Contribution of Climate Change and Urbanization to the Variation of Extreme Precipitation in the Urban Agglomerations over the Loess Plateau

Na Wei^1,2, NinglianWang^1*, Yurong Zheng¹

¹Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, Northwest University, Xi’an, Shaanxi Province, China, 710027

² Climate Centre of Shaanxi Province, Xi’an, China, 710049

Abstract: With global warming and rapid urbanization, urban agglomerations over the Loess Plateau (LP) are suffering from various urban disasters. Urbanization has aggravated the decreasing trends of extreme precipitation in Taiyuan and Xi’an urban agglomerations (UAs) and enhanced the increasing trends of extreme precipitation in Luoyang, Hohhot and Xining UAs during 1979–2018. Meanwhile, the number of light rain days decreases in almost all the cities, indicating the sensitivity of light rain days to urbanization. The climate change is a primary contributor to the change of urban precipitation during 1980–2000. However, the urbanization contribution has been increasing gradually since 2000, and the urbanization further amplifies the trend of extreme precipitation caused by the climate change. In terms of the physical mechanisms, the rapid increasing surface temperature and aerosol particles are closely related to the urban precipitation. Our findings provide a systematic understanding of the urbanization effects on the extreme precipitation over the LP and may play an important role in the mitigation of urban disasters.

Key words: Urban Extreme Precipitation Climate Change Urban Heat Island

Global warming and urbanization are both rapidly changing the urban climate environment, causing a series of urban climate problems, such as urban heat island (UHI), urban waterlogging and water shortage, and air pollution (Della-Marta, Haylock et al. 2007, Han, Baik et al. 2014, Ma, Li et al. 2018, Paul, Ghosh et al. 2018). The UHI has been a common phenomenon resulted from the urbanization, but it demonstrates different impacts on the precipitation under different climatic backgrounds (Wang, Zhou et al. 2007, Daniel, Lemonsu et al. 2018, Ma, Li et al. 2018, Yan, Chan et al. 2020, Yang 2020){Chrysanthou, 2014 #30;Mazdiyasni, 2015 #23}. Many studies revealed that the UHI can induce updrafts and trigger moist convections, resulting in more frequent extreme precipitation near the urban areas, such as Beijing and the Pearl River Delta, than in the rural areas (Della-Marta, Haylock et al. 2007, Drobinski, Silva et al. 2016). In the urban agglomerations (UAs) of the Yangtze River Delta in East China, there is a prominent urban dry island (UDI) effect, which is characterized by reduced humidity and increased vapor pressure deficit in the urban core area (Du, Wang et al. 2019, Luo and Lau 2019). Some researchers concluded that the urbanization tends to augment the precipitation trends caused by the climate change at national scale in China (Gu, Zhang et al. 2019). Moreover, the UHI effects are enhanced more obviously in wet climates than in dry climates (Zhao, Lee et al. 2014). It can be found that the urbanization impact on temperature and precipitation may also be different under different climatic backgrounds and terrain conditions (Liao, Liu et al. 2018). Climate change is a dominant contributor for the urban precipitation trends, but the urbanization has been exhibiting a relatively strong influence since the reform and opening up in China. Assessing the contributions of climate change and urbanization to the changes in urban precipitation is important for the disaster risk management.

Previous studies mainly evaluated the urbanization effect by analyzing the difference between urban and rural meteorological observation stations (Zhao and Wu 2017, Luo and Lau 2018, Ma, Li et al. 2018). Because of the urban expansion, the distance between urban and rural stations and the number of urban and rural stations vary greatly, which may cause uncertainty of the urbanization effect evaluation (Song, Zhang et al. 2014). High-resolution gridded precipitation data and artificial impervious area have been well applied in urban precipitation researches (Su, Li et al. 2019). Most of these studies focused on individual UAs such as Beijing, Shanghai and the Pearl River Delta. There are also a few studies that give an overall landscape of the urbanization impact on climate at the national scale in China (Gu, Zhang et al. 2019, Lin, Gao et al. 2020). Therefore, it is very interesting to study the variation characteristics of the precipitation in the UAs within a unique climate zone.

The Loess Plateau (LP), located over the central-northern part of China and covering seven provinces and the middle reaches of the Yellow River, belongs to the wet-arid transition area and hence is sensitive to climate change. In recent decades, it has suffered severe water and soil losses and ecological environment changes, with decreasing trends of precipitation extreme in the Fen-Wei River valley and increasing trends of rainfall intensity and frequency in the northwestern LP, which exert negative effects on the social and economic developments (Zhao, Li et al. 2017, Zhang, Gao et al. 2020). Meanwhile, the LP has experienced drastically rapid urbanization in the last 20 years, and six typical UAs (Taiyuan, Xi’an, Hohhot, Yinchuan, Xining-Lanzhou and Luoyang) home to 70 percent of the LP population play an important role in the economic development and the ecological security(ＭＡ ＢＢ 2019). Precipitation over the LP is primarily affected by the complex terrain, the land-atmosphere interaction and the monsoon. During July to September, the East Asian summer monsoon has a dominant effect on the LP at a spatial scale far beyond the urban areas, which may overwhelm the local urban impacts. However, during April–June, the rainfall is much weaker and thus local urban effects may be more evident.

In this study, we present the first analysis of the changes of urban extreme precipitation over the LP in different climate zones based on a set of high-resolution gridded precipitation data. We also evaluate the contributions of climate change and urbanization to the urban precipitation across several UAs over the LP during different periods by using an effective attribution method.

The paper is structured as follows. Section 2 introduces the study area, the data, the definition of extreme climate events and the methods used in the study. Section 3 presents and discusses the analysis results. Conclusions are made in Section 4.