Analysis Of The Characteristics Of Extreme Temperature Changes And Its Causes In Megacities Of China

Based on the daily temperature data of 61 meteorological stations in megacities of China, NCEP/NCAR reanalysis data from 1958 to 2013, NOAA provides global sea surface temperature (SST) data, and Chinese statistical yearbook data using the methods of conventional statistics, wavelet analysis, SVD, CCA etc., the spatial-temporal characteristics of extreme temperature changes and relationships with atmospheric circulation, SST and urbanization were analyzed. The results show that:(1) Daytime extreme low temperature and nighttime extreme low temperature was a growing trend in most cities,but extreme low temperature days during the day and extreme low temperature days at night, showed a decreasing trend, and reduced more at night. Extreme high temperature and days increased in most of cities, but decreased from Northeast to Southwest of China. At night, the extreme high temperature and days almost all showed increasing trend, and the increase is larger at night, and the increase was more obvious than during the day. The trend of extreme low temperature changes was positive with time, but more significantly at night than daytime. The extreme low temperature change is tiny during daytime, the largest increase of the extreme low temperature was in Central of China, about 1.6℃/10a. Extreme temperature changes with time during the day:the Northeast and Central of China showed negative growth, showed positive growth in others, the most significant increase in east China, about 0.24℃/10a. Extreme high temperature showed no significant trend at night. Daytime extreme high temperature days were the largest increase in South of China, with an average increase of about 4.7 days per decade; Extreme high temperature days showed significantly increasing trend at night, North of China was largest,about 4.4 days per decade, East of China was minimum, about 1.7 days per decade. And most extreme temperatures were mutated, extreme temperature days cycles of each region have emerged 4a annual cycle around.(2) The Northern Hemisphere 500hPa geopotential height showed strong association with extreme temperature events in the mega-cities of China.. In the same period the Northern Hemisphere 500hPa geopotential height with extreme low temperature was a positive correlation. When the Northern Hemisphere 500hPa geopotential height in the same period enhancement, the number of extreme temperature days was increasing. The Northern Hemisphere 500hPa geopotential height with extreme high temperature days,the most of cities showed positive correlation, but there was also some negative correlation. Western Pacific SST were also related to extreme temperature events in the mega-cities of China. Extreme low temperature days were closed related to pre-summer western Pacific SST. When early summer SST in equatorial and China Offshore rise, high-latitude SST reduce, the number of extreme low temperature days are reducing in the mega-cities of China; When the low-latitude early winter SST of western Pacific decrease, the number of extreme high temperature days is decreasing in most cities,but increasing in East and North of China. Changing in extreme high temperature days is more complex than extreme low temperature days of diurnal variation, which may be due to high changing about temperature is the result of many factors, the atmospheric circulation or SST is just one factor only.Urbanization factors had a significant impact on extreme temperature events in the mega-cities of China. Demographic factors was the most significant to extreme temperatures in factors on urbanization, and extreme temperature changes rate at night was more obvious than during daytime, Changes in the number of days of extreme low temperatures during the day was minimum.