URBANIZATION EFFECTS ON OBSERVED CHANGES IN SUMMER EXTREME HEAT EVENTS OVER ZHEJIANG PROVINCE, EAST CHINA

Although the urban heat island(UHI) is a well-documented phenomenon, relatively little information in the literature is available about its impact on summer extreme heat event(EHE). As UHI is characterized by increased temperature, it can potentially increase the magnitude and duration of EHEs within cities. Based on daily maximum temperature records from 62 observation stations in Zhejiang province from the period 1971-2011 and satellite-measured nighttime light imagery from the Defense Meteorological Satellite Program(DMSP) Operational Linescan System(OLS) during 1992-2010, we analyzed the long-term change of summer EHEs and its association with the rapid urbanization process. The results could be concluded as follows:(1) Zhejiang has experienced rapid urbanization and dramatic growth in urban areas in the past two decades, especially after 2000.(2) The summer mean maximum temperature and the 95 th percentile of summer daily maximum temperature in most of its stations have increased, with the most significant increase occurring in the highly urbanized areas including the city belt around Hangzhou Bay, Taizhou-Wenzhou and Jinghua-Yiwu city belts.(3) The hot days and hot-day degrees, defined by both daily 95 th percentile and the threshold of 35℃, show that the UHI effect causes additional hot days and heat stress in urban stations compared to rural stations.The results in this study suggest that the UHI effect should be determined and incorporated in preparing high temperature forecasts in cities.

Regional changes in extreme heat events in China under stabilized 1.5℃ and 2.0℃ global warming

Extreme heat events(EHEs)have a significant impact on the social economy and human health.China is a country with a large population and diverse terrain,and it is necessary to project future extreme heat changes in the sub-regions.This study used a specially designed dataset,the Community Earth System Model(CESM)simulations,namely CESM low-warming,to investigate the EHEs in China under 1.5℃ and 2.0℃ global warming.The results indicate that the regional mean warming over China will exceed the global average,about 1.63℃ and 2.24℃ in 1.5℃ and 2.0℃ warmer futures.Compared to the present-day(1976–2005),the frequency and duration of the EHEs in South China are projected to increase the most among the sub-regions.For example,the frequency of EHEs in South China at 1.5℃ and 2.0℃ warming will exceed 3 and 3.5 times the present-day level.However,when global warming rises from 1.5℃ to 2.0℃,the increased impacts relative to the 1.5℃ warming level will be the lowest in South China(less than 40%),and the highest increased impacts are projected to appear in Northeast China(53%-84%)and Northwest China(53%–107%).The main reason for this situation is that compared with the 1.5℃ scenario,the upper zonal westerly in northern China weakens and the continental high pressure enhances under the 2.0℃ scenario.Therefore,limiting global warming at 1.5℃ instead of 2.0℃ is beneficial for eliminating extreme heat events,especially for Northeast China and Northwest China.