How does weather radar detect the weather?

In ancient myths of China, there are stories of clairvoyance and ears with the wind. Radar is a clairvoyance of science. Radar is a kind of radio equipment, and its English name is radar (abbreviation of radio detecting and ranging), which means radio detection and positioning. Early radars were mainly used for military purposes. Now it has been widely used in aviation, navigation, meteorology, scientific research and other aspects. The radar used to observe clouds and measure rain, that is, weather radar, has become a powerful tool for meteorological departments to monitor weather conditions and predict future weather.

The radar specially used for atmospheric detection belongs to active microwave atmospheric remote sensing equipment. The high-altitude wind radar used with radiosonde is only a special equipment for positioning displacement balloons, and it is generally not considered as such radar. Meteorological radar is one of the main detection tools for early warning and forecasting small-scale weather systems (such as typhoons and rainstorm clouds).

Weather radar

The weather radar emits pulsed radio waves into space through a highly directional antenna, and interacts with the atmosphere during the propagation. For example, the water vapor condensate in the atmosphere (clouds, fog, precipitation) scatters and absorbs radar waves; The depolarization caused by the scattering of circularly polarized waves by non-spherical particles, the uneven structure of air refractive index of radio waves and the scattering of incident waves by ionized media formed by lightning discharge stabilizes the partial reflection of incident waves by stratified atmosphere; And the Doppler effect of the motion of the scattering target in the scatterer on the incident wave.

Weather radar echo can not only determine the macroscopic characteristics of the detection target, such as spatial position, shape, scale, movement and development, but also determine various physical characteristics of the target according to the amplitude, phase, frequency and polarization of the echo signal, such as water content in the cloud, precipitation intensity, wind field, vertical airflow speed, atmospheric turbulence, precipitation particle spectrum, cloud and precipitation particle phase state, lightning and so on. In addition, the vertical gradient of refractive index can be obtained from the vertical distribution of troposphere temperature and humidity by using the law that the refractive index changes with height caused by the change of troposphere temperature and humidity, and the detection distance of radar can be predicted by analyzing the propagation conditions of radio waves, or the stratification of atmospheric temperature and humidity can be inferred from the abnormal phenomenon of radar detection distance (such as superrefraction phenomenon).

The basic principle of radar positioning is not complicated. Radar directionally emits electromagnetic waves (called transmitted waves), which are received after encountering the target, and some scattered waves are received by radar. The radar adopts polar scanning mode to record the radar direction and determine the direction of the target relative to the radar. With the distance and bearing, the point of the target is fixed. Sex and rain are a special goal. Rainfall particles such as cloud drops, raindrops, snow crystals and ice particles in cloud rain can scatter radar waves. When the energy scattered by them is large enough, the radar can identify them, and the color image of clouds and rain is displayed on the radar screen, which is more intuitive and quantitative. According to these images, the trained radar staff can judge the nature of the cloud and rain target, whether it is rain or hail, whether it is accompanied by lightning and strong wind, where it will move in the future, whether it is strengthened or weakened, etc.

Doppler weather radar works by using Doppler effect. I don't know if you have had such an experience. When you are on the platform of the railway station, there is a train coming towards you in the distance, and you will hear the whistle of the train getting sharper and sharper. When the train passes in front of you and fades away, the whistle of the train will get deeper and deeper. This is because the sound source has relative motion to you, and the frequency of the sound source you feel has changed. This is the Doppler effect. When Doppler weather radar detects clouds and rain, precipitation particles such as cloud droplets will also move relative to the radar. This motion has a component in the warping direction pointed by the radar, that is, towards or away from the radar. Due to the Doppler effect, the frequency of radar waves scattered by precipitation particles (called echoes) is different from that of transmitted radar waves. Doppler radar can measure this frequency offset, and through this frequency offset, we can understand the movement of precipitation particles in the cloud and extract rich meteorological information from it. Therefore, weather radar with Doppler function is more conducive to monitoring strong convective weather such as thunderstorm and gale, hail and gale, tornado, downburst and low-level wind shear. Doppler weather radar also has more advanced functions in quantitative measurement of precipitation, and can also enhance our ability to monitor heavy rain.