The Medium Range Formation Mechanism Of Persistent Heavy Rainfall Over The Yangtze-Huai River Valley

Persistent heavy rainfall (PHR) events over the Yangtze-Huai river valley are one of the weather related high-impact events. To improve the forecast skill of the PHR events, it is necessary to find out the global-to-regional influences on the initiation and evolution of this kind of events. Based on daily rainfall data measured in China and NCEP/NCAR daily reanalysis for 54 years from 1951 to 2004, the climatology characteristics of the PHR events over the study area were firstly investigated, then the significant medium range signals from the global circulation prior to the PHR events were identified by the synoptical meteorology-correlation analysis, then the spatial-temporal distribution features and synoptic meanings of these signals were revealed, finally a tentative concept model of the medium range formation mechanism of the PHR events over the Yangtze-Huai river valley was constructed by synthetically analyzing the linkage between the signals.There are three chains of medium range signals for the PHR events. The first one, "Equatorial Pacific signals chain" originates from the development of upper level warm high over the eastern equatorial Pacific 4-3 weeks before the onset of the PHR events. By zonal teleconnection, it propagates its effects westerly thereafter to influence the developments of upper level cyclonic troughs near Australia and west Asia respectively 1-2 weeks later in succession, finally, to influence significant extending westward and jumping northward of the subtropical high near southeast China coast 1 week prior to the events. The second one, "South Indian Ocean signals chain" stems from the cold trough with cyclonic shear at periphery of polar vortex over southwest Atlantic 4 weeks before the onset of the PHR events. Then in company with the trough development at Australia, it causes the enhancing of the trough with cyclonic shear at south of Madagascar by energy dispersion mechanism. The signal feature of cyclonic shear near Madagascar then causes the anticyclonic shear in Central Europe and cyclonic shear in west Asia respectively by meridional teleconnection of zonal wind. The impacts of above two chains on the west Asia cyclonic trough trigger off the third signals chain, "Eurasian signals chain". The development of upper level cyclonic trough in west Asia disperses energy quickly downstream, which causes the development of two troughs at the East China coast and southern Bering Sea separately and induces significant Rossby wave-train activity over Eurasia during two weeks before the onset of the PHR events. With the disappearance of Eurasian Rossby wave train, significant adjustment of large scale circulation occurs. The west retreat of the trough at southern Bering Sea contributes to the occurrence and stability of Okhotsk blocking high. The trough in west Asia shifts eastward and simultaneously the trough in East China coast weakens and retreats westerly. Both of them connect to form a wider and flatter trough extending from Balkhash lake to northeast China, which favors propagating southeasterly of multi-minor-troughs with weak cold air in sequence. As the trough in east China coast shrink northward, the Subtropical West Pacific High (SWPH) extends westward along with low level southerly jet on the west frank of the SWPH and simultaneously the South Asia High (SAH) extends eastward along with upper level strong northerlies on the east frank of SAH. As a result, a local meridional circulation is developed in the southeast China with upper level northerly and lower level southerly. Obviously, it is above significant adjustment of circulation that results in the formation of stable circulation characteristics favoring the formation of the PHR events over the study area.As the last part of this thesis, an experiment of ensemble numerical simulation was conducted for a typical PHR event in 1991. To carry out the ensemble simulation, a series of initial wind disturbance which is based on the characteristics of Rossby wave train signals in Eurasian were imposed on the initial conditions of control experiment. The results indicate that the simulation of PHR event and corresponding circulation characteristics are sensitive to the Rossby wave train signals in Eurasian. The subsequent influences of the initial disturbances with the characteristics of that signals was closely relate to the establishment of the stable circulation during the PHR event.