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- Title
Analysis of Uncertainties and Associated Convective Processes in Simulations of Extreme Precipitation Over Cities With a Regional Earth System Model: A Case Study.
- Authors
Li, Zhenghui; Luo, Yali; Chen, Feng
- Abstract
This study utilizes the Weather Research and Forecasting model coupled with an atmospheric chemistry model, a multi‐layer urban canopy model (UCM), and a building energy model to simulate the extreme rainfall event influencing the Guangzhou city in South China on 7 May 2017. By employing small variations in the longwave emissivity of buildings within the UCM, 11 convective‐permitting experiments are conducted. The maximal 18‐hr and hourly rainfall accumulation vary from a 2‐year return period to as high as a 20 or 40‐year return period with notable spatial differences. Comparisons between the more accurate group (GOOD) and less accurate group (POOR) simulations highlight that some minor differences in the near‐surface air thermodynamic conditions in urban area could lead to substantial differences in local convection and its impacts on subsequent convective systems. With persistent transportation of warm, moist airflows from the northern South China Sea, formation of a slow‐moving mesoscale outflow boundary to the north of the urban agglomeration leads to the development of a quasi‐stationary, compactly structured meso‐γ‐scale rainstorm in the GOOD simulations. The stronger low‐level to near‐surface convergence and mid‐level cyclonic shear within this system substantially enhance low‐level updrafts, leading to increased microphysical production and stronger horizontal advection of rainwater within the system. These findings offer some process‐based understanding about the uncertainties in simulating urban extreme rainfall and underscore the need to develop ensemble forecasting methods for convection‐permitting numerical models that incorporate increasingly complicated representations of anthropogenic influences. Plain Language Summary: Precise forecasting of extreme precipitation in cities is an urgent need for hazards mitigation, but very challenging. It is unclear how well the state‐of‐the‐art numerical models with the most advanced representations of natural and anthropogenic physical processes can simulate urban extreme precipitation. Therefore, we conduct 11 experiments with small variations in a physical parameter of buildings to simulate an extreme rainfall influencing Guangzhou city using a state‐of‐the‐art coupled model with very high resolutions to resolve the initiation and propagation of extreme rainfall‐producing convection. We find the small changes in the setting led to substantial differences in the location and intensity of rainfall, for example, the simulated rainfall accumulations vary from once every 2 to 40 years. By comparing those experiments with good and poor results, we illustrate why the simulated rainfall differ so much in the new perspective of convection evolution. The small differences in near‐surface air conditions over cities can lead to huge bifurcation in convection development. Whether the continuous lifting of warm onshore airflows can be reproduced dictates whether a well‐organized rainstorm can subsequently form. The dynamical structure of the rainstorm can substantially modulate the microphysical production and transportation of rainwater and thus the maximal hourly rain rates. Key Points: The simulated rainfall shows high uncertainties in 11 experiments with only minor changes in an urban canopy parameterSmall differences in the urban near‐surface air conditions lead to huge variations in local convection and subsequent rainstormsThe uncertainties in simulating mesoscale and convective‐scale processes lead to the uncertainties in predicting extreme rainfall
- Subjects
GUANGZHOU (China); CHINA; RAINSTORMS; CITIES &; towns; METEOROLOGICAL research; ATMOSPHERIC chemistry; WEATHER forecasting; RAINFALL
- Publication
Journal of Geophysical Research. Atmospheres, 2024, Vol 129, Issue 9, p1
- ISSN
2169-897X
- Publication type
Article
- DOI
10.1029/2023JD040606