Characteristics and Causes of Extreme Rainfall Induced by Binary Tropical Cyclones over China

Binary tropical cyclones (BTC) often bring disastrous rainfall to China. From the viewpoint of the extreme of the BTC maximum daily rainfall, the characteristics of BTC extreme rainfall (BTCER) during 1960-2018 are analyzed, using daily rainfall data; and some representative large-scale mean flows, in which the associated BTCs are embedded, are analyzed. Results show that the frequency of BTCER shows a decreasing trend [-0.49 (10 yr)(-1)] and is mainly distributed within the BTC heavy rainstorm interval (100 mm <= BTCER <250 mm). BTCER occurs mostly from July to September with a peak in August. Three BTCER typical regions- Minbei, the Pearl River Delta (PRD), and Taiwan-are identified according to the clustering of stations with high BTCER frequency and large BTCER. A further analysis of the 850-hPa BTC composite horizontal wind and water vapor flux over the PRD region shows the existence of two water vapor transport channels, which transport water vapor to the western tropical cyclone. In the first of these channels, the transport takes place via the southwest monsoon, which accounts for 58% of the total moisture, and an easterly flow associated with eastern tropical cyclone accounts for the remaining 42%.