Characteristics of tropical cyclone extreme precipitation and its preliminary causes in Southeast China

Extreme precipitation induced by a tropical cyclone (TC) is of great concern to Southeast China. Regional characteristics of daily TC-induced extreme precipitation (TCEP) between 1958 and 2016 and the associated preliminary causes over Southeast China (Zhejiang, Fujian, and Shanghai) were examined by applying the objective synoptic analysis technique, TC track similarity area index, daily precipitation observations, and reanalysis data. The intensity and frequency of high-intensity TCEP (≥ 100, ≥ 200, ≥ 300 mm) have had an increasing trend over recent decades. Most of TCEP occurs from July to September, with frequency peaks in August for TCEP at all intensity levels, apart from the frequency for TCEP ≥ 300 mm that peaks in September. Regions with high frequency and large TCEP (R-HFLTs) (relatively high frequency for TCEP ≥ 100 mm) were concentrated along the coastline of the southern coastal Fujian (Southern R-HFLT), the regions from northern coastal Fujian to southernmost coastal Zhejiang (Central R-HFLT), and central coastal Zhejiang (Northern R-HFLT), decreasing from the coastline to inland. The Central R-HFLT region had the highest TCEP intensity and frequency for TCEP ≥ 100 mm compared with the other R-HFLTs. Further analysis showed that the special terrain of Southeast China matched the spatial distribution of TCEP, which highlights the significance of the topography of Southeast China. To discover other factors responsible for the heavy TCEP, we compared two TC groups that influence Central R-HFLT. Under a more northerly direction and slow movement combined with the unique terrain, TCs with stronger vortex circulation generated heavier TCEP during landfall in Central R-HFLT. Heavy TCEP occurred with easterly and southeasterly winds interacting with terrain over the eastern coast for Central R-HFLT. Although large changes in the internal and external environment were sensitive to the observed TCEP intensity, the interaction between TC circulation and the complex topography in Southeast China under the northerly track was the dominant factor.