Cause analysis of snowstorm in South China in winter of 2007

The global climate background this year is La Nina. We are familiar with El Nino, but we are still unfamiliar with her sister La Nina.

La Nina is a climate phenomenon contrary to El Ni? o, which means that the seawater in the eastern equatorial Pacific (near Peru) continues to cool abnormally, and the surface temperature of seawater is lower than the climate average by more than 0.5℃. It is 0.5 degrees smaller and far away from South America, but it has a great influence on China.

1. How did the low temperature in the South Pacific spread to the whole world? The ocean current movement on the ocean surface is mainly restricted by the sea surface wind. The existence of trade winds makes a large amount of warm water blow from the eastern Pacific (Peru) to the equatorial western Pacific (China and Japan), where it is blown away, mainly supplemented by cold water below the sea surface. The SST in the eastern equatorial Pacific is obviously lower than that in the western Pacific, which makes the airflow in the eastern equatorial Pacific sink, thus intensifying the upward movement of the airflow in the western Pacific, actually weakening the subtropical high in the western Pacific, strengthening the trade winds and westerly winds, and further aggravating the cold water in the eastern equatorial Pacific. Finally, La Nina's influence spread from Peru to the whole world, especially to China.

2. The influence of the western Pacific upward movement on China. The intensification of the ascending movement in the western Pacific leads to the subtropical high that has been far away from Chinese mainland and entered the Pacific Ocean in winter and half a year, and further away from the mainland, further reducing its resistance to the cold air mass in the mainland. Obviously, without the obstruction of the subtropical high, the wind belts on both sides of the subtropical high, one is the strong cold westerly belt in the north and the other is the trade wind belt in the South Indian Ocean, can directly drive the mainland and sweep across Asia, and China is under the control of these two wind belts.

3. The influence of the strengthening of the wind belt on China

(1) westerlies. The influence of westerlies on China mainly comes from two directions: one is to add fuel to the strong cold air from Siberia in the north. The cold air mass comes from the polar direction. It's colder and stronger. Because the evaporation in the Arctic Ocean is small and the air is extremely dry, mainly due to strong winds and cooling processes, there will be no large-scale snowfall. It is this cold air that has the greatest impact on China and impressed us the most. There is little snowfall in the northeast this year, which is mainly affected by the northern air mass, as before. Second, it comes from the West Road, which we seldom pay attention to, but comes from Iceland and the Atlantic Ocean with the help of the westerly belt, and sweeps across the whole Eurasia through the European Mediterranean. Because it comes from the humid Atlantic Ocean, this cold air brings a lot of snowfall. Facts have proved this. This year, because of this strong cold and humid airflow, it snowed for the first time in a hundred years in an extremely dry place like Baghdad. The snow in Central Asia, especially in the mountainous areas of Afghanistan, exceeds two meters. The snowfall in relatively arid Gansu this year is more than twice that of normal years, the highest in 60 years. Interestingly, the snow in our sky actually comes from the Atlantic Ocean.

(2) trade winds. As the direct point of the sun moved southward, the trade winds had little influence on China in the past. However, due to the strengthening of La Nina and the westerly wind blocked by the Qinghai-Tibet Plateau turning to the southern wing of the Himalayas, the air mass is gradually heated and humidified, exposing the south of China to a large amount of warm and humid air from the Indian Ocean, resulting in a higher temperature in the Yunnan-Guizhou Plateau than normal.

(3)*** has the same influence. Due to the weakening of the Pacific subtropical high, two cold air streams from the north and south can directly drive the inland of China, and the warm and humid air stream brought by the trade winds controls the south, such as Shenzhen, with humidity as high as 90%. In the past, cold air in the north controlled the whole country, and the whole country was cold and dry. But this year La Nina led the trade wind to bring a lot of water vapor, which met the dry and cold air mass in the north in the middle and lower reaches of the Yangtze River, forming a large-scale blizzard. Snow falls on the same principle as rain. Now, as soon as the temperature rises, it rains heavily, even rainstorm. From the scope of influence, the degree of disaster is decreasing from east to west, and the coastal provinces have little influence. It can be seen that the water vapor mainly comes from the Indian Ocean rather than the Pacific Ocean, and the trade winds have a great influence.

Second, local analysis.

1. Middle and lower reaches of the Yangtze River, Anhui, Hubei. This area is located near 30 degrees north latitude, where the westerlies and trade winds meet. The intersection of north and south air currents will inevitably cause a lot of snowfall. In addition, there are many lakes in the middle and lower reaches of the Yangtze River, and a large amount of water vapor evaporated from the local water surface is also one of the sources of snowfall.

(1) Hunan. Due to the blocking of Nanling mountain range, the remaining cold air is concentrated in Chenzhou and Hengyang, Hunan Province, that is, at the foot of Nanling mountain, thus releasing all the last strength of Hunan cold air.

(2) Guangxi and Guizhou. In the past, the Yunnan-Guizhou Plateau was always affected by two air masses in the north and south, but this year it was extremely strong, too wet and too cold, causing severe freezing rain.

(3) the north. In the past, the scenery in the northland was frozen for thousands of miles, and the snow in Wan Li was gone with the wind, which remained unchanged. The northeast is mainly affected by the dry and cold air mass in the north, so the snowfall has not changed much. From the satellite cloud picture, it hardly snows these days. However, due to the strong westerly wind, the northwest region brought a lot of water vapor from the Atlantic Ocean, and the snowfall was more than normal.

The third is the impact of snowfall on the south.

Snow is a natural phenomenon, and it rains every year in the north. Even if there is a snowstorm, it will not cause serious problems. Why does snowfall in the south cause so many problems? This paper mainly makes a comparative analysis with the cold north.

1. freezing rain. Now the ground temperature in the north and south is below 0℃ for a long time. But the north is a steady cold air from low altitude to high altitude, so all the snow falls.

In the south, due to the weakening of cold air, warm air is extremely strong, forming a warm air layer above 0℃ over 1500m to 3000m, and below 0℃ above 3000m. The vertical structure of the atmosphere is cold up and down and warm in the middle, that is, there is an inversion layer near the ground. The atmosphere is divided into ice crystal layer, warm layer and cold layer from top to bottom. In this way, the snow falling from the ice crystal layer in the sky melts into raindrops through the warm layer, and then enters the cold air layer near the ground, and the raindrops quickly cool down. Although they haven't turned into ice yet, the temperature has dropped below 0℃, forming freezing rain.

2. Don't move. It snows in the north. Because of the low temperature, it won't melt when landing, so it won't freeze. Even if the temperature is high and melts during the day, because the north is completely controlled by the extremely dry cold air mass, the melted water will soon evaporate back into the air and the ground will always be dry. It is very rare for trees and wires to freeze.

In the south, when freezing rain falls on the ground, branches, wires and other objects, it will gather and spread to all parts of the object surface. Because the temperature of an object is below zero, it immediately freezes into ice. When the temperature of snow falling during the day is higher than zero, the ice on the surface will melt, but due to the control of warm and humid air mass, the humidity is so high that it can't evaporate at all, and ice water remains on the ground objects; At night, the temperature dropped below zero again, and the water immediately turned into ice again.

3. Snowfall. It's as if the north is frozen for thousands of miles, and Wan Li is snowing, but the less thick snow has accumulated for a winter. Because the cold air in Siberia is extremely dry, there is not much snow every time, and it has always been light snow and medium snow, that is, it only snows about 5 mm per hour. There is only a few centimeters of snow in winter.

The humidity in the south is high, and the trade winds brought by La Nina make the snowfall very heavy. There are frequent blizzards in Hunan and Anhui, and more than 14mm of snow fell in an hour. At present, the snow in many places is more than 20 centimeters thick, far exceeding the annual snowfall in the north, so the snowstorm in the south is more serious.

4. Frostbite caused by overweight. Snow in the north is not easy to freeze, so although there is snow on the wire and the roof, once it is too big to keep balance, it will automatically fall from the wire to the ground, so there will only be a small amount of snow on the surface of the wire, which has little effect on the strength of the wire, will not exceed the weight of the wire and will not collapse the roof.

Snowfall and rainfall in the south will eventually turn into ice. Rain and snow fall on the wires, and freezing at night will wrap the wires tightly and the ice will be fixed on the wires. In this way, the ice can wrap the wires layer by layer every day, forming a situation like tree rings. After several days of accumulation, the thickness can not be ignored. It is also true that the thickness of ice on the wire is more than twice the diameter of the wire, the density of iron is 7, and the ice is 0.9. The iron tower and high-voltage line on the roof were wrapped in thick ice and eventually collapsed. This situation mainly occurred at the foot of Nanling Mountain with extremely high humidity and extremely low temperature, so nearly one-third of more than 70 high-voltage towers in Chenzhou and Hengyang were crushed. High-voltage steel towers will bear 2-3 times the weight in snowy days, but if they are frozen, they will bear 10-20 times the weight of wires. There is another problem with wires and water pipes, that is, because the temperature is unusually low, the wires have been stretched between towers beyond the design limit due to thermal expansion and cold contraction, and the strength has been greatly reduced, which directly leads to serious paralysis of the power system. Chenzhou has been without water and electricity for five consecutive days, especially the Shaoguan-Chenzhou section of the Beijing-Guangzhou railway. The electric locomotive is out of power, so it can be replaced by diesel locomotive, but the signal lamp is out of power and the turnout is out of power. Therefore, even the diesel locomotive can only pass slowly by the manual command of the roadside, and the turnout must be moved manually, which makes the diesel locomotive unable to run at its usual speed in this section, which increases the difficulty of railway transportation.

5. Freezing injury of highway. There will also be snow in northern expressway, but it will not melt, and the amount will not be great. Even if it melts, it will evaporate quickly, leaving no water on the ground and freezing into ice. The friction of snow compacted by cars is much greater than that of ice. Even if you want to climb a mountain, you can advance on the snow by installing snow chains.

On the one hand, the snow in the south is particularly thick, and many places have not crossed their feet. More importantly, snow melts into water. Due to the extremely high humidity, water quickly freezes into ice. Like wires, it covers the road layer by layer. Some places are as thick as 10 cm. The friction of the car on the ice is very small, so it will naturally slip. Therefore, the Beijing-Zhuhai Expressway can't pass completely for the safety of the people, and the icing is totally irresistible. On the other hand, since ice is much harder than snow, it is useless to install snow chains on wheels. This most practical method in the north has lost its function in the south, but it will make friction less and driving more dangerous.