Classification of meteorological radar echo

Different precipitation systems have different radar echo characteristics. It is usually composed of many echo cells closely arranged in a strip shape (Figure 1[ Cloud System and Precipitation Echo of Cold Front on Plane Position Indicator]). When the cold front moves from a distance of about 300 kilometers from the radar station, the discrete echo blocks arranged in a row are usually first seen on the plane position indicator (PPI). This is due to surface curvature and atmospheric refraction. Even if the beam with elevation angle close to 0, it can only detect the higher part of the cloud at 300 kilometers. When the cold front approaches, the radar beam can scan a wider part of the lower part of the cloud. At this time, the monomer of the echo area becomes larger, forming a more coherent echo area. When the cold front moves far away through the radar station, the change of echo is opposite to the above process. Usually, the length of a complete cold front precipitation system can reach more than 600 kilometers, so one station can only detect a part of the whole cold front system. Sometimes the cold front system observed by radar contains more than one rain belt. The echo area of the cold front generally moves from northwest to southeast, but southwest wind is often blowing in the warm area before or above the cold front, so the echo area monomer often moves northeast or east, which has an angle with the overall moving direction of the echo area.

The vertical structure of cold front echo unit is similar to moving isolation. In China, the top of this echo unit is usually more than seven or eight kilometers. In the fast moving cold front, there is a big cloud anvil in the front and upper part of the monomer. Monomer is always in constant rebirth, development and dissipation, and its life cycle is about tens of minutes, while the life cycle of the whole cold front echo area is much longer.

The echo structure of thunderstorm area in air mass is similar to that of cold front, but it moves faster. Sometimes more than two squall echo bands can appear on the radar screen at the same time. It consists of a large range of continuous precipitation. The precipitation area of warm front almost always exceeds the effective visual range of radar station, so only part of the precipitation area can be seen on the plane position indicator. In the stable warm front precipitation area, the echoes on the screen are connected into pieces, and the edges are in the shape of silk thread or cotton wool, and the intensity distribution is quite uniform. In the unstable warm front precipitation area, there are strong convective cells in the large uniform precipitation echo. The moving direction of these echo units may be slightly different from that of the whole precipitation system. Careful observation of the positions of these stronger units shows that they are usually arranged in bands (Figure 2[ Cloud system and precipitation echo of warm front on plane position indicator]).

In the echo image on the distance height indicator (RHI), we can see that there is a strong echo band near the height corresponding to the atmospheric temperature of 0℃, which is called the zero-degree layer bright band (Figure 3 [typical echo image of stable warm front precipitation on the distance height indicator]). Its formation is the result of the increase of reflectivity caused by the melting of slowly falling ice crystals and snowflakes near the zero layer. Below the bright band, the particles melt into raindrops, which fall at a faster speed, reducing the particle concentration and reflectivity. The zero-layer bright band observed on the radar screen can estimate the height of the 0℃ layer and verify the stability of the atmosphere to some extent. In the echo image of the distance height indicator of the unstable warm front precipitation, we can see that the horizontal zero layer bright band and the vertical columnar convective cell echo structure coexist. In addition, zero-degree bright areas can also be seen in the residual precipitation after the thunderstorm is weakened. The variation of precipitation intensity in warm front system is slow, and the temporal and spatial variation of radar echo is also small, which is helpful to verify the quantitative relationship between precipitation intensity and echo power. The echo range of large-scale low-pressure precipitation system is very wide. In the range of radar detection ability, the echoes are almost connected, but the intensity structure is very uneven, such as cotton wool. This echo lasts for a long time.

The echo of the thermal convection thunderstorm in the air mass is generated internally, and the echo of the convective monomer is often scattered and irregular on the plane position indicator (Figure 4 [Echo of the thermal convection thunderstorm in the upper air mass in the plane position indicator]). This kind of convective echo block often appears over the plain hills or islands on the lake. The scale of convection unit is usually between several kilometers and ten kilometers, and its life cycle is about tens of minutes.

Typhoon echo is a strong convective weather system. The characteristic structure of typhoon echo can be clearly seen on the radar plane position indicator (Figure 5 [Cloud System and Precipitation Echo of Typhoon in Plane Position Indicator]). About 400 ~ 600 kilometers before the typhoon center, there are often some strong convective echo areas, which are called the squall line echo areas before the typhoon. Its trend is roughly perpendicular to the moving direction of the typhoon center, but the moving direction is consistent with the moving direction of the typhoon center. Within two or three hundred kilometers around the typhoon eye behind the squall line echo belt, there are large continuous precipitation echoes and spiral convection precipitation echoes. This area is the main precipitation area for typhoons. The spiral rain belt is arranged in a logarithmic spiral shape with the typhoon eye as the center. By carefully observing the moving path of each monomer in the spiral rain belt, we can find that the moving path of the monomer is not consistent with the instantaneous spiral direction, but approximately moves around the eye of the typhoon and slowly approaches the center.

In the center of the spiral rain belt, there is a round strong echo circle around the echo-free hole, which is called typhoon eye wall echo. At this position of the eye wall, convection develops most vigorously, and the top of the echo is as high as more than ten kilometers. The echo-free area in the eye wall echo corresponds to the clear sky in the typhoon eye. Many times, the echo of the eye wall is incomplete, and it is a notched ring. After the typhoon landed, it gradually weakened, the typhoon eye gradually filled with precipitation echoes, and the spiral characteristics of typhoon rain belt gradually disappeared, turning into a large area of low-pressure precipitation echoes.

By observing the weather radar echo, we can find the typhoon earlier, determine the location of the typhoon center, detect the precipitation intensity and wind speed of each part in the typhoon rain belt, and study the detailed structure of this severe convective storm. Strong thunderstorm cell, whether isolated or mixed in convective precipitation system, often has the following remarkable characteristics: the reflectivity of the strong echo core (the region with the highest echo) is very large; The horizontal scale of monomer is also large, generally 10 ~ 30 km. On the distance height indicator, the echo body is upright and thick, with the top reaching the tropopause and sometimes reaching the lower stratosphere. The upper part of the cloud body is provided with a cloud anvil extending forward, and a front hanging echo is hung on the anvil; The air flowing in from the lower level in front constitutes an ascending air column, which causes a weak echo dome in the cloud; The continuous heavy precipitation in a single body mainly appears behind the inflow rising area, forming a "echo wall" with strong echo intensity and steep shape (Figure 6 [Typical vertical structure of thunderstorm with strong mobility in distance height display]); Sometimes, it can be seen that the sharp echo generated by the strong echo signal entering the antenna sidelobe appears directly above the core of the main strong echo. This kind of strong thunderstorm will not only produce thunderstorms and sudden strong winds, but also produce peace.

Through the analysis of radar echo, we can judge the transition stage from convective clouds to severe thunderstorms, but it is difficult to reliably judge whether strong thunderstorm clouds will produce tornadoes or hail on the ground simply based on the echo shape and structure. It is generally believed that the height of the echo top and the reflectivity of the strong echo core can be used as the criteria for identifying hail clouds. For example, in North China, the echo peak of hail clouds in summer often appears at the height of 10 ~ 12 km, and the reflection factor of 3 cm radar is checked by strong echo in disastrous hail clouds (see), which often exceeds10 mm/m. When radar observes non-precipitation clouds, millimeter waves are often used to effectively receive echo signals because of the small size of cloud droplets. In practical application, the antenna usually points vertically to the zenith to measure the lower and upper limits of the cloud above the radar station. In addition, millimeter wave radar is also beneficial to observe the birth of precipitation particles and the expansion of the particle area, which is very valuable for the study of precipitation mechanism.

On the highly sensitive weather radar display, we can occasionally observe some echoes that are not produced by water vapor condensation. They were once called "fairy waves" because they couldn't explain the cause of this echo before. Some of this echo is caused by birds or insects, and some are caused by the clear sky atmosphere with strong and uneven refractive index distribution (Figure 7 [Clear Sky Echo]). On the centimeter-band weather radar, the clear sky echoes observed mainly appear in front of the thunderstorm or near the low-altitude inversion layer during the thunderstorm dissipation period. Clear sky echo is mainly used for detection and research. The echo received by radar is the sum of all scattering elements (such as clouds and precipitation particles) in the spatially effective scatterer irradiated by radar waves. Due to the relative displacement between the scattering elements, the echoes arriving at the radar antenna have different phases, and the superposition of these waves leads to random fluctuations of the echoes. Through the analysis of fluctuation parameters, the motion information of particles and the turbulence intensity of the measured space can be obtained.