A pair of "wise eyes" to identify sand and dust

Recently, the north has been hit by a series of severe sandstorms, with yellow sand "willfully" entering, flowing back and leaving, causing many friends to sigh: "The season of eating dirt has come again!" This year's sandstorms are so willful and erratic, are there any signs of them? Today, let's use the "eyes" of satellites to get to know this "Internet celebrity" that has caused Beijing and more than a dozen other provinces and cities to "eat dirt."

Artifact to capture dust trails

Dust storms are not uncommon weather phenomena in the spring in northern my country. In addition to bringing inconvenience to people's lives, they also seriously affect air quality and cause environmental pollution. Monitoring and early warning have long been one of the important tasks of the meteorological department. At present, the most effective means of monitoring sandstorms is to use satellite remote sensing technology. So, where is the source of the sandstorm? Where does it move to? How strong, fast, large, and harmful is it? Why does it look so much like a typhoon? In the face of questions of public concern, satellite cloud images can actually find the answers.

Early in the morning of March 27, satellite technology workers discovered sandstorms in Mongolia on cloud images. Affected by the eastward-moving Mongolian cyclone, the sandstorms had spread to southern Mongolia, western Inner Mongolia, and the north-central region of my country in the afternoon. Continuous dynamic monitoring of cloud images shows that the sandstorm area continues to spread to the southeast, and by 3 pm, the visible area of ​​sandstorms is about 398,000 square kilometers. Watching the sandstorms continue to move eastward and southward with the Mongolian cyclone, combined with other ground monitoring methods and years of forecasting experience, weather forecast experts from the China Meteorological Administration issued a forecast to the public that sandstorms will also occur in northern North China, western Northeast China, northern Hebei, Beijing, Tianjin and other places. After a media report that "the sandstorm is coming! Satellite monitoring of the visible area of ​​sandstorms has reached 398,000 square kilometers", the event instantly became a hot search, and this day was only 12 days away from the news on March 15 that "the sandstorm is coming, and the visible area of ​​sandstorms visible to satellites is about 466,000 square kilometers."

The monitoring results will be available in just ten minutes

How do satellites identify yellow sand?

As we all know, sand and dust contain a lot of minerals. These sand and dust particles absorb and scatter solar radiation and long-wave radiation from the ground and clouds, etc., and affect the earth, which gives them their own spectral characteristics. By comparing the differences between the spectral characteristics of sand and dust storms and the spectral characteristics of other substances, the sand and dust storm information can be identified through the combination of different detection channel values ​​on the satellite-borne detector. In the three-channel color composite satellite image, technicians identify the sand and dust storm area and its area through characteristics such as color, tone, texture and shape. Then, through multi-time dynamic image and remote sensing data processing and analysis, the origin, movement path, movement speed, affected area and future advancement direction of the sand and dust storm and related dynamic information can be determined.

Of course, the accuracy of satellite monitoring of sandstorms is affected by three factors: cloud cover, ground desertification and exposure, and low frequency of satellite monitoring.

Cloud cover will cause the information of sandstorms to be blocked; ground desertification and exposure will make it more difficult to identify sandstorm information; low satellite monitoring frequency will miss some sandstorm processes or a certain stage in a process. The first two factors exist objectively, and there is currently a lack of better means to circumvent them, but the last factor can be said to have been effectively solved. Because my country's Fengyun-3D and Fengyun-4A satellites can respectively complete higher-resolution and higher-frequency observations in space operations, especially the Fengyun-4 meteorological satellite can obtain a monitoring image every five minutes, the monitoring of sandstorms has almost reached the quasi-real-time standard. After the spectral information is captured and transmitted to the ground, it is processed through data to form monitoring images and processed data. Basically, the monitoring results can be obtained in more than ten minutes.

Dust storms and typhoons are not close "brothers"

People who often pay attention to satellite cloud images may have a question: Why does the shape of this sandstorm look so much like a typhoon? Are they "brothers"? I had this question 20 years ago, but today I can give a clear answer: they are not really "brothers", but they are indeed related. Because they belong to the same cyclone family, the sandstorms that affect my country belong to "Mongolian cyclones" and typhoons belong to "tropical cyclones"; they belong to the same meteorological satellite "photographer"; they are both air vortices and low-pressure systems; they both have similar-looking vortex shapes and rotation directions.

Comparing the satellite cloud images of the 2019 "Southern Lekima" and this "Northern Dust Storm", it is not difficult to see that they have distinct southern and northern "painting styles": the southern style is free and easy, while the northern style is delicate; the southern style is free and loose, "with hardness and softness inside", while the northern style is tight and compact, "with exposed muscles outside"; the southern style has a deep blue sea background, while the northern style has a clear texture of sand dunes; the southern style seems to focus on freehand painting, while the northern style seems to be a carving technique. Image provided by Fang Meng

However, they have more differences.

First, they were born in different places. One is in the north and the other is in the south. The former are usually born in the central and eastern plateaus of Mongolia at 40°-50° north latitude. The western and northwestern parts of the region are mostly mountainous, while the central and eastern parts are on the leeward slope, which is particularly conducive to the formation and development of cyclones. The latter are usually born in tropical and subtropical oceans, the vast tropical Pacific Ocean with water temperatures above 26°C, and the South China Sea.

Second, they are born at different times, one occurs more often in spring and the other occurs more often in summer.

Third, the two have different structures and means of survival. The back of a sandstorm (the west side) has cold air falling down, and when it encounters warm air heading north, the cold and warm air rotate and clash, often reaching their strongest in the northeast or north China. The "package" of survival usually includes strong winds, dust, cooling and even precipitation; a typhoon is a positive pressure, front-free warm-heart system, which mainly relies on water vapor and heat from the tropical ocean to "fill its stomach" to maintain and develop. When it is strong, there will be a clear sky area in the center (the eye of the typhoon).

Fourth, the movement paths are different. The former tend to move eastward after birth (after entering Inner Mongolia, it continues to move eastward along the lower reaches of the Songhua River through the Northeast Plain) or southeastward (entering North my country, entering the Bohai Sea and then going east through the Korean Peninsula). The latter usually moves northwestward after birth. Fifth, the affected areas are different. The former has a great impact on the weather in northern my country. It is often accompanied by strong winds, cooling and sandstorms along its way. Severe convective rainfall may also occur. If it is very strong, it may even affect the air visibility in the south. The clouds and rain caused by the latter mainly appear in the area around the eye of the typhoon, and usually have a greater impact on South China, but if the typhoon in late summer and early autumn is very strong, it will also affect the north. Lekima in 2019 arrived in the northeast to show its power.

Obviously, sandstorms and typhoons are at most "relatives" that look somewhat similar.