Why is the weather in Xi 'an as humid as in the south, except that it is hot in recent two years?
At the entrance of the unit building of a residential area in Xishaomen, Xi, a notice against cockroaches was recently posted. Some owners who live on the lower floors say that cockroaches have often crawled out of the kitchen sewer in the past few months. Cockroaches are dirty and disgusting, but they are difficult to eradicate.
In the monitoring of vectors in Xi City CDC, a cockroach named Periplaneta fusca was also found. This cockroach is mainly distributed in Fujian, Guangdong and other places, and it is relatively rare in the north. With its appearance, almost all common cockroach species in southern China have appeared in Xi 'an.
Cockroaches like humidity and are common pests in rainy season in southern China. This summer, the cockroach infestation in the ancient city made many people feel that the weather in Xi 'an suddenly had a southern flavor.
Where does the rainstorm come from?
In the impression of Shaanxi people this year, lightning, strong winds and hail are much stronger than in previous years. According to the statistics of Shaanxi Lightning Protection Center, lightning strikes occurred in xi 'an from the early morning of September 4th to 3: 00pm on the same day, with a total of 1007, which actually reached 1085 in the whole province. Affected by these strong convective weather, various disasters followed. From July to September, Hanzhong, Ankang and Shangluo in southern Shaanxi were successively affected, and some parts of Guanzhong were also seriously affected.
For example, in Gaoling County next to Xi 'an, after more than ten hours of rainfall, the precipitation reached 1 19.6 mm, and the deepest water in the urban area 120 cm, which became the largest precipitation record in the county. The heavy rainfall also caused 90% of the autumn fields in the county to be affected. 1444 solar greenhouses collapsed, 474 houses collapsed, 5,822 trees were blown down or blown down, and some villages were cut off or tap water could not be supplied.
"Rainstorm, lightning, strong wind, hail, etc. It's all strong convective weather. The climate anomaly this summer is mainly due to the concentration of strong convective weather such as heavy precipitation. " Yang Wenfeng, director and senior engineer of Shaanxi Meteorological Observatory, said.
Yang Wenfeng, 40, has been studying climate for 16 years, and many papers have been published in core journals. Summing up his work experience over the years, he felt that he had too much to learn.
Yang Wenfeng said that the so-called strong convective weather is "the weather caused by the formation of huge cumulonimbus clouds driven by strong convective air movement". When air is heated rapidly, it will produce a strong upward movement. When the airflow rises and falls to a certain speed (10 m/s), it is a strong convective movement. In this case, cumulonimbus clouds will form heavy precipitation and other weather.
"The formation of severe convective weather is influenced by weather system and climate background." Zhang Hong, chief forecaster and senior engineer of Shaanxi Meteorological Observatory, said that the heavy precipitation in China this summer was mainly due to the northward location of the western Pacific subtropical high. "The subtropical high is an atmospheric circulation system that transports warm and humid air from the ocean to the mainland. After the mainland encounters cold air, it is easy to form precipitation. "
"In July this year, the subtropical high performed strongly in our province and occupied it for a long time. Affected by it, the troposphere (where strong convective weather such as precipitation and lightning forms) is cold and hot, and the air structure is unstable. The warm and humid airflow from Southeast Asia continues to encounter cold air from Xinjiang and Inner Mongolia over our province, and the rain will be more concentrated. "
Zhang Hong said that there is another reason for the formation of local strong convective weather, that is, the ground is strongly radiated by the sun during the day, and the energy in the air accumulates rapidly, so that it reaches extremes at night. In the process of hot air rising, its temperature drops continuously. If it is triggered by cold or night temperature drop, it will easily lead to high-intensity release of energy in a short time, and it will also cause strong convective weather.
"From a big background, strong convective weather is affected by global warming. Because climate warming affects the abnormal changes of the whole atmospheric circulation system, it will inevitably lead to the increase of extreme weather events, "Zhang Hong said. In the case of global warming, water evaporation intensifies, atmospheric circulation accelerates and its intensity increases. When transported to some mountainous areas, it is also easy to be stimulated by terrain to form strong convective weather. "Like the Bashan area in southern Shaanxi, there will be strong convective weather affected by the terrain. On August 8 th and 9 th, the warm and humid airflow from Bangladesh suddenly encountered cold air from the south, and there was heavy rain in Guanzhong. " He said that the formation of such strong convective weather often takes only a few hours.
Will Xi 'an become a "southern climate"?
Since strong convective weather is sudden, short-lived and local, why does the ancient city always have the southern weather characteristics of "rain at night and sunny during the day" this summer? Will Xi Gu Chenghui transition to "Southern Climate" in the near future?
In the impression of many people, it rains mostly during the day in Xi 'an, and at least for a day or two. But this summer, intense thunderstorms and heavy rains at night often cleared up in the morning, as if nothing had happened.
Yang Wenfeng explained that the rain stopped in the morning, on the one hand, because the energy accumulated in the atmosphere has been exhausted by the rapid release of thunderstorms and heavy rains; Second, the temperature is low in the morning, and new energy has not been accumulated-"but there are also cases where energy accumulation is particularly large and it continues to rain during the day," he said.
Due to the sudden formation and strong intensity of strong convective weather, it is difficult for meteorologists to make the most timely and accurate forecast according to the previous weather laws and forecasting experience. So the weather forecast this summer is very stressful for the staff.
"The weather anomaly is a cumulative process from quantitative change to qualitative change, but this accumulation is often' lightning'." Wu Shuo, Director of Weather Service Research Office of Shaanxi Meteorological Observatory.
Wu used to do weather forecasting, but now he mainly studies forecasting business. He said that in view of the uncertainty of strong convective weather, meteorologists must closely monitor weather changes 24 hours a day.
Zhang Kexiang, chief of the short-term forecasting section of Shaanxi Meteorological Observatory, introduced that the routine process of weather forecasting is: the forecaster on duty summarizes and analyzes the data returned by meteorological satellites, radars and ground observation stations; Then make a meteorological conclusion and pass it to the chief forecaster; If there is no problem, it will be released to the public through media and other channels.
"Strong convective weather often occurs within 2 hours or 6 hours, requiring short-term forecasting to be very fast. This short-term forecast is now sent by SMS. " Zhang Kexiang said that this kind of short message is mainly sent to the leaders of party and government organs and some units with special needs for the weather, so that they can respond to the upcoming disastrous weather in time.
"At the busiest time, while we are staring at the satellite images on the screen, we should also observe the changes in the echo images of Doppler radar. At the same time, it is necessary to analyze the data, make judgments, and promptly notify the meteorological department in the area where the rainstorm is about to occur. " From the evening of September 3 to the early morning of the next day, Zhang Kexiang issued three short-term rainstorm warnings to Meixian, Chunhua and Huxian, and the rainstorm weather arrived as scheduled two hours later.
At present, there are 5 radar stations, 97 conventional observation stations and hundreds of automatic and encryption stations in the province. Even so, with the completion of Baoji radar station, Shaanxi Province will build 1800 ground observation stations in the future. In this way, the "net" for monitoring the weather will be denser and the accuracy of forecasting will be higher.
Experts from Shaanxi Meteorological Observatory have not given the conclusion whether xi will become a "southern climate". However, they believe that although Xi 'an is very humid this summer and has some characteristics of the southern weather, the weather is not a climate, and it cannot be said that it will definitely transition to the southern climate in the future.
"Since 2003, the climate in Shaanxi has experienced a quasi-three-year cycle, that is, abnormal weather will occur every other year. For example, after 2003, it will be more stable in 2004. It was slightly better in 2006, but it was abnormal in 2007. However, the current observation time is still relatively short. Whether it is a' quasi' three-year cycle and what kind of trend it will be in the future remains to be further observed. " Yang Wenfeng said.
Chang Xingyuan, former deputy chief engineer of Shaanxi Hydrology and Water Resources Survey Bureau, has been observing precipitation changes in his spare time since 13. "This year's precipitation from 65438+ 10 to September 8 is 605 mm, slightly less than last year's 688 mm, but this summer's precipitation is relatively concentrated and impressive." Chang Xingyuan, 70, thinks that although the precipitation in Xi 'an has been increasing slowly since 1995, the overall difference is not great. "It is obviously unscientific to say that Xi 'an will become a southern climate only from precipitation. "
Abnormal weather began in warm winter.
However, this year's weather anomalies did not begin in summer. Before July, there were "signs". "This year is the second warm winter after 1998. From February 2006 to February 2007, the average temperature in the province was 2 C higher than normal, especially in February this year, the temperature was as high as 3 C to 6 C. "
Yang Wenfeng said that from June to June 2007, the climate in all parts of the province was also abnormal. For example, after the warm winter, there was a severe "cold spring" weather in March, and the drought in April-May was serious, which was different from previous years.
The unusually warm winter caused the wheat seedlings in the wheat field to grow wildly. This will inevitably lead to excessive consumption of soil nutrients, which is not good for its later growth. At the same time, the high temperature at the beginning of the year, the low frost resistance of plants and the serious diseases and insect pests will also have adverse effects in the future.
With the continuous low precipitation from early April to late June, severe droughts occurred in Weinan, Shangluo, Tongchuan and Yulin. In China, the highest temperature in Beijing since February is 1840- 16℃, which is the highest temperature in Beijing since 167 in winter. Heilongjiang, the northernmost part of China, experienced the warmest winter since 195 1. "Since 2003, the global climate has been obviously abnormal, mainly from global warming." Yang Wenfeng said that global warming is the background of abnormal weather in our province. With global warming, the ocean temperature rises, evaporation accelerates, and cold air is blocked from going south. El Niñ o phenomenon affects the atmospheric circulation system to some extent. The change of atmospheric circulation system makes the climate in many places abnormal. Expert say that that most important factor leading to global war is human activities. As more and more greenhouse gases are emitted by human activities, there are more and more greenhouse gases such as carbon dioxide in the atmosphere.
"At present, all countries are making efforts to reduce greenhouse gas emissions." Yang Wenfeng said that some ruminants in nature will also release methane, and rice in plants will also release some. But these have little impact on the atmosphere, and the most important thing is the emissions caused by human activities, such as human agriculture, industrial activities, urbanization, deforestation and so on.
According to some data, the 20th century is the warmest one hundred years in the last thousand years, and the 20 years at the end of the 20th century are the warmest 20 years in the last century 100. With the intensification of global warming, increasing heat will not only cause more severe and frequent climate changes, but also trigger a series of natural disasters. For example, heavy precipitation and drought coexist; The melting of glaciers and snow is accelerating, the sea level is rising, the ocean area is expanding, rivers are diverted and dried up, and lakes are shrinking or even disappearing-when storms, blizzards and tropical storms become more frequent and rapid, the bad weather in the movie The Day After Tomorrow may really come. ...