Abstract
Abstract In this study, a nocturnal extreme rainfall event induced by the urban heat island (UHI) effects of the coastal city of Guangzhou in South China on 7 May 2017 is examined using observational analyses and 18-h cloud-permitting simulations with the finest grid size of 1.33 km and the bottom boundary conditions nudged. Results show that the model reproduces convective initiation on Guangzhou’s downstream side (i.e., Huashan), where a shallow thermal mesolow is located, the subsequent back-building of convective cells as a larger-scale warm-moist southerly flow interacts with convectively generated cold outflows, and their eastward drifting and reorganization into a localized extreme-rain-producing storm near Jiulong under the influences of local orography. In particular, the model produces the maximum hourly, 3- and 12-hourly rainfall amounts of 146, 315, and 551 mm, respectively, at nearly the right location compared to their corresponding observed extreme amounts of 184, 382, and 542 mm. In addition, the model reproduces an intense meso-γ-scale vortex associated with the extreme-rain-producing Jiulong storm, as also captured by Doppler radar, with organized updrafts along cold outflow boundaries over a semicircle. A comparison of sensitivity and control simulations indicates that despite the occurrence of heavier rainfall amounts without the UHI effects than those without orography, the UHI effects appear to account directly for the convective initiation and heavy rainfall near Huashan, and indirectly for the subsequent formation of the Jiulong storm, while orography plays an important role in blocking cold outflows and enhancing cool pool strength for the sustained back-building of convective cells over the semicircle, thereby magnifying rainfall production near Jiulong.
Highlights
An extreme rainfall (ER) event occurred over the coastal city of Guangzhou during the early morning hours of 7 May 2017, with record-breaking 3- and 12-h accumulated amount of 382 and 542 mm, respectively, for Guangdong Province
The results presented in the preceding two sections indicate the possible roles of the urban heat island (UHI) effects and northern trumpet-shaped mountains in determining convective initiation and the subsequent development of the ER-producing storms
We examine the impact of the UHI effects and orography on the convective initiation and subsequent ER generation over the southern coastal city of Guangzhou during the nocturnal period of 7 May 2017 using a series of cloud-permitting simulations with the Weather Research and Forecasting (WRF) Model
Summary
An extreme rainfall (ER) event occurred over the coastal city of Guangzhou during the early morning hours of 7 May 2017, with record-breaking 3- and 12-h accumulated amount of 382 and 542 mm, respectively, for Guangdong Province. Heavy rainfall events occurred frequently in South China (Luo et al 2017), there were three distinct features associated with Guangzhou’s ER event: (i) its initiation and growth on the downwind side of Guangzhou; (ii) the ER production from localized convective storms with life spans of more than 10 h; and (iii) the quasi-stationary nature of the ER production near Huashan and Jiulong. The major objectives of the present study are to (i) determine to what extent the record-breaking rain-producing storms over Guangzhou on 7 May 2017 could be simulated with the most advanced weather prediction model; (ii) explore the multiscale processes, especially the UHI effects of Guangzhou and its northern mountains, governing the nocturnal convective initiation and subsequent ER production in the warm sector; and (iii) examine any nonobservable storm-scale flow structure(s) that are associated with the record-breaking rainfall production. A summary and concluding remarks are given in the final section
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