Stronger exacerbation of extreme rainfall at the hourly than daily scale by urbanization in a warming climate

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Journal Article
Despite the substantial progress in understanding the response of extreme precipitation to climate warming across different time scales, the response of these events to urbanization remains uncertain. Here, we evaluate changes in hourly and daily extreme precipitation during 1981–2020 in the urbanized Pearl River Delta, China, using observations and numerical simulations. The results of observations show that hourly rather than daily extreme precipitation in urban areas is more intense and frequent than in rural ones. Hourly extreme intensity increases ∼134 % faster than the daily one, and the urban downwind increase is most evident in hourly extreme precipitation frequency. The numerical simulations also show that the presence of urban areas increases hourly extreme intensity by ∼26 %, but increases daily extreme intensity by only ∼13 % when averaged over urban areas. The increases of precipitation intensities are local, and the increases across the whole region are much smaller than those over urban areas. Both observations and simulations indicate that urbanization exacerbates extreme precipitation and redistributes its spatial pattern, which is more pronounced at the hourly scale, potentially leading to a higher probability of hazardous urban flash floods. Our analysis of meteorological conditions elucidates that urban areas tend to create a drier and warmer surface environment and a more unstable atmosphere compared with surrounding rural areas. Moist air from surrounding rural areas is brought in to compensate for moisture anomalies and low pressure over the cities. This provides the air above urban areas with moisture, generating stronger updrafts, thus intensifying extreme precipitation over the cities. Urban areas provide a more favorable environment for the enhancement of hourly rather than daily extreme precipitation, and their differential responses to urbanization are mainly due to the mechanisms associated with enhanced-Clausius-Clapeyron behavior and the greater enhanced urban–rural thermal and moisture contrast in hourly extreme precipitation. Similarities can be found between the mechanisms of climate warming-induced and urban-induced intensification of localized hourly extreme precipitation Our results highlight the importance of understanding scale difference brought by urban environment, and thus provide a scale-based understanding of urban precipitation modification.
Journal of Hydrology
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