Extremity Analysis on the Precipitation and Environmental Field of Typhoon Rumbia in 2018

Using a variety of observational and analytical data, the evolution of heavy precipitation, features of extreme precipitation and physical characteristics of environmental field causing extreme precipitation of Typhoon Rumbia in 2018 are analyzed. Obvious extremity can be seen with the daily rainfall at many national observational stations breaking historic records. The rainstorm process of Typhoon Rumbia can be divided into three stages: Landing, moving further inland and turning, and extratropical transition due to cold air intrusion. In the second stage, the typhoon moves very slowly and the circulation of Typhoon Rumbia stays over Henan Province for very long time, which makes the second stage the strongest precipitation period. Influenced by Typhoon Rumbia, historical daily precipitation records of many national stations in eastern Henan, southwestern Shandong and northern Shandong are broken. The observed maximum hourly precipitation is 127.7 mm, and as many as 74 stations experience an hourly precipitation of more than 80 mm. Furthermore, short-term heavy rainfall feature is very obvious, for more than 14 hours, the hourly accumulated precipitation exceeds 20 mm. Influenced by both the high precipitation efficiency and the long duration, extreme precipitation happens. Due to the atmospheric circulation characteristics of Typhoon Rumbia, there is an abnormal low pressure circulation with the standardized anomaly smaller than -4 times climate standard deviation in middle and lower troposphere, which results in extreme low-level convergence. At the same time, extreme upper divergence, induced by the combined actions of both upper tropospheric jet and high-pressure edge, can be seen in upper level. Therefore, there are obvious extreme characteristics in dynamic conditions. Compared with top 30 precipitation days in recent 30 years, 200 hPa divergence and 850 hPa convergence of Typhoon Rumbia are either close to or far beyond the historical maximum. Substantial water vapor is brought and converged in the rainstorm area continuously through the water vapor transport belt on the east side of typhoon, resulting in an extreme water vapor environmental condition. Significant extremity can be seen in many moisture-related physical parameters, such as pseudo-equivalent potential temperature, atmospheric precipitable water and vapor flux divergence, compared with both climatic mean state and historical heavy precipitation days in recent 30 years, and the extreme vapor condition lasts for up to 30 hours.