Abstract
An extraordinary heavy rain event caused by Typhoon Mangkhut occurred in the Yangtze River Delta region on 16 September 2018, with the maximum of 24-h accumulated rainfall at a single station reaching 297 mm. However, numerical models and subjective forecast failed to predict this typhoon remote rainstorm accurately. In this study, multiple observational data, an analysis dataset, and a trajectory model are used to analyze the causes of this severe rainstorm. The results show that the circulation situation provides a favorable large-scale background condition for the generation of the rainstorm. The coupling of the upper-level westerly jet and the low-level southerly jet is beneficial to the development of strong convections. In the rainstorm area there is a positive vorticity center connected to the main body of the typhoon. The cooling and humidifying effect of dry-cold air saturates the formerly unsaturated wet air, leading to the increase of precipitation. Besides, there is a lower-tropospheric moisture transport path connecting the typhoon and the rainstorm area, providing abundant moisture for the development of rainstorms. The backward trajectory simulation shows that the moisture mainly originates from the lower troposphere over the Philippine Sea, the southern South China Sea, and the sea south of the Philippines.
Highlights
Typhoons are one of the major disastrous weather systems causing extraordinary heavy rainfall in China
The typhoon-induced rainstorm areas can be roughly divided into the typhoon remote rainstorm (TRR) area and the rainfall area within the typhoon circulation
Cote [2] gave a statistical standard for typhoon remote rainstorm: first, the radar reflectivity factor exceeds 35 dBZ in the precipitation area, and the precipitation lasts for more than 6 h; second, the 24-h precipitation is ≥100 mm; third, there is a clear boundary between the remote rainfall area and the precipitation area within the typhoon circulation in radar images, and there is a moisture transport path connecting the typhoon and the TRR area
Summary
Typhoons are one of the major disastrous weather systems causing extraordinary heavy rainfall in China. After studying 28 TRRs to the east of the Rocky Mountains in the United States from 1995 to 2008, Galarnea and Bosart [5] found that TRRs are more likely to occur in the regions to the east of the upper trough at 700 hPa, the west of the ridge line in the potential temperature field at 925 hPa, and at the entrance of the upper level jet at 200 hPa. Byun and Lee [6] analyzed TRR events on the Korean Peninsula from 1981 to 2009 and concluded that the average annual rainfall of remote rainstorms caused by tropical cyclones during this period accounts for 49% of the 29-year-average total annual rainfall. The circulation situation during these remote rainstorm events shows that tropical cyclones are generally located over the southwestern Korean Peninsula, with a trough to its northwest, and the western Pacific subtropical high lies to the southeast of tropical cyclones. An extraordinary heavy rain event occurred in the Yangtze River Delta region on 16 September 2018, later we will show that it was a TRR caused by Typhoon Mangkhut (2018). We aim for a better understanding of TRRs, especially in the Yangtze River Delta region
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