All the cold you have experienced has been captured by China's Fengyun satellites

"Cold" is undoubtedly the keyword of the coldest months of winter. People in the north feel the dry cold, with the whistling north wind blowing on their faces; while people in the south feel the wet cold, with the freezing rain knocking harshly against the doors and windows.

Can the cold in different places be "seen"? Of course. In space, the "telescope" of satellites can "remotely see" the cold air. Its extremely wide field of vision allows us to clearly and intuitively see the changes in the weather when the cold air arrives.

The following precious satellite cloud images are the space perspectives of two types of Fengyun meteorological satellites. The polar-orbiting meteorological satellite is at an altitude of about 900 kilometers, and the geostationary meteorological satellite is at an altitude of about 36,000 kilometers, respectively providing the most dazzling "selfie images" and the most dynamic "live broadcast" of the last cold air in 2021.

What does cold air look like in the eyes of Fengyun meteorological satellites?

The following pictures show how a strong cold front appeared in the eyes of satellites at the end of last year: Picture 1 is the cold front in the eyes of FY-3E Dawn Satellite: The wind radar showed that the wind speed on the western sea surface of Bohai Sea in the north exceeded 20 meters per second, indicating that a cold front has begun to move southward in a large scale.

Figure 1: Sea surface wind vector monitoring image of FY-3E wind field measurement radar

Because Figure 1 is a regional map, its resolution is a bit low and it has mosaics. In contrast, in global observations, the wind field in the eyes of the Fengyun satellite looks like Figure 2, which looks much more detailed.

Figure 2: The first monthly average ocean wind product image from FY-3E

Figure 3 shows what it looks like from the perspective of the Fengyun-3D polar-orbiting meteorological satellite: The streaky clouds (also called cloud streets and horizontal convective rolls) shown in the image are amazing. This type of cloud system is often formed when cold air is blown to the relatively warm ocean surface and is "heated and humidified". It can only appear when strong cold air enters the sea in winter.

Figure 3: True color monitoring image of FY-3D meteorological satellite

The same cold front, under continuous monitoring by the Fengyun-4 multi-channel scanning radiometer, looks like Figure 4. Cloud images one after another dynamically broadcast the cold front "moving southward". Does it look like the "flying waterfall plunging three thousand feet" described in Li Bai's poem?

Figure 4: True color monitoring animation image of FY-4A meteorological satellite

Obviously, in the eyes of the satellite, the static cold air shows a beautiful landscape painting of mountains, rivers, seas, land, white clouds and snow-capped mountains; the dynamic cold air fully demonstrates the strength, speed and beauty of the cold wave, just like a live broadcast of "water-like flowing clouds" with a waterfall hanging in front of a river.

From the time the cold wave starts in the north to the time it crosses the sea, climbs over mountains and ridges all the way south, the Fengyun satellites track and monitor the generation, movement, strength and range of the cold air in all directions, just like a human mother paying attention to the growth of her children, and send back information on weather conditions at any time to the earth.

How do Fengyun satellites know that a cold wave is coming and going?

The ability of Fengyun satellites to monitor cold air comes from their "clairvoyance": advanced satellite payload observation instruments.

There are currently eight Chinese Fengyun satellites on duty in the sky. They work 24 hours a day, 365 days a year to help us monitor various meteorological information on the earth and the atmosphere.

Among the many observation tools carried by the eight satellites, there are several advanced instruments that can effectively monitor cold air:

The first one is the "wind field measurement radar", which can actively obtain information on wind speed and direction on the ocean surface with high accuracy. It is very capable of monitoring cold air crossing the sea and going south. The previous Figures 1 and 2 were taken by them.

The "wind and clouds" seen by the "wind field measurement radar" are just like the flow of blood in our human capillaries. It is the first instrument used by my country's Fengyun satellites to actively detect "wind" information. It can be said that it has truly realized the original intention and long-cherished wish of Fengyun satellites to measure "wind", "clouds" and "wind and clouds".

The second is the "multi-channel scanning imaging radiometer", which can provide various satellite cloud images such as day, night, and water vapor distribution, and intuitively show the appearance of southward cold air of different intensities, ranges, and origins. It can also capture dynamic information such as cloud movement through minute-level image synthesis. It is a magic tool for cold air selfies and live broadcasts. The previous Figures 3 and 4 are static and dynamic pictures taken by them.

The "multi-channel scanning imaging radiometer" is like the "color ultrasound" commonly used by doctors to take pictures of patients and the "dynamic electrocardiograph" to monitor and record the patient's electrocardiographic activity. It can scan the cold air image in a single frame and continuously monitor the entire process of cold air under daily working conditions and transmit it back to the ground in real time.

Doctors judge the patient's physical condition based on "color ultrasound and electrocardiogram", while satellite meteorological technicians diagnose the intensity, speed, range and other information of cold air from "satellite cloud map" data, providing important scientific basis for cold air forecasts.

The third one is the "atmospheric vertical detector", which can monitor temperature changes at different altitudes. As long as the temperature drops, there will be a good response near the ground.

The function of the "Atmospheric Vertical Sounder" is similar to that of the "PET-CT" instrument that performs CT scans on the entire body of a person. It can perform diagnostic scans of the atmosphere at different altitudes and obtain information on temperature drops at different altitudes from the ground to the top of the troposphere.

It is worth mentioning that my country is the first country in the world to place the "Atmospheric Vertical Sounder" and "Multi-channel Scanning Imaging Radiometer" on a geostationary satellite platform and successfully launch it into a geostationary orbit at an altitude of 36,000 kilometers.

These precision instruments are "the best in observing cold air" and the soul of the Fengyun meteorological satellite, a national weapon.

Figure 5: 24-hour temperature change chart of different altitude layers of the vertical sounder of FY-4A meteorological satellite (degrees Celsius)

Figure 5 shows the same cold front as seen by the satellite "PET-CT". The vertical axis Pressure refers to the atmospheric pressure, which is related to the altitude. The smaller the number, the higher the altitude. 1000 hPa corresponds to the surface, 850 hPa corresponds to an average altitude of about 1500 meters, and 300 hPa corresponds to an average altitude of about 9000 meters.

When cold air comes, different altitudes will experience different degrees of cooling, and the lower the altitude, the greater the cooling. The top of the near-ground boundary layer, about 1,500 meters, is a very important altitude, which can determine the cooling extent of the middle and lower layers of the troposphere and judge the strength of the cold air.

It is these "telescope" instruments carried by the Fengyun satellites that transmit first-hand cold air information monitored in real time back to the ground in a timely manner, ensuring that forecasters can accurately predict the "coming" and "going" of cold waves.

As early as 1969, Premier Zhou called for "we must develop our own meteorological satellites." In the more than half a century since then, China's Fengyun satellites have started from scratch, from experiments to operations, from the first generation to the second generation, from following to running side by side to partially leading. 19 satellites have been launched, and 8 are currently in orbit, providing monitoring data and service products to 121 countries in the world.

Whenever you look up at the sky, weather satellites are right above your head, diligently taking beautiful and practical cloud pictures and transmitting them back to the ground!

Author: Cao Jing

Photo credit: Jia Xu, National Satellite Meteorological Center

Editor: Emeria, You Shiyou

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