Meteorological study on freezing rain
1) Under the freezing rain load, the tension of the transmission line may exceed its ultimate tensile stress, resulting in the transmission line being pulled off;
2) The design strength of the power tower is not enough, which may cause great top displacement under the tension of the transmission line, thus causing instability and damage.
The design values of mid-span deflection and span of transmission lines generally meet the condition that f/L ≤0. 125, so the transmission tower-conductor model can be simplified to the micro-cable model with flexible support for calculation.
A section of 1 10 kV transmission line is selected for example analysis. The tower is 34 meters high and has a steel truss structure. The wire span is 400 m, the two ends are equal in height, and the hinged bracket is adopted. The simplified method of using several split wires to be equivalent to a cable is adopted. The conductor material parameters are as follows: L GJ2240/ 40, horizontal span 400 m, sag 6. 128 m, dead weight 0. 96 kg/ m, diameter 2 1. 66 mm, the elastic modulus is 76 000 N/mm2, and the initial Zhang Liwei is 3 1. 47 thousand cattle. The calculation method of conductor freezing rain quality adopts the method in reference [6], and the quality of conductor freezing rain is mi = 7. 19 kg/m. The weight of freezing rain can be simply regarded as being added to the conductor in the form of static load and becoming a part of its own weight.
N = 1, 2,3 for calculation.
From the statistical results, it can be seen that the dynamic response of transmission lines under freezing rain load is much greater than that under no freezing rain load. This increase in tension makes the tension of transmission lines under freezing rain load reach its failure stress more easily than that under no freezing rain load. At the same time, under the action of freezing rain, the displacement of the top of the tower also increases greatly, and the maximum value can reach about1m. Such a large displacement will cause great bending moment and shear force in the tower, thus causing instability and damage of the transmission tower. It can also be seen that in the tower-line system, the displacement of conductor increases by 265,438 0.8%, the tension increases by 65,438 0.8%, and the displacement at the top of the tower increases by 65,438 0.79%, indicating that the tower-line system has a significant influence on the dynamic response of conductor and tower. Conclusion:
1) The dynamic tension of transmission lines under freezing rain load is larger than that without freezing rain load, and the increase can reach 4-5 times. The reason of transmission line disconnection under freezing rain load is explained preliminarily.
2) The tensile force under freezing rain load will make the top of the tower move more than that without freezing rain load, which will lead to the increase of bending moment and shear force in the tower, thus increasing the probability of instability and collapse of the transmission tower.
3) The tower-line system has a significant influence on the dynamic response of conductors and transmission towers. Comparison of Freezing Rain, Rime and Rime
Formation condition
After freezing rain, rime or rime landscape will generally form. The formation conditions of rime and rime are very harsh, which can only be formed in rainy and foggy weather at 0 ~- 10℃. Due to the abundance of condensation nuclei and the lack of freezing nuclei in the atmosphere, fog drops and raindrops floating in the air under the constraint of curvature can often be as low as -40℃ without freezing. Such fog drops and raindrops are called supercooled fog drops and supercooled raindrops, commonly known as supercooled water. The only difference between them is the size of supercooled water. The droplet size is 3 ~ 100 micron, while the raindrop size is 0. 1~ 8 mm. Although they can remain liquid in the atmosphere, once they come into contact with any object below 0℃, they will quickly freeze on it, form various structures and grow with the wind. Because of the amount of frozen water, different ways, different colors and shapes, everything around is covered with a clean coat, creating a fairy tale world like a crystal palace. Rime and rime often appear at the same time or alternately, causing great damage to power supply lines and forming serious meteorological disasters. If the plane flies in the supercooled cloud and accidentally enters the supercooled water enrichment area, it will hit a lot of supercooled water at a high speed of 60 ~ 100 m/s, and the fuselage will be covered with a lot of ice, which will easily lead to an air crash.
Brief introduction of rime and rime
Rime can be divided into two types. Crystalline rime is a droplet that is supercooled at a certain temperature.
At this temperature, the rime is supercooled raindrops or Mao Mao raindrops.
The causes of rime and rime
According to the principle of cloud physics, I personally think that there are two key points about the cause of rime and rime compound ice accumulation: one is the temperature of the underlying object.
The content of supercooled water in the atmosphere and the transport speed of supercooled water droplets are the two main factors that affect the ice accumulation intensity. For rime, it is the wind speed, and for rime, it is the final speed of raindrops. Secondly, it also involves the capture coefficient and freezing coefficient of supercooled water droplets. In addition, if the near-surface layer keeps a high humidity in the case of long-term low temperature and rainy weather, although it is unsaturated for the water surface, if it is saturated for the ice surface, the water vapor will condense and adhere to the original structure. If this situation continues for a long time, the amount of condensation and adhesion will be amazing. This is how the transmission lines seen in this freezing disaster are covered with almost concentric ice.
Freezing rain and inversion layer
Inversion layer is a low-level stratification commonly existing in typical stratiform cloud precipitation such as frontal area and frontal surface, and it is not a necessary and sufficient condition for freezing rain. As long as the near-surface layer (below 3 000 m) is rich in supercooled water, the gas layer temperature is 0 ~- 15℃, and there are abundant condensation nuclei and water vapor supply. Chai Morski, a scientist from the former Soviet Union, suggested that freezing rain in high latitudes in Europe was formed in1940s. At that time, the understanding of the physical process of cloud precipitation was still hazy, and it was understood by the physical knowledge of cloud precipitation.
The possibility that high-level snowflake ice crystals enter the melting layer and melt into raindrops, and then enter the negative temperature layer to cool to a supercooled state. First of all, after the snowflake ice crystals melt into raindrops, they fall faster. The final falling speed of typical raindrops (diameter 1 ~ 3 mm) is about 1 ~ 5 m/s, and raindrops tend to keep the upper temperature low during falling (thermal hysteresis effect). Raindrops falling 2 000 meters in the atmosphere of thermal insulation have no time to exchange heat with the environment. The raindrop temperature of 3 mm can be 4 ~ 5℃ lower than the ambient temperature, and the raindrop temperature of 1 ~ 2℃ is lower than the ambient temperature. Therefore, in the conceptual model of inversion layer to explain the formation of freezing rain, it is assumed that 3 000~2 000 m is a cloud composed of snowflakes and ice crystals, 2 000 ~ 65 438+0 000 m is an inversion layer, the ambient temperature can be 2℃, and snowflakes and ice crystals melt into raindrops; From the previous discussion, before the raindrop line comes, it is basically a continuous "ND" area from the bottom to the sky.
When the squall line passes through the station, the "nd" area in the middle layer disappears rapidly, and the "ND" areas in the bottom layer and the upper layer gradually rise and shrink until they disappear, indicating that the water vapor content in the atmosphere around the station is unbalanced in heat exchange with the environment when the squall line falls, which has the thermal lag effect of maintaining the temperature trend in the upper layer, and the temperature of raindrops falling in the inversion layer will be higher than that in the environment, even if the ground temperature is low, it will not freeze instantly. In addition, after the snow ice crystals melt into raindrops, there are ice cores in the raindrops. Once they enter the negative temperature zone, they will immediately freeze into ice particles in the air, and will not form freezing rain or collide with objects to form rainbows.
Therefore, the formation of freezing rain can only be rich in supercooled water in the cloud layer of1000 ~ 3 000 m. Through a large number of observations in China, there are usually abundant supercooled water in the 0 ~- 10℃ layer of typical precipitation stratiform clouds, and a large amount of supercooled water can form freezing rain and then collide with objects to form a rain top. Judging from this serious persistence in several provinces in southern China, if it lasts for such a long time, it needs bad warm front inversion conditions (the freezing rain in Guizhou, Guangxi, Hunan, Jiangxi and other provinces in China is not affected by warm front), and it is difficult to last for more than 10 days. However, under the background of stable atmospheric circulation weather, it is possible if freezing rain rich in cooling water often passes through Leng Yun to form rainbows, especially when the amount of ice covered in this process is so large, which also shows that there must be ice.
A "hodgepodge" of precipitation was staged on the weather stage in Beijing, with sleet, heavy snow, ice particles and freezing rain competing to appear. Experts from the Municipal Meteorological Observatory pointed out that freezing rain is rare in Beijing and has a great impact on traffic. According to statistics, as of yesterday morning 1 1, 122, more than 2,000 accident alarms were received, and three major traffic accidents involving multi-vehicle rear-end collision occurred within an hour from 8 am to 9 am, resulting in three deaths, including an 8-year-old boy.
Yesterday afternoon, the Municipal Meteorological Observatory predicted that cold air would arrive in Beijing in the second half of the night, bringing a northerly wind of about level 4. It is expected that the smog weather that lasts for five days today will completely withdraw from Beijing, and the air quality will turn into second-class good, which is suitable for window ventilation and outdoor activities.
Haze and heavy snow drove away heavy pollution.
"It's snowing, but the color is a little gray." Yesterday's light snow slightly improved the air quality in Beijing, and the city changed from the previous six-level serious pollution to five-level serious pollution. However, under the air quality that has been shrouded in smog for four days, even if the concentration of pollutants drops, it will not make people feel comfortable.
The monitoring information of the Municipal Environmental Monitoring Center shows that since yesterday morning, the diffusion conditions of pollutants have been greatly improved, and the hourly concentration of PM2.5 at each monitoring station has continued to decline. At 9 o'clock, the hourly concentration of each station in the urban area dropped to about 150 μ g/m3, and at 19, the air quality index of each station in the urban area dropped to about 200, and the air quality level was five-level heavy pollution, and the pollution degree was reduced compared with the day before yesterday.
Under the strong wind, it is expected that the foggy weather will bid farewell to Beijing completely today, and the air quality will turn to second-class good.
Call for early warning of freezing rain
Yesterday morning, the interaction of cold front and warm and humid air over Beijing brought precipitation, but the precipitation pattern was "rich". Rain turns into ice when it falls to the ground, and a thin layer of ice forms on the ground in Beijing, which is also commonly known as "ground armor piercing".
Yesterday, the yellow warning of smog has not been lifted. The Municipal Meteorological Observatory also issued a yellow warning for road icing at 8: 00. "At present, precipitation has occurred in most areas, the surface temperature is below 0℃, and roads will freeze. Please take precautions. " However, many office workers who traveled in the early hours of the morning still fell down. Some people laughed at themselves, "I slipped all the way to work." Zheng also said in Weibo: "The roads are frozen and traffic accidents can be seen everywhere. I walked three kilometers and saw twelve motor vehicle traffic accidents, many non-motor vehicles skidded. "
Some citizens have called for whether the meteorological department can start an effective emergency warning against the sudden freezing rain, so that the public can understand the harm of freezing rain and the severity of slippery roads earlier and more widely. "Maybe not so many people will fall."
Gene inversion layer brewing frozen rainy days.
Ma Xuekuan, chief forecaster of the Central Meteorological Observatory, said that there were obvious freezing rains in parts of Beijing and Hebei yesterday. Freezing rain seldom appeared in Beijing before.
Ma Xuekuan explained that the surface temperature in Beijing is 0℃ or lower, and the surface temperature is lower than 0℃. There is warm advection at low altitude, forming an air layer with a temperature higher than 0℃, that is, the "inversion layer". At this time, the falling ice crystals will melt to form water droplets when passing through the inversion layer, and the water droplets will cool when they reach the ground, forming supercooled water droplets (the temperature is below 0℃, but they remain liquid), and the supercooled water droplets will freeze immediately when they collide with the ground or objects with the temperature below 0℃, forming freezing rain.
Experts from the Municipal Meteorological Observatory said that there are more freezing rains in the south and less in the north. Guizhou is the province with the most freezing rain in China, and Weining is also known as the "hometown of freezing rain".
Emergency measures to reduce emissions were lifted today.
According to the air quality forecast, today's air quality will be excellent. The Office of the Emergency Coordination Agency of the Municipal Heavy Pollution Day announced yesterday that the emergency measures for air pollution reduction in Beijing will be terminated at 0: 00 today, and I would like to express my heartfelt thanks to all enterprises and institutions, the general public and the news media for supporting the propaganda work of emission reduction.
In the evening, the city launched an emergency plan for heavy pollution day and set up an emergency coordination agency office for heavy pollution day. For two consecutive days, three groups of joint inspection teams of the municipal government were divided into three roads to supervise and inspect the construction sites, road cleaning and sewage discharge of factories and enterprises such as boiler rooms and power plants.
abstract
Freezing rain is composed of supercooled water droplets, which collide with objects with a temperature below 0℃ and freeze immediately.
Overview of freezing rain
Rain that freezes immediately after raindrops collide with the ground, ground objects, airplanes and other objects is called freezing rain.
Freezing rain is a transparent or translucent ice layer formed by supercooled raindrops or Mao Mao rain whose temperature is lower than freezing point, and the water drops quickly freeze on the ground and objects. This kind of ice layer can form a magnificent scene of "thousands of cliffs in the ice and thousands of peaks in the crystal". If it rains in Mao Mao, particles will appear with rough surface and clear particle structure. In case of heavy rain or heavy rainfall intensity, clear ice mounds often form. The surface of the ice clearing mound is smooth, transparent and dense, and it often freezes when flowing on wires, branches or ships, forming long ice hangings.
The cause of freezing rain
When freezing rain occurs, the ground is often not too cold (0℃ ~ 3℃), the sky is inverted and there is a warm layer with a temperature higher than 0℃. Precipitation is a raindrop in the warm layer. When it falls into the atmosphere near the ground, it becomes supercooled and freezing rain, which often causes some harm. If raindrops keep hitting the surface of these frozen objects, they will gradually form icicles.
Freezing rain hazard
After the wire freezes, it will shrink when it is cold, and it will break under the influence of the weight of freezing rain. Sometimes, rows of telephone poles are knocked down, resulting in interruption of telecommunications and transmission. Ice on the ground hindered road traffic. The thickness of freezing rain can generally reach 10 ~ 20mm, and the thickest is 30 ~ 40 mm.
When the northwest airflow and southwest airflow are strong, cold air invades the ground, and warm air flows northward over 1500 to 3000m, vertical structure of the atmosphere is cold from top to bottom and warm in the middle, with ice crystal layer, warm layer and cold layer from top to bottom. That is to say, the atmospheric temperature above 3000 meters is often below-10℃, and at an altitude of about 2000 meters, the atmospheric temperature is generally around 0℃, while below 2000 meters, the temperature is below 0℃, which is prone to freezing rain. Most of the freezing rain in Jianghuai area occurred from early June to early and middle February in 5438, and lasted for more than one month. The start date has the characteristics of north early and south late, mountain early and plain late, while the end date is the opposite.
In mountainous areas with higher terrain, freezing rain starts early and ends late, and the freezing rain period is slightly longer. Once every 2-3 years along the Huaihe River and once every 7-8 years south of the Huaihe River. However, in mountainous areas, there is a big difference between valleys and mountain tops. There is almost no freezing rain in some valleys in mountainous areas, while there is freezing rain in higher mountainous areas almost every year. Freezing rain is a disastrous weather, and a large amount of freezing will break power lines and telephone lines. Severe freezing rain will cause houses to collapse. When an airplane flies in a cloud with supercooled water droplets, the wings and propellers will accumulate water, which will affect the aerodynamic performance of the airplane and cause a crash.
Freezing rain prevention
The method to eliminate freezing rain disaster is mainly to mobilize residents along the power line to continuously knock and scrape the rain on the power line when freezing rain occurs; Freezing rain falls on telephone poles, forming icicles. Install deicing equipment on the plane or simply fly around the freezing rain area.
An example of freezing rain
1at the end of February, 972, there was a large-scale freezing rain in China, and the telecommunications from Guangzhou, Changsha, Nanjing, Kunming, Chongqing, Chengdu and Guiyang to Beijing were once interrupted, causing extremely serious economic losses.
About freezing rain and rime and rime
Generally, it can only be formed in rainy and foggy weather at 0 ~- 10℃; Because there are a large number of condensation nuclei in the atmosphere, there is often a lack of freezing nuclei, so the fog droplets floating in the air and the raindrops falling under the constraint of curvature can often be as low as -40℃ without freezing, which are called supercooled fog droplets and supercooled raindrops, commonly known as supercooled water. The only difference between the two is that the size of supercooled water is different, the fog drops range from 3 to 100 micron, and the raindrops range from 0. 1 to 8 mm; Although they can remain liquid in the atmosphere, once they come into contact with any object with a temperature below 0℃, they will quickly freeze on it, form various structures and grow in the wind; Because of the amount of frozen water, different ways, different colors and shapes, everything around is covered with a clean coat, creating a fairy tale world like a crystal palace. Rime and rime often appear at the same time or alternately, causing great damage to power supply lines and forming serious meteorological disasters. If the plane flies in the supercooled cloud and accidentally enters the supercooled water enrichment area, it will hit a lot of supercooled water at a high speed of 60 ~ 100 m/s, and the fuselage will be covered with a lot of ice, which will easily lead to an air crash.
Discrimination between freezing, freezing and freezing
"Freezing" and "freezing" are the nicknames for the weather phenomenon of "freezing rain" in some areas of China. When the weather is cold, when rain drops on anything below 0℃, it will immediately form ice and attach to this object. Meteorology calls this freezing phenomenon "freezing rain".
The common people have given "freezing rain" many common names-the ice on the tree is called "tree hanging", the ice on the wire or roof is called "hanging ice", and the ice on the ground is called "freezing". Some areas in southern China also call these phenomena "falling ice", "sky ice" or "cowhide ice".
Freezing rain is one of the important weather phenomena leading to freezing disaster. In winter or early spring, continuous low temperature, rainfall or snowfall make the ice and snow stick to objects for a long time without melting, resulting in serious consequences such as broken wires, destroyed trees, frozen crops and blocked traffic and communication. , forming a freezing disaster.
Freezing rain and inversion layer
As long as the near-surface layer (below 3000m) is rich in supercooled water, the gas layer temperature is 0 ~- 15℃, and there are abundant condensation nuclei and water vapor supply, freezing rain without condensation nuclei (ice nuclei) can be formed. At this time, freezing rain can be formed no matter whether the atmospheric stratification is wet saturation decreasing or isothermal decreasing, or even there is an inversion layer.