| 2026001 |
On the Extreme Rainfall Event of 7 May 2017 over the Coastal City of Guangzhou. Part I: Impacts of Urbanization and Orography |
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Guangzhou |
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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 cor responding observed extreme amounts of 184, 382, and 542 mm. In addition, the model reproduces an intense meso-g-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. |
| 2026002 |
A Possible Dynamic Mechanism for Rapid Production of the Extreme Hourly Rainfall in Zhengzhou City on 20 July 2021 |
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Zhengzhou |
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In this study, the unprecedented extreme rainfall event during 19–20 July 2021, which caused devastating flooding in Zhengzhou City and its nearby areas, is examined based on observational data analysis and WRF model 40-h simulations on 1-km horizontal resolution. The results show that the model successfully reproduces (i) major synopticscale weather systems (i.e., the western Pacific subtropical high, the Tibetan high, two typhoons, and the Huang–Huaicyclone), (ii) convective initiation along the east to north edge of the Songshan Mountain, where orographic lifting is obvious, and (iii) subsequent formation of the convective storm producing the extreme rainfall in Zhengzhou. In particular, the model generates the maximum rainfall rate of 233 mm h−1 and 40-h accumulated rainfall of 704 mm, corresponding well to the observed extreme values of 201.9 mm h−1 and 818 mm, at nearly observed timing and location. Importantly, the model reproduces an intense quasi-stationary, well-organized meso-γ-scale convective system,surrounded by an arc-shaped convergence zone, allowing the development of convective updrafts in a three-quarter circle around the convective system, in a way similar to multidirectional pumping, attracting all associated precipitation overlaid and concentrated into the same trailing region to generate the extreme hourly rainfall over Zhengzhou.Our study emphasizes the significant contribution of the unique dynamic structure of the well-organized meso-γ-scaleconvective system to the record-high hourly rainfall. A possible dynamic mechanism for short-time extreme rainfallproduction is proposed. That is, the arc-shaped convergence zone of the mesoscale convective system, acting like multidirectional lifting pumps, transports precipitation from different directions into the same region, and thus prosixduces the extreme rainfall. The results gained herein may shed new light on better understanding and forecasting of short-time extreme rainfall. |
