying.

The strong wind roared in the yellow sand

(April 19 Surface barometer: central Meteorological Observatory) ▼

(photo: central Meteorological Station) ▼

Of course, if you encounter a high-altitude trough moving eastward, the cyclone in front of the trough will stimulate strong winds to a certain extent, and if the cyclone cooperates with cold high pressure, a larger pressure gradient will cause more violent winds (on the west side of the cyclone). This situation tends to cause more violent sand.

If there are only these two steps, we can only get a "mixture" of wind and sand, and there is no way to "affectionately circle the horizon." once the surface wind decreases slightly, it will soon fall to the ground because of friction close to the surface. In order to cause strong dust weather, a third condition is needed: unstable atmospheric structure (or ascending motion conditions).

To put it simply, it requires that the atmosphere is cold and warm in the upper and lower parts in the vertical direction, so that the air below is lighter and easier to shake up; the high-altitude wind is faster and less affected by ground friction, so the dust entering the upper air is enough to fly thousands of miles.

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Of course, in addition to this thermal condition, strong dynamic conditions can also be provided directly by the weather system-for example, the cyclone mentioned in the previous paragraph can itself bring about a wide range of convergence and ascending motion toward the center of the cyclone, which can have a similar effect.

Therefore, according to the analysis of the above conditions, spring is the most frequent period of sand and dust weather-when the thin snow in winter in the desert dissipates, the fixing capacity of sandy soil is the weakest and sand is the easiest to rise; in spring, the desert surface heats up quickly and heats up the atmosphere near the surface, which is easy to cause unstable stratification of upper and lower warm, which is easy to make a large amount of sand and dust rise.

There are multiple reasons for the occurrence of sandstorms, ▼

In summer, with the withdrawal of cold air, there is a lack of strong wind conditions; and although autumn is a period of intense exchange of cold and warm air, the ground cools rapidly, unable to form unstable stratification and ascending movement of upper cold and warm lower; although cold air is often active in winter, but the surface freezing is not easy to cause sand.

Therefore, the sand and dust weather is relatively rare in summer, autumn and winter, which is also reflected in the observation over the years.

Sandstorm has become the standard ▼ in spring in the north.

Sandstorm does not require a visa. We can apply the above points to the analysis of severe dust weather from April 9 to 14 this year.

The sandy weather from April 9 to 14 indicates ▼

Among them, the most direct source of sand-raising power is a significantly developing Mongolian cyclone, which is induced by a short-wave trough in the upper westerly belt at low altitude.

(April 9 Surface barometer: central Meteorological Observatory) ▼

The meteorological conditions at that time were very suitable for the development of this Mongolian cyclone: first, the local and upper air temperature in the Mongolian Plateau was higher in the early stage, while the temperature in Siberia in the north was obviously lower, and the great temperature gradient contained the effective potential energy to promote the development of the cyclone.

You can see that this small cyclone forms ▼ at the junction of high and low temperatures.

On the other hand, on the terrain, the cyclone is generally in a state of downhill in the process of eastward movement, which will also promote the development and enhancement of the cyclone according to the principle of conservation of local vorticity.

The circulation of this Mongolian cyclone continues to expand in the process of development. Because the path is northward, the first sand wall path carried by it is also northward, which mainly affects the northeast region on April 10.

Only one day later, it was blown to ▼ in the northeast.

Mongolia cyclone continues to strengthen

By the way, it carried the sand and dust to the ▼ in Northeast China.

However, the movement speed of the Mongolian cyclone slowed significantly until the early morning of April 11, when the large-scale uplift movement and strong northerly winds on the western side were maintained for a long time, and dust was poured into the southward North China Plain at night from April 10. so that it swept across the Central Plains and finally drifted across the river.

Take a day off after a day of travel

Pass the Central Plains Lower Jiangnan ▼ as soon as you have a chance

It can also be seen from the inversion images of meteorological satellites at night on April 8 and late at night on April 8 that the dust mainly originated in southwestern Mongolia and swept across the Gobi Desert on the border between China and Mongolia, and further enhanced after a large number of sand sources were obtained.

The flying cyclone and white spiral cloud system in the picture

It is the Mongolian cyclone that causes strong sand and dust weather.

The bright yellow on the south side of the cyclone center is sand and dust.

(figure: CIRA) ▼

The dust enters the Inner Mongolia Plateau and even the North China Plain with the wind, and finally the strongly developed Mongolian cyclone provides the key conditions for unstable stratification and ascending movement (and leads to significant frontal areas and gale areas in the middle and low altitude). Let the dust fly thousands of miles over the rivers and mountains.

Overall, Mongolia is the source of most of the dust, a small part of which may come from the Badain Jaran Desert in western Inner Mongolia. The sandstorm also had a significant impact on Mongolia. Such extreme weather never has national boundaries, and we need to deal with it together.

One earth, one home.

There is no difference between you and me in the face of extreme weather changes.

(figure: NASA) ▼

Why are sandstorms so frequent this spring? The above analysis is the analysis of short-term weather process conditions for a single event. If we want to analyze the causes of the overall excessive sand and dust this spring, it is necessary to analyze the longer-term climatic factors and the circulation response.

Three strong dust weather processes have occurred in Beijing since March.

Can the dust blow through in the end?

(shooting in the same place) ▼

We take the period of sand and dust concentration from March 16 to April 15 for analysis.

First of all, according to the anomaly that the potential height of the middle troposphere (500hPa isobaric surface) represented by the color below deviates from the mean value of the same period of the year, the height field anomaly in the high latitudes of Eurasia shows an obvious "+-+" structure, that is, a significant abnormal high pressure occurs from the Eastern European Plain to the Ural Mountains and the North Pacific. The Mongolian plateau along the Siberian Arctic Ocean and southwest of Lake Baikal is extremely low.

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From the corresponding relationship between the middle troposphere and the surface circulation system, we can see that there will be abnormal low pressure development on the surface directly below and to the east of the high-altitude abnormal low vortex on the Mongolian Plateau (the middle line of the figure is sea level pressure anomaly).

This long-term low pressure, like a background plate, will promote the Mongolian cyclone, which is also a low-pressure system, to be stronger.

At the same time, it will also strengthen the southwesterly winds on its southeastern side, sending warm air at lower latitudes to places such as the Mongolian Plateau, leading to significantly warmer local temperatures in early spring this year. Unusually high temperatures occurred in most of Mongolia and western Inner Mongolia, which led to the early melting of light snow and seasonal permafrost in the desert.

The cold air retreated northward earlier than in previous years

The dust in the desert "awakened" from the land earlier.

(photo: national Climate Center) ▼

On the snow monitoring map on March 18, it can be seen that there is no snow cover in the Gobi in the southern Mongolian Plateau, which melts earlier than in recent years. In the season when sparse vegetation is not yet growing, bare and unusually dry sand can easily become a source of sand.

Global mean temperature anomaly from February 15 to March 16 this year

Lay the foundation for rapid melting and sand removal of snow and seasonal permafrost

(photo: national Climate Center) ▼

Satellite monitoring map of snow cover on March 18, 2023

White areas represent snow.

You can see that there is no snow cover in the Gobi Desert in southern Mongolia.

(photo: satellite inversion of American Ice and Snow Center) ▼

What is the origin of this abnormal circulation situation?

Generally speaking, the middle and high latitudes of the Northern Hemisphere have two active centers, the North Pacific and the North Atlantic, in winter and spring, which not only have synoptic-scale processes that last several days, but also affect low-frequency processes that last longer (up to several months), longer wavelengths (thousands of kilometers) and slowly change, and affect the distance beyond the skyline through excited long atmospheric waves. This phenomenon is often called "teleconnection".

The most representative teleconnection is the Southern Oscillation.

The Southern Oscillation in the atmosphere is reflected in the ocean.

Is the El Ni ñ o phenomenon that we are familiar with.

(figure: NOAA Climate.gov) ▼

From the basic wind direction of the upper westerly belt, it can be seen that more low-frequency process disturbance sources start from the North Atlantic and spread along the Eurasian jet stream affecting eastern Asia.

In order to find the most direct relationship between the propagation of the long-wave disturbance and the upper and lower reaches of the westerly belt, we have made the flow function anomaly in the upper troposphere (300hPa isobaric surface, about 9000 meters above sea level) and the wave flux propagation represented by the arrow.

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It can be seen that there are three north-south anomaly centers in the North Atlantic, including the high latitudes of the Greenland-Norwegian Sea, the low pressure on the south of Greenland and Iceland and the high pressure on the western side of the Strait of Gibraltar.

The propagation of the arrowhead wave flux indicates that these long-term anomaly systems in the North Atlantic are the source of influence, although the mainstream continues to spread southward into Africa, but a small part of the wave flux in the Mediterranean Sea to the northeast to the Eastern Europe-Ural Mountains abnormal high, and further spread to the southeast, resulting in the formation of the abnormal low pressure center of the Mongolian Plateau.

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This means that the longer-term circulation anomaly in the North Atlantic region is one of the most important reasons for the frequent occurrence of sand and dust this year. Such circulation anomalies also correspond to the negative phase state of the North Atlantic Oscillation (NAO)-in fact, the NAO has been in a negative phase state for a long time since March.

The North Atlantic Oscillation is also a teleconnection.

Is the most prominent mode of the atmosphere in the North Atlantic.

(figure: NOAA) ▼

There are many reasons for the negative phase of NAO, but this year, the sea surface temperature in the high latitudes of the North Atlantic and the Arctic sea ice in the adjacent areas of the Atlantic Ocean (such as the Norwegian Sea and the Barents Sea) are two important climate factors.

They all promote the heat exchange between the sea surface and the atmosphere in the high latitudes of the North Atlantic, and let the local atmosphere get more heat and lift to form an abnormal high pressure, and finally affect the Mongolian Plateau and northern China by affecting the long-term atmospheric circulation anomaly in the North Atlantic. This is an important direct cause of the frequent occurrence of sand and dust this spring.

The reduction of Arctic sea ice

Finally, it directly led to the frequent occurrence of sand and dust in Mongolia.

The overall power of global climate change should not be underestimated.

(figure: National Snow and Ice Data Center) ▼

Finally, does shelterbelt (grass) really play little role in preventing sand and dust? The answer is obviously no.

First of all, the area where shelterbelt (grass) is planted is the arid and semi-arid area around the desert. Although this is not the core sand area, the climate is quite different and changeable year by year, and there is a significant dry and warm trend under climate change. Ecologically fragile areas are prone to desertification and topsoil desertification.

The scope of the three-north shelterbelt project roughly corresponds to the area of wind-blown sand belt in China.

(approximate scope of the three-North Shelterbelt Project) ▼

After planting shelterbelt (grass), the vegetation root system effectively fixed the soil in this area, which greatly weakened the sand-raising process and effectively restrained the further expansion of the sand-raising area.

In addition, although the shelterbelt (grass) has little effect on strong sandstorms as high as thousands of meters, there is no unstable stratification support, the dust thickness is thinner and the wind speed is weaker. Ordinary sand can be fully developed-in fact, this kind of ordinary process accounts for the majority of the sand, and can be seen almost every time cold air invades in autumn, winter and spring.

We can't guard against the high ones and the short ones.

And more than 40 years of history has proved

The three-north shelterbelt has a remarkable effect on resisting sandstorm.

According to the data of the National Climate Center, the number of sand and dust days in China has decreased significantly since 1961, which is partly related to the weakening of cold air sources caused by natural climate change. the other part also verified the great contribution of forest and grass protection and other ecological protection and management projects.

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