Urbanization enhanced summertime extreme hourly precipitation over the Yangtze River Delta

Abstract:

Extreme short-term precipitation of a few hours often floods cities and causes huge damages. Understanding how urbanization influences the sub-daily rainfall extremes is of great importance for urban planning, regional climate studies, and daily weather forecasts. Using historical land-use and local statistical data, a rapid-urbanized city cluster over the Yangtze River Delta (YRD) region of East China is identified. With the long-term (i.e., 1975-2018) hourly rain gauge observations, a 44-yr climatology of the summertime (JJA) extreme hourly precipitation (EXHP; ≥90th percentile) over the YRD region is analyzed. Moreover, 113 local extreme rainfall events during 2011-18 summers are analyzed to examine the relationship between the rapid urbanization and the EXHP changes. Results show apparent EXHP differences in diurnal variation and storm type roughly before and after middle July. That is, tropical cyclones (TCs) account for 16.4% of the total EXHP hours, and the TC-induced EXHP mostly (80.5%) occurs during the late summer, whereas the non-TC EXHP accounts for 94.7% during the early summer. Statistically significant increasing trends in occurrence frequency and amount of both the non-TC and TC-induced EXHP are observed over the urban agglomeration, with larger increasing trends over the urban stations than the nearby rural stations, collocating with those of surface temperature. The analysis of the 113 events reveals contrasting rainfall distributions between strong- and weak-UHI events, suggesting the contribution of the UHI effects to the more EXHP occurrences, especially over a band-shaped region where several major cities are distributed. This study reveals a significant correlation between the increased EXHP and the rapid urbanization during the past several decades over the YRD region.