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

A record-breaking rainfall event occurred in the coastal metropolitan city, Guangzhou, China during 06–07 May 2017, causing devastating flooding. Observation analysis and the Weather Research and Forecasting (WRF) model nested large-eddy simulation (LES) were conducted to investigate the roles of various forcing factors including synoptic weather pattern, topographic effect, cold pool and urban effect. Results exhibiting the mechanism of this event is as follows. The sustaining warm moist southerly flow in the lower troposphere over the trumpet-shaped topography of the Pearl River Delta is the key to the development of this severe rainfall event. After the initial convection developed, the southerly flow in the convection area was strengthened by the convergent inflow of the convection. Thereafter, the cold outflow boundaries associated with the convective precipitation stimulated the initiation and development of new convections. The near opposite directions of the convective system propagation and the movement of convective cells within the rain band resulted in very slow convective system movement, which caused the extreme amount of the local-scale rainfall. The result of sensitivity experiments on the urban land-use indicates that the urban affects the timing and location of convection initiation and helps concentrate the maximum rain core. WRF nested LES simulated this deep convection successfully, indicating its promising use for forecasting such severe weather event.