Assessing summertime urban warming and the cooling efficacy of adaptation strategy in the Chengdu-Chongqing metropolitan region of China.

Western China has experienced rapid urbanization since the Chinese reform process began in the late 1970s. It is essential to study the spatiotemporal patterns of warming induced by historical and future urban expansion and to evaluate adaptation strategies for the Chengdu-Chongqing metropolitan region (CCMR) in western China. The observed urban heat island intensity was ~1.5K in July 2009-2011. We employed the weather research and forecasting (WRF) model using real and projected urban land-use data to simulate near-surface air temperatures for a crop, urban in 2010 and urban in 2030 scenarios in summer over the CCMR. The difference between urban 2010 and cropland scenarios is 0.93K. Warming induced by urban development in 2010-2030 is in the range of 1-1.5K, but warming induced by future urban development will be less intense than historical warming over eastern China. We increased roof albedo to 0.8 to assess the difference in near-surface air temperature between cool roofs (CR) and urban 2030 scenarios, which represents the maximum potential impact of CR; we also assessed the cooling caused by green roofs (GR) (i.e., the difference between the GR and urban 2030 scenarios). Greater cooling occurs during the day due to reflection of solar radiation by CR and additional water evaporation by GR. We provided an evaluation criterion, cooling efficiency (CE), to measure the local performances of CR and GR. CE represents the local cooling capability based on urban warming rather than absolute cooling over a larger spatial scale. CE reveals a lower nocturnal cooling capability, which poses a significant challenge to the applications of CR and GR at night. CR has a better cooling capability across CCMR than GR, only when roof albedo of CR exceeds 0.68. Measures enacted should be appropriately adjusted to optimize for cost, technology and energy savings.