Impact of urban clusters on spatial pattern of extreme high temperature events over Yangtze River Delta

Abstract

The Yangtze River Delta (YRD) urban agglomeration, the most prosperous region in China with hot and humid summers under the control of the Western Pacific Subtropical High (WPSH), has experienced rapid urbanization and dramatic economic development in recent decades. The combined effects of rapidly urbanization and global warming have resulted in increased frequency, intensity, and duration of extreme high temperature events (EHTEs), which poses significant challengesfor risk management over dense urban populations and intensive economic activities areas. Here we present evidences of significant impacts of the massive urbanization process on the EHTEs based on long-term meteorological observations and satellite data, by applying daily percentile, Chow test, linear tendency and spatial similarity analysis methods. The result shows that massive urbanization occurred in the period of 2003–2013 at unprecedented speed and scale, in which the urban agglomeration expanded at the speed of 1206.48 km2 per year. As a result, the core of urban heat island effects has enlarged about 18154.0 km2 from 2003 to 2013 over Nanjing, Yangzhou, Shanghai, Hangzhou, and Ningbo, leading to a significant change of spatial pattern of the EHTEs over the Shanghai metropolitan area (SHMA). The extent of SHMA has been exceeding the minimum distance with spatial gradient more than 1°C of the EHTEs climatology, inducing strongest anomalies about 2.6–2.8°Cin intensity and 30–40 d in duration compared with EHTEs climatology. Furthermore, significant correlation between the EHTEs anomalies in intensity and duration and urban areas in station-Voronoi, with the largest coefficients of about 0.65–0.68 in extreme cases of 2007, 2010 and 2013, shows that growing size of the city can amplify EHTEs. There is a significant positive correlation between the EHTE intensity/duration and urban area. This suggests that the growing size of the cities tends to amplify and extend the length of EHTEs. Through a linear regression analysis, we noted that the strongest impacts appeared in those cities with area of about 492.8–932.6 km2. Although the WPSH anchors the EHTE inter-annual fluctuation, massive urbanization has played an important role in modulating the EHTE spatial pattern over the YRD region. From a pure dynamical point view, the WPSH circulation system is the fundamental driving force of the extreme high temperature events in the eastern China. However, owing to the superimposed effect of global warming and heated island, the EHTEs became more frequent and stronger and lasted longer over the YRD urban agglomeration in the past decades. Under the similar condition of the WPSH, the EHTE days in 2003–2013 were 1.5 times as large as those in 1959–1978. There is a significant positive linear trend in EHTE days and maximum temperatures from 1979 to 2014. The change of spatial pattern of EHTEs is to a large extent caused by rapid urbanization over the YRD region. Thus future extreme high temperature risk assessment should focus not only on the strength and duration changes but also on the spatial pattern change. It is anticipated that the current trend of EHTE frequency and strength will be magnified with continuing global warming and urban agglomeration. Changes in strength, duration and spatial pattern of the EHTEs will no doubt induce more complex, adverse effects on environment, human health and economics. The diagnosis tool above may be easily implemented to assess the climate effect of the urbanization over other major metropolitan regions in China.