This article is selected from the journal of the Chinese Academy of Engineering, China Engineering Science, No. 9, 2012
Author: Zhai Panmao, Liu Jing.
Source:Extreme weather and climate events and disaster prevention and mitigation under the background of climate warming[J].Strategic Study of Chinese Academy of Engineering,2012,14(9):55-63,84.)
Editor's note
In the context of global warming, extreme events are frequent and intensifying, and the risk of disasters faced by human society is increasing. The frequency of heavy precipitation in the Yangtze River basin tends to increase, while the drought range in North China and Northeast China tends to expand, and the drought in Southwest China tends to be frequent. Therefore, it is urgent to assess the risk factors of disasters according to local conditions and formulate risk management measures to reduce the impact of extreme weather and climate events.
In the 9th issue of 2012, the journal of the Chinese Academy of Engineering, Chinese Academy of Engineering, published an article entitled "Extreme Weather and Climate Events and Disaster Prevention and Mitigation in the Context of Climate Warming" by the team of researcher Zhai Panmao from the State Key Laboratory of Severe Weather of the Chinese Academy of Meteorological Sciences. This paper first summarizes extreme weather and climate events and the relevant definitions of "climate extremes", and divides extreme events into single-element extreme events, extreme events related to weather phenomena, multi-element extreme events and extreme climate events. This paper points out that under the background of climate warming, heavy precipitation events in the middle and lower reaches of the Yangtze River in mainland China are more frequent, and the high temperature and heat wave weather in the eastern part of the mainland is more obvious, the drought trend in Northeast China and North China is increasing, especially in the late 20th century and early 21st century, and the drought in southwest China tends to be more frequent in recent years. In order to alleviate the losses caused by the increasing number of major meteorological disasters, it is necessary for the mainland to strengthen the monitoring and early warning capacity of high-impact extreme events, and at the same time, it is also necessary to strengthen engineering defense measures in accordance with the law of extreme weather and climate events, so as to prevent and respond to floods and urban floods caused by heavy precipitation, as well as major droughts and high temperature and heat waves related to the continuous lack of precipitation.
I. Preface
In the context of global warming, the changes of extreme weather and climate events have attracted extensive attention from scholars at home and abroad. According to the IPCC (Intergovernmental Panel on Climate Change) 4th Assessment Report, the global average temperature has risen by 0.74 °C in the past 100 years, and the global temperature has risen at a rate of 0.13 °C/10 a in the past 50 years. Global warming appears to have made summers hotter and winters milder, accompanied by an increase in the frequency and intensity of heatwaves, with a more pronounced reduction in the number of frost days, fewer cold extremes, and more pronounced nights. At the same time, the frequency of heavy precipitation has increased in regions that account for more than half of the world's land area over the past 50 years. Heavy precipitation events have increased in the United States, China, Australia, Canada, Norway and Mexico, Poland and the former Soviet Union. Droughts are more intense and last longer due to higher temperatures and lower regional precipitation.
The intensification of extreme weather and climate events will have a serious impact and losses on society, economy and people's lives. China is a country prone to natural disasters such as droughts and floods, typhoons, and cold waves, with frequent droughts, snowstorms, cold waves, and sandstorms in the northern part of the mainland, and typhoons, high temperatures, and rain and floods in the southeastern region (see Figure 1). According to statistics, since 1949, the loss of meteorological disasters has tended to increase, and the annual loss since the mid-90s of the 20th century has been close to or more than 2 000 trillion yuan, of which in 1998, affected by extreme events such as continuous heavy precipitation in the Yangtze River and Nenjiang River basins, the national meteorological disaster loss reached 3 000 trillion yuan, and in 2008, the continuous low temperature rain, snow and freezing events in the southern part of the mainland also caused serious economic losses of more than 3 000 trillion yuan, 2010 The annual losses caused by debris flows and landslides caused by the sudden heavy precipitation in Zhouqu amounted to 5 000 trillion yuan (see Figure 2).
Fig.1 Comprehensive schematic diagram of the distribution of major meteorological disasters in China
Note: Quoted from the 2007 Atlas of Severe Weather and Climate in China
Fig. 2 Changes in losses from meteorological disasters closely related to extreme weather from 1949 to 2010 Meteorological disasters are closely related to the frequency and intensity of extreme events on the one hand, and the economic development level and disaster prevention and mitigation capacity of the mainland on the other. Strengthening research on changes in extreme events related to climate change is conducive to national disaster prevention and mitigation and response to climate change. Given that the impacts of different types of extreme events vary widely, monitoring, predicting and evaluating extreme events and their impacts requires rigorous definitions and quantitative indicators for each type of extreme event. This paper summarizes the indicators and applications of extreme events such as extreme temperature, extreme precipitation, typhoon, hail, fog and haze, drought, cold wave, and sandstorm, and discusses the existing problems and key tasks that need to be paid attention to in the future.
2. Definition of extreme weather and climate events
Extreme weather and climate events are rare meteorological events that occur in a specific area and time, and are considered to have occurred when the weather and climate state of a place deviates significantly from its climatic average. In a statistical sense, extreme events are considered to be low-probability events, and some people believe that they are once in 50 years or even once in 100 years. In terms of time scale, extreme weather events are rare or high-impact meteorological events with a short time scale (generally less than a week). Extreme climate events, on the other hand, are usually the result of the accumulation of extreme weather events. Extreme events can be divided into two categories in terms of the nature of extremism. The first category is defined by relying on the extremes of basic weather and climate elements, such as extreme heavy precipitation, extreme heat and low temperature events. In climate change research, the evolution of extreme events is usually analyzed by analyzing the trends of these elements. Another type of extreme event is a catastrophic extreme event (such as drought, flood, heat wave, etc.) that has a significant impact on the natural environment and usually brings greater economic losses, these extreme weather and climate events are also called comprehensive extreme events, because generally speaking, they are not caused by the anomaly of a single meteorological element, but due to the joint action of two or more meteorological factors, such as the reduction of precipitation and global warming are two important factors that cause drought intensification.
At present, the research and business is mainly based on the definition of single station monitoring technology, using "climate extremes" to quantify and characterize abnormal weather and climate phenomena, and when climate factors (such as temperature, precipitation, etc.) reach the defined climate extremes, extreme events can be considered to occur. There are two main ways to deal with the selection of extreme value thresholds, namely absolute extreme value and relative extreme value. Although the definition of this threshold is relatively simple, it can also better characterize the characteristics and change laws of extreme events to a certain extent, so it is still favored by scholars and staff from all walks of life. At the same time, "extreme weather and climate" has a spatiotemporal relativity, for example, 30 mm of precipitation per day is normal precipitation in the coastal areas of South China and the lower reaches of the Yangtze River, while extreme precipitation has reached the level of extreme precipitation in Northwest China, even in the same region, the impact of the same precipitation in different seasons may be different. Therefore, it is necessary to determine the relative thresholds for certain events, such as daily precipitation, etc. At present, a certain percentile value (such as the 95th percentile value) is often used as the threshold value of the extreme value in the world, Zhai Panmao et al. used the percentile method to analyze and study the changes of extreme events of temperature and precipitation in the northern part of the continent in the past 50 years, and this method has been widely recognized and applied in the study Γ of continental extreme climate 3), Jiang Zhihong et al. used the Γ distribution model to fit the probability distribution of regional precipitation percentage, and then deduced the probability of drought and flood in the summer half year (April to October) in the Huanghuai River Basin, and discussed the spatial and temporal distribution characteristics of drought and flood probabilities at all levels in this region. Li Wei and Zhai Panmao used the Γ distribution function to fit the probability distribution of precipitation on rainy days and define extreme precipitation events based on the daily precipitation observation data of ground stations in China from 1951 to 2004, and then analyzed the relationship between extreme precipitation days and ENSO (EL Nino Southern Oscillation).
Fig. 3 The probabilistic density function and frequency distribution curve of daily Γ precipitation distribution in Beijing and Guangzhou are mostly based on daily climate data. Since the late 90s of the 20th century, some scholars in mainland China have begun to use China's daily meteorological data to carry out analysis of the change pattern of various extreme weather indicators, and these studies are still of general significance today.
3. Extreme weather and climate event indices and their changes
In recent years, the Intergovernmental Panel on Climate Change (IPCC) has evaluated the changes in extreme weather and climate events and their understanding of the impact on the natural and physical environment. Three areas are emphasized: a. Changes in weather and climate extremes in the atmosphere (variables such as temperature, precipitation, wind, etc.); Weather and climate phenomena affecting the occurrence of extreme weather and climate events (monsoon, El Niño, other variability modes, tropical cyclones, extratropical cyclones); Impacts on the natural physical environment (droughts, floods, extreme sea levels, waves, coastal impacts, glacial, topographic and geological impacts, changes including high-latitude permafrost, sandstorms). This classification links extreme weather and climate events with their weather, climate, environment and impacts, but it is also easy to confuse the conditions that form extreme events with the disasters caused by extreme weather and climate events. This paper modifies the above classification methods, mainly starting from the influencing factors of extreme events, and divides them into single-element extreme events, extreme events related to weather phenomena and multi-element extreme events, and further analyzes the climate change characteristics of these extreme events. In terms of the duration of extreme events, extreme events are sometimes divided into extreme weather and extreme climate events. Most of the extreme events covered in this article should fall under the category of extreme weather events due to their short time scales. Due to their long duration, drought is the result of extremely low precipitation over a long period of time, and is a typical extreme weather event.
(1) Single-element extreme events
In order to effectively promote countries around the world to carry out extreme weather and climate event change detection research, WMO (World Meteorological Organization) Climate Committee and other organizations jointly established the Expert Team on Climate Change Detection and Indicators (ETCCDI) and defined 27 typical climate indicators, including 16 temperature indices and 11 precipitation indices (see Tables 1 and 2). Scholars at home and abroad have used these basic indices to discuss various extreme temperature and precipitation events. Manton found that in Southeast Asia and the South Pacific, hot days and warm nights have increased significantly since 1961, while cold days and cold nights have decreased. Kunkel, Kostopoulou, and Sen analyzed extreme precipitation in the United States, Italy, India and other countries, respectively, and showed that the amount and frequency of extreme precipitation increased in most parts of the world. Zhai Panmao et al. pointed out that the number of days with extremely low nighttime temperature in the northern part of the mainland decreased significantly, and the number of days with high daytime temperature tended to increase. The number of frost days on the continent has decreased significantly as the continental temperature has risen significantly, especially since the mid-80s of the 20th century, with a significant increase in the rate of warming (see Figure 4). Wang Xiaoling and Zhai Panmao analyzed the linear trends of annual precipitation, precipitation frequency and average precipitation intensity in eight regions of China from 1957 to 2004 according to different levels of precipitation intensity. The northeast and north-west parts showed a weakening trend, while the rest of the region showed an intensifying trend. The number of days of extreme heavy precipitation per year decreases in Northeast China, North China and the Sichuan Basin, while increasing in the western region and the middle and lower reaches of the Yangtze River to South China (Fig. 5). Alexander is the first to give a global trend of extreme temperature and precipitation over land, based on the results of expert studies in different regions, but there are still spatial gaps in regions such as Africa and the Middle East.
Table 1 Extreme temperature index
Table 2 Extreme precipitation index
Fig. 4 Changes in the number of frost days from 1951 to 2010 based on daily minimum temperatures
Fig. 5 Trends in the number of days of extreme heavy precipitation from 1951 to 2004
Note: The solid (hollow) dots represent increases (decreases) and depending on the size of the icon, the trend is greater than 7.5% and less than 7.5%, respectively, and the cross represents a significant level above 0.05
Wind is an important meteorological factor affecting human safety, maritime aviation activities, and infrastructure construction. In the past 50 years, the annual average wind speed in China has been decreasing year by year, and Jiang et al. pointed out that the number of windy days in China has also shown a decreasing trend. Although the concept of extreme wind speed is not given in the 27 indices of ETCCDI, some studies at home and abroad have also carried out studies on extreme wind speed changes.
(2) Extreme events related to weather phenomena
In addition to extreme events related to individual elements such as temperature, precipitation and wind, extreme weather and climate events closely related to severe weather phenomena such as typhoons, hail, fog and haze also have a greater impact on socio-economic and human activities.
A typhoon is a strong cyclonic eddy with a warm central structure that occurs in the tropical ocean, always accompanied by violent storms and rains, and often causes serious disasters in the affected areas. China is one of the countries with the most tropical cyclone (TC) landfall and the most severe disaster in the world, with an average of 7~8 landfalls per year. The intensity of a typhoon is usually judged based on the maximum average wind speed on the ground at the center of the typhoon and the minimum pressure at sea level at the center of the typhoon.
Based on the data of tropical cyclone yearbooks from 1949 to 2006, Liu et al. analyzed the climatic characteristics of TC landfall in continental TC according to the newly formulated TC classification standard in 2006, and the results showed that the average intensity of TC landfall showed a weakening trend, but since the beginning of the 21st century, the average intensity has increased significantly, especially the extreme value of TC intensity is more obvious in the 21st century.
Hail is a kind of violent meteorological disaster caused by strong convective weather system, it has a short duration, a small range of action, but the intensity is generally very large, and the damage to agriculture is the greatest, Xie et al. counted the interannual variation characteristics and trends of hail frequency in China from 1960 to 2005, and the results show that there was no significant change in the annual average hail days before the 80s of the 20th century, and the annual average precipitation days had a significant decreasing trend after that.
Fog and haze severely affect atmospheric visibility and have important impacts on transportation, human health, and crop production. According to the visibility, the fog is divided into heavy dense fog, dense fog and heavy fog, Chen Xiaoxiao et al. analyzed the interdecadal variation of different grades of fog, and the results show that for most areas, heavy dense fog has an increasing mutation in the 70s of the 20th century, while dense fog and fog do not have this characteristic. Gao Ge used the statistics of haze days in China from 1961 to 2005 to discuss the change trend of haze, and the study showed that the haze days in most parts of the eastern part of the mainland mainly showed an increasing trend, while most of the western and northeastern regions showed a decreasing trend.
(3) Multi-element extreme events
1. Cold tide
The cold wave weather process is a large-scale strong cold air activity process, which is mainly characterized by severe cooling and strong winds, sometimes accompanied by rain, snow, rime or frost, which will have a great impact on agricultural production, human activities and transportation. The Central Meteorological Observatory delineates the intensity of cold air activity by combining process cooling and temperature negative anomaly, and when the process cooling reaches more than 10 °C and the absolute value of the temperature negative anomaly reaches more than 5 °C, it can be regarded as a cold wave event. Lau et al. defined cold wave in three criteria, namely, cooling greater than 5 °C, the ground pressure difference between Chinese mainland and coastal areas greater than 5 hPa, and prevailing northerly wind speed greater than 5 m/s in the northern part of the South China Sea. Central Siberia (70°~90° E, 43°~65° N) is the key area of the cold wave, where most of the cold air accumulates and strengthens and invades the continent. From the perspective of interdecadal variation, the average cold wave frequency in China from 1961 to 2010 showed a significant decreasing trend, and its linear trend coefficient was -0.3 times/10 a, which passed the significance test of 95% (Fig. 6). Wei Fengying's results show that the frequency of nationwide cold wave disasters in winter and spring decreases significantly after climate warming, and points out that this decreasing trend is related to the background of the strengthening trend of AO phase. The results of Qian Weihong et al. showed that the cold wave that occurred in the mainland was the most common in the north, the cold wave in the northeast began in October, and the cold wave in Hetao and Jiangnan was more frequent in April.
Fig. 6 Variation curve of average cold wave frequency in China from 1961 to 2010
2. Sandstorms
Dust weather mostly occurs in arid areas, and the air is full of sand and dust, which will reduce visibility and have a negative impact on transportation and health. If strong winds are sustained, sandstorms can reach winds of 12 or more, and their destructive power is far greater than that of ordinary winds of the same magnitude.
People generally delineate the level of sandstorms according to visibility and wind speed, and the sandstorm intensity level standard of the China Meteorological Administration divides sandstorms into three grades according to visibility: sandstorms, strong sandstorms, and extra-strong sandstorms correspond to visibility of 500~1000 m, 50~500 m, and respectively<50 m。 On this basis, Kang Ling et al. considered the severity of the sandstorm disaster, and divided the sandstorm into four grades, namely, weak sandstorm, sub-strong sandstorm, strong sandstorm and extra-strong sandstorm, and the corresponding visibility was 500~1000 m, 200~500 m, 50~200 m and ≤50 m, respectively, and used this index to use the sandstorm, visibility, The results show that in a year, sandstorms, strong and extremely strong sandstorms in Inner Mongolia are concentrated from March to May in spring, with the most in April, and the sandstorms are relatively concentrated in the second half of the year, and the first and second months are the periods when large and small strong and extremely strong sandstorms are prone to occur. Li Dongliang et al. defined sandstorm events with horizontal visibility less than 1000 m, selected 185 conventional meteorological observation stations in China, analyzed the climatic characteristics of sandstorms in northern China in the past 50 years, and showed that the number of sandstorm days in China showed a decreasing trend, and the 90s of the 20th century was the least in the past five decades, and pointed out that China's sandstorms are closely related to the surface heat of the Qinghai-Tibet Plateau in summer.
(4) Drought
Droughts are typical extreme weather events caused by chronic lack of precipitation. Drought and its formation mechanism is an ancient but challenging research topic, and it is a major natural disaster faced by mankind, and the trend of aridification has become one of the main issues concerned by scholars at home and abroad under the background of global warming. There are various definitions of the drought index, but in general, one is an indicator that only considers precipitation as a single factor, and the other is an indicator of comprehensive water profit and loss.
Precipitation drought indicators (such as low precipitation index, precipitation anomaly percentage index, etc.) are to study the statistical distribution of precipitation through meteorological methods or to reflect the intensity and duration of drought by the number of days without precipitation. The standardized precipitation index (SPI) has been widely used in drought and flood monitoring by the National Climate Center of the Continent.
Precipitation-evaporation, evaporation/precipitation, precipitation-crop water requirement, crop water requirement/precipitation and other drought indicators consider the influence of precipitation and temperature changes, precipitation data are easy to obtain, and there are many calculation schemes for evaporation, usually replaced by potential evapotranspiration, common methods for calculating potential evapotranspiration include Thomthwaite, saturation difference, Penman and other models, among which Penman model and its modification scheme are the most widely used in the mainland.
The national standard of "Meteorological Drought Grade" recommends the use of the comprehensive meteorological drought index Ic, which is obtained by combining the standardized precipitation index and the relative wetness index of precipitation, which reflects the climatic anomalies of precipitation on short time scales (monthly) and long time scales (seasonal), and also reflects the water deficit on short time scales (affecting crops). The comprehensive meteorological drought index was used to analyze the drought changes in China and different regions in the past half century, and the results showed that the aridification trend in Northeast China and North China was significant (Fig. 7), and major drought events in Southwest China have also occurred frequently in recent years.
Fig. 7 Changes in drought area in Northeast China and North China from 1951 to 2010
There are many indicators related to the mechanism of drought occurrence, among which the most used is the PDSI indicator proposed by Palmer, which is a comprehensive indicator that considers water, possible evapotranspiration, soil moisture in the early stage, and runoff. As early as the 70s of the 20th century, the index was introduced into China, and the index was revised according to the actual situation of the mainland, and many meaningful research results were obtained. Zhai Panmao et al. analyzed the characteristics of drought in China from 1951 to 2003 using the Drought Monitoring Precipitation Index (PDSI), and the results showed that in the past half century, the drought occurred in the continent mainly in the 60s, late 70s to early 80s of the 20th century, and from the late 90s to the early 21st century in the late 90s of the 20th century. In the past 10 years, droughts have occurred frequently in southwest China, which has had a significant impact on the production and life of local people.
For drought, according to statistics, there are a total of 55 indicators in various countries around the world, and some people divide drought into four categories: meteorological drought, hydrological drought, agricultural drought and socio-economic drought, and define their corresponding drought index. Some indices have regional and field of application limitations. Therefore, the selection of extreme indicators for the monitoring of drought events should be context-specific, and the misuse of indicators is likely to lead to conclusions that do not correspond to reality.
(5) The trend of extreme events in the context of climate warming
Since the 70s of the 20th century, the variation characteristics of extreme weather and climate events in the continent have shown obvious differences (see Table 3), with a significant decrease in frost and cold waves, no obvious changes in high temperature and heat waves, and a large spatial difference in the trend of extreme precipitation events, and a decreasing trend in wind, tropical cyclones, hail, fog and haze, and sandstorms.
Table 3 Definitions of climate indices for various extreme weather and their trends in the context of global climate change
The study of the relationship between climate warming and extreme weather and climate events involves complex questions about the formation mechanism. Trenberth points out that an increase in surface temperature increases surface evaporation, increasing the ability of the atmosphere to retain water and increasing the amount of moisture in the atmosphere. Increased surface evaporation will make droughts more susceptible to evaporation, and precipitation will increase to balance evaporation, making it more prone to flooding. Liu et al. discussed the relationship between precipitation and temperature at different intensities, and pointed out that when the global average temperature increases by 1 K, the extremely heavy precipitation (90% ~100% of the most intense precipitation) increases by about 94.2%, while the 30% ~60% percentile precipitation decreases by about 20%, and the increase of global average precipitation intensity with temperature is significantly greater than the change of atmospheric water content. Therefore, it is indeed worth paying attention to the impact of climate warming on precipitation extreme events through the atmospheric water cycle. Due to the weakening of cold air activity, the frequency and intensity of cyclones in Mongolia decreased, and the average number of gale days and maximum wind speed in China showed a weakening trend, corresponding to the decrease in the frequency of sandstorms in China. Large-scale warming may also reduce the frequency of fog, making the number of fog days in most parts of China decrease in half a century.
In addition, the frequent occurrence of extreme weather and climate events is closely related to ENSO events, monsoon anomalies and different climate anomaly modes, and extreme weather and climate events are more prominent in large-scale circulation anomalies.
4. Response to extreme weather and climate events
In the context of global warming, various extreme events in the continent show different spatiotemporal evolution characteristics. Since the middle of the 20th century, although not all extreme events have increased, the frequency of heavy precipitation in the Yangtze River basin of the mainland has tended to increase, while the drought scope in North China and Northeast China has tended to expand, and drought has occurred frequently in Southwest China in the past 10 years.
Extreme weather and climate events such as extreme drought, persistent heavy precipitation, super typhoons, strong cold waves, and regional high temperature and heat waves will cause huge economic losses and casualties, which can be called major meteorological disasters. Climate warming will cause the intensity and frequency of some extreme events to increase, human society will face higher disaster risks, and with the development of the economy, the economic losses of major meteorological disasters continue to increase, so it is necessary to strengthen research, response and defense. Since the severity of extreme weather and climate events depends on the nature and intensity of the event itself, as well as the disaster-bearing environment, the vulnerability of the carrier and the ability to prevent disasters, it is necessary to respond from the following aspects.
(1) It is necessary to establish an index and indicator system for high-impact extreme events, and strengthen the monitoring, research and early warning of the occurrence and development of high-impact extreme weather and climate events. With the deepening of research, scientific and technological workers have gradually realized the importance of exploring the formation mechanism of extreme weather and climate events. In future work, it is necessary to further strengthen the monitoring and research of high-impact extreme events, deeply explore the formation mechanism and prediction technology of extreme weather and climate events, especially high-impact extreme events, and continuously improve the level of early warning of severe weather and climate on the mainland.
(2) It is necessary to strengthen the defense capacity of high-impact extreme events. According to statistics, the economic losses caused by extreme events in developed countries are relatively large, while the casualties caused by extreme events in developing countries are more serious, which is largely related to the weak ability of developing countries to resist disasters and the insufficient ability to warn of extreme events. Therefore, improving human vulnerability and exposure to extreme events can help improve a country's capacity for disaster prevention, resilience and mitigation. For example, in view of the frequent occurrence of high temperature and heat waves, we can deal with them by strengthening the construction of early warning systems, improving refrigeration facilities in public places, and improving urban infrastructure; in view of the increase in the frequency and intensity of heavy precipitation, in order to reduce the harm of sudden floods, we can strengthen the construction of flood control projects, improve the quality of construction, improve urban drainage systems, and other risk prevention measures; and reduce the impact of drought by strengthening the development of water-saving agricultural technology and the construction of drought-resistant projects for water resources management.
Meteorological disasters faced by human beings have great regional characteristics and economic and social impact attributes, and it is necessary to assess the risk factors of disasters according to the actual local conditions, and formulate risk management measures according to local conditions to reduce the impact of extreme weather and climate events.
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