Mechanisms for a Record‐Breaking Rainfall in the Coastal Metropolitan City of Guangzhou, China: Observation Analysis and Nested Very Large Eddy Simulation With the WRF Model

Abstract

Record‐breaking rainfall of 524.1 mm in 24 hr occurred in the coastal metropolitan city of Guangzhou, China, during 6–7 May 2017 and caused devastating flooding. Observation analysis and a nested very large eddy simulation (VLES) with Weather Research and Forecasting (WRF) model were conducted to investigate various factors that contributed to the heavy rainfall, including synoptic weather pattern, topographic effects, cold pool, and urban effects. First, the warm and moist southerly flow in the lower troposphere over the trumpet‐shaped topography of the Pearl River Delta continuously provided fuel for the development of the severe rainfall. Consequently, the southerly flow from the sea in the south strengthened with the development of the convection. Meanwhile, the precipitation‐produced weak cold pool supported a stationary outflow boundary, where new convective cells were continuously initiated and drifted downstream. The interaction between the cold outflows and the warm moist southerly flows in the lower troposphere formed a back‐building convective system, which produced local persistent heavy rainfall that lasted for more than 5 hr and reached record levels. Sensitivity experiments in which the urban area was removed from the model indicate that the urban forcing affected the timing and location of convective initiation and helped concentrate the maximum rain core. The nested WRF‐LES successfully simulated this heavy rainfall, and the model's advantages are noted for forecasting such local severe weather.

Budget Analyses of a Record‐Breaking Rainfall Event in the Coastal Metropolitan City of Guangzhou, China

“…The ACCB was the main force for supporting the development and maintenance of updrafts, corresponding to the warm anomaly areas of perturbation potential temperature (Figure a). This result is consistent with that of Huang, Liu, et al (), who showed that high convective available potential energy (CAPE) was persistently observed in this region. The existence of abundant moisture in the lower troposphere (mentioned in section ) and the latent heat removed in the upper troposphere (discussed in section ) were important to maintain the total buoyancy.…”

“…Both positive and negative total potential temperature tendencies existed in the warm anomaly (Figure a), but their values were an order of magnitude smaller than those of latent heating and advection cooling. Therefore, the cooling from advection in the upper troposphere helped maintain the CAPE of the rising parcels that originated in the lower troposphere, which is consistent with the large CAPE generation rate attributable to the large‐scale advection shown by Huang, Liu, et al (). This process was crucial for the persistent development of updrafts.…”

“…However, Schumacher and Johnson () indicated that the typical scale of BB MCSs is smaller and thus BB MCSs are less predictable than TL/AS MCSs. Many case studies that examine extreme‐rainfall‐producing MCSs in China also found similar MCS patterns of organization (e.g., Huang, Liu, et al, ; Luo et al, ; Luo & Chen, ; Wang et al, ; Zhang & Zhang, ).…”

“…The strong inward moisture flux across the western boundary (positive QFlux W ) in the upper troposphere (Figure a) was mostly canceled by the strong outward moisture flux at the eastern boundary (negative QFlux E ) in the upper troposphere (Figure b). This mainly resulted from the prevailing westerly flows in the upper troposphere (Huang, Liu, et al, ). The positive net zonal moisture flux was concentrated in the lower troposphere, and the maximum reached 2,250 kg·kg –1 m 2 ·s –1 (Figure c).…”

“…Comparing the model output with global reanalyses and observations from automatic weather stations, rain gauges, and Doppler radar confirmed that the model simulation reproduced the initiation and development of the moist convection and heavy rainfall generally well. Please refer to the paper by Huang, Liu, et al (2019) for more details of the model configuration and verification. Figure 1 shows observed and simulated hourly accumulated precipitation in Guangzhou city from 2200 UTC 6 to 0000 UTC 7 May 2017.…”