Rainfall Mechanisms of a landfalling Western North Pacific Tropical Cyclone: Bilis (2006)

Forecasts of heavy rainfall and often disastrous flooding associated with tropical cyclones have long been a challenge for both the operational and research communities. Though many contributing factors for tropical cyclone rainfall have been recognized, dominant factors could change dramatically from storm to storm and even at different stages of the same storm.

In July 2006, western North Pacific tropical storm Bilis made landfall in China, bringing heavy rain to a large area in south China and resulting in at least 654 fatalities and a direct economic loss of $4-5 billion. Operational rainfall forecasts were inaccurate after Bilis made landfall, and this study investigates possible reasons for the excessive rainfall through examining NCEP final analyses fields of the Global Forecasting System and numerical simulations using the Weather Research and Forecasting (WRF) model.

It is found that rainfall associated with Bilis after its landfall can roughly be divided into three stages by location and timing. The first stage occurred in the Fujian and Zhejiang provinces from 1200 UTC 13 July 13 to 1200 UTC July 14 and was largely caused by the storm's main circulation interacting with coastal terrain. The second stage of the inland rainfall occurred around extended boundaries between the Jiangxi, Hunan and Guangdong provinces from 0600 UTC July 14 to 1200 UTC July 15. The third stage occurred with a narrow rain band along the coastal area of the Guangdong and Fujian provinces from 1200 UTC July 15 to 0000 UTC July 16. The rainfall mechanisms for the second and third stages that induced the most damage are explored extensively in this study. A 60-h cloud resolving WRF simulation successfully captured the lanfalling storm and its attendant heavy rainfalls.

Results show that large moisture flux convergence and a convectively unstable environment provide necessary conditions for the occurrence of rainfall in the whole process. For the second stage, vertical motion is mainly associated with mesoscale low-level convergence, associated frontogenesis and favorable vortex-shear interaction. Consistent with prior studies, rainfall in outer area of Bilis appeared mainly downshear and/or downshear right. Counter-intuitively, through sensitivity experiments with and without rough local topography, it is found that topography does not have apparent impact on the rainfall during the second stage. The rainfall during the third stage seems to result from interaction between Bilis and southwestly flow from the East-Asian monsoon circulation, topographical lifting and frontogenesis.