Title: Rapid Urbanization and Heavy Rainfall over the Yangtze-River Delta
Lecturer: Dalin Zhang, University of Maryland
Time: Tuesday January 16, 2024 at 2:00 PM
Venue: Lecture Hall D103, School of Atmospheric Sciences, Xianlin Campus
Abstract: The urban heat island (UHI) effects tend to produce more rainfall on its downwind side than that on the other sides, but alone could hardly account for the generation of heavy rainfall. In this talk, I will first present a 44-yr (i.e., 1975-2018) climatology of the summertime heavy rainfall over the Yangtze-River Delta, showing a significant correlation between rapid urbanization and increased heavy rainfall events during the past two decades. Then, I will present a case study of the influences of the UHI effects associated with the Shanghai-Suzhou-Wuxi-Changzhou city belt on generating an afternoon heavy rainfall event. This event occurred under the influences of weak southwesterly monsoonal flows on 26 July 2018 over coastal Nantong that is 70–100 km downwind from the city belt. An analysis of both observations and model simulation shows the continuous propagation of deep convection from the city belt to Nantong through moist downdrafts, which is referred to as “convective relay”. Several cloud-permitting model simulations with the finest grid spacing of 1 km are performed. Results show i) the initiation of convective storms along the city belt, and their subsequent downstream propagation, leading to the generation of heavy rainfall over Nantong; and ii) the generation of little rainfall over the plain area on the north of the city belt, after replacing urban and built-up land uses by croplands over the region; and iii) only small contribution of Nantong’s UHI effects to the heavy rainfall event. The above results support the recent IPCC (2021) Sixth Assessment Report, stating that rapid urbanization has contributed to “increases in mean and extreme precipitation over and downwind of the city, especially in the afternoon and early evening (medium confidence)”.