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

Every morning we open our eyes and see the sun. have we ever wondered what "obstacles" solar radiation has experienced in the atmosphere during the distance from the sun to the earth's surface? Why didn't the high-energy shortwave radiation in solar radiation hurt us? Is there some mysterious force in the path of light that is invisible to the naked eye?

Magical atmospheric window We regard the sun as a blackbody with a temperature of 5250 ℃. Most of the solar radiation is emitted at a wavelength of 0-25 microns. Its radiation spectrum is shown by the solid black line in figure 1 (theoretical curve). The solar radiation received at the top of the atmosphere is shown in the yellow area in figure 1, but at sea level, the actual radiation received is only the red part of the picture, and the radiation curve is jagged. There are many grooves of different sizes, indicating that a lot of solar radiation is consumed or blocked in the atmosphere from the top of the atmosphere to the ground. Why does this happen?

Fig. 1 Solar radiation spectrum, in fact, the solar radiation through the atmosphere or the earth's atmospheric radiation will be absorbed by some gases in the atmosphere, which are almost completely absorbed in some bands. for example, oxygen and ozone absorb a large amount of shortwave ultraviolet rays in the upper atmosphere, thus protecting human beings. In other bands, solar radiation is unimpeded and will not be significantly absorbed. Solar radiation and Earth's atmospheric radiation can pass through the atmosphere like light through windows. These bands are called atmospheric windows.

02. The classification of atmospheric windows, simply speaking, atmospheric windows are the part of the electromagnetic spectrum that can be transmitted through the atmosphere, while the absorption band is the part of the electromagnetic spectrum that is difficult to transmit through the atmosphere. According to the different bands, the atmospheric window is divided into visible window region, infrared window region and radio window region.

In the entire atmosphere, no gas can effectively absorb radiation at the wavelength of 0.3-0.7 microns, which corresponds to the visible band, accounting for 43% of solar radiation energy. Therefore, visible light can penetrate the atmosphere and be captured by vision, so that we can see the colorful world, which is the visible light window. It is worth noting that although atmospheric gases absorb little in the visible band, atmospheric molecules and aerosols scatter strongly in this band, so the visible radiation received near the ground is less than that at the zenith.

Under the heating action of the sun, the earth emits long-wave infrared radiation, and the atmosphere is usually an effective absorber for the earth to emit long-wave radiation, in which water vapor and carbon dioxide are the main absorption gases. water vapor has a series of absorption bands near 1.1, 1.4, 1.9, 2.7, 6.3 microns and above 13 microns, which can absorb 60% of the radiation emitted by the earth's surface. The absorption band center of carbon dioxide is located around 2.7, 4.3 and 14.7 microns, and the absorption band center of ozone is around 4.7, 9.6 and 4.1 microns. The gaps between these absorption bands form some infrared window regions, of which the main infrared window region is the 8-12 micron band (shown in figure 2). The atmosphere does not strongly absorb the infrared light in this band, thus showing a very high transmittance. In the infrared band, the scattering attenuation of atmospheric molecules and aerosols is much smaller than that in the visible window.

Fig. 2 the atmospheric transmission line in the infrared band increases as the wavelength continues to increase, and we find another atmospheric window-the radio window from 300GHZ (1 mm) to 10MHZ (30 m). The short wave end of this window region belongs to the microwave range, so it is also known as the microwave window region. In this window region, there are absorption bands of 2.53mm and 5mm, which are mainly absorbed by oxygen molecules, and absorption bands of 1.64mm and 1.35cm, which are mainly absorbed by water vapor. In order to avoid these absorption bands, the commonly used wavelengths in the microwave window region are 3.3mm, 8-9mm and 3cm.

Fig. 3 the application of the earth's atmospheric window 03. Due to the absorption of gas, engineers have to use the atmospheric window when designing satellites. For example, if we want to place a radiometer on a satellite to receive radiation from the earth, how should we choose the receiving channel of the radiometer? Or which band of radiation received by the instrument is absorbed less by the atmosphere and closer to the real radiation emitted by the earth?

According to the above knowledge, we will immediately think of the visible window region and infrared window region, because if you choose the band at random, it is easy to be attenuated by the atmosphere, resulting in the dimness of the remote sensing image. So yes, our satellite cloud images mainly include visible light cloud images, infrared cloud images, water vapor images and so on.

Visible light cloud images are very similar to those when we take pictures with cameras, both are reflective imaging principles, and the degree of black and white on the cloud images reflects the ability to reflect sunlight: White represents strong reflective ability of ground objects, such as snow, frozen lakes, or thick clouds; black or gray indicates weak reflection, such as vegetation areas and land forest cover areas. The advantage of visible light cloud image is that it can distinguish between ground and low clouds, and can capture cloud contours, textures and other details. For example, in figure 4a, we can see low clouds in the eye of typhoons and clear spiral clouds in the periphery. In figure 4b, you can see cellular clouds in a low-pressure system. The boundary shapes of all kinds of clouds are different, some are straight lines, some are curved, some are neat and smooth, and some are blurred.

Fig. 4 visible light cloud map, a: vortex cloud system, b: cellular cloud system, but after the sun goes down, no sunlight can be reflected back into space, our visible light cloud map can not see anything, so it is only suitable for daytime, can not be used at night.

Fig. 5 visible light cloud image at night, nothing can be seen in infrared cloud image is the cloud image obtained by the satellite in the 10.5-12.5 micron channel, its principle is temperature imaging, the radiation values emitted by objects at different temperatures are different, and the temperature of the object can be roughly inferred from the signals received by the satellite. The degree of black and white on the infrared cloud image reflects the temperature of the measured object. Black indicates high temperature and white indicates low temperature. In figure 6, due to the strong convection inside the typhoon, the cloud development is very high, and the cloud top temperature is very low, so it appears white in the infrared cloud image. similarly, mature cumulonimbus clouds and cirrus clouds are also bright white on the infrared cloud image; the transition zone between the clear sky area and the cloud-rain area is a gray cloud system; if the surface temperature is very high, such as desert areas and warm ocean surfaces in summer, the cloud image hue will be very dark. The advantage of infrared cloud image is that it can be used during the day and night, while the disadvantages are poor resolution, low contrast, low signal-to-noise ratio and blurred visual effect.

Fig. 6 Typhoon Sura captured by Fengyun 4A infrared cloud image so what if we do not want to know the radiation emitted by the ground objects, but want to know the content of gases such as water vapor and carbon dioxide in the atmosphere? The answer is also very simple. We only need to use their absorption bands as observation channels, and then according to the radiative transfer equation, we can retrieve atmospheric parameters such as water vapor, cloud water content and precipitation.

The radio window area is also widely used. The wavelengths of meteorological satellites and satellite communications are chosen in the radio window region. Take the spaceborne rain radar, for example, its detection target is precipitation particles on the earth. If the wavelength emitted by the radar is very short, it is easy to be absorbed and scattered by atmospheric molecules and meteorological objects, resulting in weak signals reflected back into space, so we have to use millimeter or centimeter waves. Enhance its ability to penetrate clouds and rain.

Fig. 7 schematic diagram of radar detection of precipitation 04, conclusion here we already know the concept of atmospheric window, but the editor would also like to remind you that the atmospheric window area is not completely transparent. In fact, there are still atmospheric molecules, aerosols, clouds and precipitation droplets and ice crystal particles of radiation absorption and scattering, resulting in detection data pollution. Therefore, atmospheric correction has always been an extremely important and difficult task in scientific research and practical application.

Finally, let's test everyone. From the cloud picture on our home page of Wechat, what shapes of clouds can you see? Can you try to find out some weather systems?

Fig. 8 references of satellite cloud images on the home page of Wechat

1. Meteorology and Life, 2016, Electronic Industry Press

two。 Wang Feixiang, Guo Jie, Xu Fangyu, Zhang Yuchen, Chen Shuangyuan, Xiao Jianguo, Jia Yuchao, Luo Hong, Zhao Zhijun. Calculation and measurement of infrared atmospheric transmittance at different elevations [J]. Chinese Optics (Chinese and English), 2019, 12 (4): 843,852.

3. Baidu encyclopedia: atmospheric window

This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), by Yang Liu

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