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2025-01-18 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >
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Figure 1. As we all know, for every 100-meter drop in height in the troposphere, the temperature rises by about 0.6 degrees Celsius. We looked up at the sky, there were many ice particles in the distant clouds, and after they fell off and fell for a period of time, the surrounding atmospheric temperature gradually rose to 0 ℃. At this time, the ice and snow particles begin to melt and become a mixed phase of ice and water, until they completely melt into raindrops. In the process of ice-water conversion, if you illuminate this area with radar, you will find that the echo here is brighter than anywhere else. Scientists have named this phenomenon as the zero layer bright band, so how is the zero layer bright band formed? Let's get to know it first.
01. What is the zero layer bright band (Bright Band,BB) is one of the radar echo characteristics of large-scale layered precipitation. It shows an obvious medium intensity color code circle or arc on the PPI (high elevation). Its intensity is often up to 30-40dBZ, which is 10-20dBZ stronger than the nearby echo (as shown in figs. 2a, b, c). On the RHI (or profile), the intensity of the first echo is obviously larger than that of the narrow strip echo bright band of the upper and lower parts (as shown in figure 2d). Due to the early display of the weather radar with a fluorescent screen, the echo in the zero layer will appear brighter than above and below it, so it is called the zero layer bright band.
(a)
(B)
(C)
(d) figure 2. However, not all precipitation can form a bright band, usually we can only see this phenomenon in layered precipitation. In meteorology, precipitation is usually divided into convective precipitation and stratified precipitation. The vertical air of convective precipitation moves very fast, and the rising air flow will bring many ice water particles into the upper and middle atmosphere. When these large particles fall, the radar echo will show a vertical column with high reflectivity factor on the profile (see figure 3, left echo column). In contrast, the falling velocity of layered precipitation particles is much faster than that of updraft, and when ice particles fall near the zero layer, they will melt and form a bright band of zero layer in the radar profile (near the height of 5km, as shown in figure 3). Therefore, the zero-degree layer bright band is the main feature of the precipitation echo of stratiform clouds.
Figure 3. Convective precipitation and layered precipitation radar echo profile 02, the cause and significance of the bright band, then, what are the main reasons for the formation of the bright band, or why the radar echo increases suddenly near the zero layer? In order to answer this question, we have to come up with the radar weather equation.
The radar echo intensity is determined by the radar parameters and the atmospheric state of the precipitation area, which is usually expressed by the radar meteorological equation.
①, where Pr is the pulse peak power of radar, R is the distance between radar and detection target (cloud, rain, snow, etc.), m is negative refractive index, Z is radar reflectivity factor, under the condition of Rayleigh scattering:
②, where D is the particle diameter and n is the numerical density of the particle.
According to the radar meteorological equation, there are five main factors for the formation of bright band.
(1) melting effect: ice crystals and snowflakes fall from high altitude to the vicinity of 0 degree layer, and the dielectric constant increases after ice melts into water, that is, in ① formula.
When the term increases, the scattering ability of particles increases. (the dielectric constant of water particles (0.93) is 5 times that of ice particles (0.16).
(2) collision effect: with the melting of particles, the snowflakes wrapped in water become more sticky, and they are easier to stick and hook together, that is, the collision and polymerization between particles is enhanced, and the diameter of particles becomes larger. It is known from ② formula that the increase of particle diameter D will lead to the increase of reflectivity factor Z, that is, radar echo enhancement.
(3) Velocity effect: when the ice crystal melts completely, it changes into spherical raindrops under the action of surface tension, the resistance decreases, and the falling speed is much larger than that of ice crystals and snowflakes, so that the number of precipitation particles n per unit volume is greatly reduced. According to the ② formula, the reflectivity factor Z decreases correspondingly, and the echo below the bright band weakens, thus highlighting the bright band.
(4) Particle shape effect: ice and snow particles are not always spherical in the process of falling, and the scattering of non-spherical particles is larger than that of spherical particles, so the scattering ability is enhanced.
(5) Particle fragmentation effect: large raindrops suffer greater resistance and usually do not last long. When they break or evaporate into small raindrops, the particle diameter decreases, and the reflectivity below the bright band decreases, highlighting the bright band.
Generally speaking, in the upper part of the bright band, the radar echo intensity increases sharply due to the change of dielectric constant and particle growth effect caused by melting, while in the lower part of the bright band, the ice crystal framework disintegrates and the velocity effect reduces the radar echo intensity, so that the melting layer near 0 ℃ forms a horizontal strong echo band: zero layer bright band.
Figure 4. What is the significance of the brightness of the zero layer in the practical meteorological application of a precipitation rain mass profile detected by dual-frequency rain radar? First of all, the zero layer reflects the obvious ice-water conversion process in the stratiform cloud precipitation. The precipitation above the bright band is dominated by ice crystals and snowflakes (solid particles), and below the bright band is dominated by raindrops (liquid particles). Detecting the presence or absence of bright bands can help us to better distinguish convection from stratified precipitation and further discuss the differences in their precipitation mechanisms. Secondly, the existence of the bright band indicates that the air flow in the stratiform cloud precipitation is stable and there is no obvious convection. Finally, according to the height of the bright band of 0 ℃ layer, we can also infer the height of 0 ℃ isotherm in the atmosphere.
03. Zero layer bright band irradiated by different instruments in recent years, with the continuous development of meteorological instruments, dual-frequency radar and dual-polarization radar can provide us with more precipitation particle information, and these two radars also have their own advantages in bright band detection.
Dual-frequency rain radar, as its name implies, is a radar that can transmit electromagnetic waves in two different bands. Different bands of radar detect different signals when they encounter the same object. For example, the solid blue and red lines in figure 5 represent the precipitation echo signals detected by Ku and Ka radar respectively, because the wavelength of Ku band is longer than that of Ka band (stronger penetration). Therefore, when it encounters the meteorological target, the signal attenuation is weak, and the detected radar echo is stronger. Using this dual-band detection signal difference, that is, dual-frequency ratio DFRm, bright band recognition can be carried out. We can see that for stratified precipitation (left of figure 5), the DFRm profile (black dotted line) has a clear peak and upper and lower boundaries, indicating the existence of bright bands, but for convective precipitation (right of figure 5), DFRm profiles do not have such characteristics, indicating that there is no bright band in convective precipitation.
Figure 5. The reflectivity factor profiles of layered (left) and convective (right) precipitation detected by dual-frequency rain radar are different from dual-frequency radar. Dual-polarization radar is a radar that can emit both horizontal (H) and vertical (V) polarization electromagnetic waves at the same time. These two different polarization electromagnetic waves illuminate various precipitation particles, and the backscattered echo can "tell" us the shape, size and direction of the particles. Dual polarization radar can also improve the ability to identify the bright band of the zero layer. Its three polarization parameters, the correlation coefficient ρ HV, the differential reflectivity ZDR and the differential phase shift KDP, are all sensitive to the bright band. Sometimes the bright band characteristics in the radar echo intensity field are not obvious (figure 6a), but ρ HV, ZDR and KDP can well indicate the position of the zero layer (figure 6b, c, d black arrow position).
Figure 6. The parameter distribution map of C-band dual polarization radar (a: reflectivity factor Zgraine b: differential reflectivity ZDR,c: correlation coefficient ρ HV,d: differential phase shift KDP) is a brief introduction to the zero layer bright band. Finally, there are a few points to pay attention to:
(1) the height of the zero layer is different in different regions and seasons, so the bright band height of the zero layer will also change (the tropospheric height is like this).
(2) during snowfall, the zero layer bright band will not appear because the near surface temperature is often lower than 0 ℃ and the particles do not have the process of ice-water conversion.
(3) since ice crystals and snowflakes begin to melt when they are greater than 0 ℃, the zero layer bright band usually appears in the region below 0 ℃.
reference
Steiner, M., R. A. Houze Jr., and S. Yuter, 1995: Climatological characterization of three-dimension storm structure from operational radar and rain gauge data. J. Appl. Meteor., 36 (7), 847-867
Fabry, F., and I. Zawadzki, 1995: Long-term radar observations of the melting layer of precipitation and their interpretation. J. Atmos. Sci., 52 (7), 838-851
Le, M., and V. Chandrasekar, 2013: Precipitation type classification method for Dual-Frequency Precipitation Radar (DPR) onboard the GPM. IEEE Trans. Geosci. Remote Sens., 51 (3), 1784-1790
Chen Jun, Wang Jinhu, Wan Fayu, Fan Pan, Han Songyu. Cause Analysis of Zero layer Bright Band based on CST Software [J]. Journal of Lanzhou University (Natural Science Edition), 2017.
Yao Xiaojuan, Wei Ming, du Aijun, Lu Meiying. Sensitivity analysis of polarization radar parameters to melting layer echo detection [J]. Science, Technology and Engineering, 2016.
This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), by Yang Liu
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