Typical Circulation Patterns and Associated Mechanisms for Persistent Heavy Rainfall Events over Yangtze–Huaihe River Valley during 1981–2020

Persistent heavy rainfall events(PHREs)over the Yangtze–Huaihe River Valley(YHRV)during 1981–2020 are classified into three types(type-A,type-B and type-C)according to pattern correlation.The characteristics of the synoptic systems for the PHREs and their possible development mechanisms are investigated.The anomalous cyclonic disturbance over the southern part of the YHRV during type-A events is primarily maintained and intensified by the propagation of Rossby wave energy originating from the northeast Atlantic in the mid–upper troposphere and the northward propagation of Rossby wave packets from the western Pacific in the mid–lower troposphere.The zonal propagation of Rossby wave packets and the northward propagation of Rossby wave packets during type-B events are more coherent than those for type-A events,which induces eastward propagation of stronger anomaly centers of geopotential height from the northeast Atlantic Ocean to the YHRV and a meridional anomaly in geopotential height over the Asian continent.Type-C events have“two ridges and one trough”in the high latitudes of the Eurasian continent,but the anomalous intensity of the western Pacific subtropical high(WPSH)and the trough of the YHRV region are weaker than those for type-A and type-B events.The composite synoptic circulation of four PHREs in 2020 is basically consistent with that of the corresponding PHRE type.The location of the South Asian high(SAH)in three of the PHREs in 2020 moves eastward as in the composite of the three types,but the position of the WPSH of the four PHREs is clearly westward and northward.Two water vapor conveyor belts and two cold air conveyor belts are tracked during the four PHREs in 2020,but the water vapor path from the western Pacific is not seen,which may be caused by the westward extension of the WPSH.

Classification of yearly extreme precipitation events and associated flood risk in the Yangtze-Huaihe River Valley

Fifty cases of regional yearly extreme precipitation events （RYEPEs） were identified over the Yangtze-Huaihe River Valley （YHRV） during 1979-2016 applying the statistical percentile method. There were five types of RYEPEs, namely Yangtze Meiyu （YM-RYEPE）, Huaihe Meiyu （HM-RYEPE）, southwest-northeast-oriented Meiyu （SWNE-RYEPE） and typhoon I and II （TC-RYEPE） types of RYEPEs. Potential vorticity diagnosis showed that propagation trajectories of the RYEPEs along the Western Pacific Subtropical High and its steering flow were concentrated over the southern YHRV. As a result, the strongest and most frequently RYEPEs events, about 16-21 cases with average rainfall above 100 mm, occurred in the southern YHRV, particularly in the Nanjing metropolitan area. There have been 14 cases of flood-inducing RYEPEs since 1979, with the submerged area exceeding 120 km2 as simulated by the FloodArea hydraulic model, comprising six HM-RYEPEs, five YM- RYEPEs, two TC-RYEPEs, and one SWNE-RYEPE. The combination of evolving RYEPEs and rapid expansion of urban agglomeration is most likely to change the flood risk distribution over the Nanjing metropolitan area in the future. In the RCP6.0 （RCPS.5） scenario, the built-up area increases at a rate of about 10.41 km2 （10 yr）-t（24.67 km2 （10 yr）-1） from 2010 to 2100, and the area of high flood risk correspondingly increases from 3.86 km2（3.86 km2） to 9.00 kin2（13.51 km2）. Areas of high flood risk are mainly located at Chishan Lake in Jurong, Lukou International Airport in Nanjing, Dongshan in Jiangning District, Lishui District and other low-lying areas. The accurate simulation of flood scenarios can help reduce losses due to torrential flooding and improve early warnings, evacuation planning and risk analysis. More attention should be paid to the projected high flood risk because of the concentrated population, industrial zones and social wealth throughout the Nanjing metropolitan area.