The Influence Of The Interaction Between Tropical Cyclone And Middle-low Latitude Systems On Pre-flood Season Rainfall In South China

Based on the data,the tropical cyclones developed in the Northwest Pacific during the pre-flood season from 2000 to 2015 are statistically analyzed.Then selecting "Noul" for analysis purpose,the characteristics of large-scale circulation of interaction between "Noul"and middle-low latitude systems are analyzed.Also the mechanism of rainfall in South China on 7-10 May 2015 is concretely diagnosed and analyzed.Simultaneously,the WRF model is used for numerical simulation analysis.The main results are obtained as following:(1)The results shows that the influence of the interaction between tropical cyclone and middle-low latitude systems on pre-flood season rainfall over South China happen frequently.Most of the tropical cyclone generate in April and May,and tropical cyclone in high intensity during rainfall.Futher more,the majority of tropical cyclones originate in the eastern of Philippines,and track easily westewards in low latitude.(2)The large scale circulation characteristics of the interaction process indicate that the earlier South China area is located at the cross convergence area of the south branch trough and the west side of the subtropical high.,and the existence of the southern branch,the high and low jet and the wind shear will form a large scale circulation situation which is beneficial to the precipitation.The intensity of the western side of the subtropical high weakened by the northward typhoon,which had an unfavourable impact on precipitation.In this process,the interaction between "Noul" and the subtropical high through the divergence wind caused the atmospheric divergence circulation change,the "Noul" to be enhanced and the subtropical high is weakened.(3)The diagnostic analysis shows that the convergence of the warm air and cold air is the trigger condition of the precipitation,the main source of the water vapor is from the South China Sea,and the location of the upper jet stream determines the precipitation area.The upward movement of the upper jet and the secondary circulation in the early stage of precipitation combined with the dynamic conditions,and the accumulation of unstable energy leads to the fornation of favorable precipitation conditions.When the "Noul" moving northward,the rise circulation circle weakened and the convergence of the lower layer weakened,and the water vapor from the South China Sea decreased,resulting in the decrease of precipitation in the later period.(4)WRf control test simulation results reproduce the formation and development of the rainstorm process well.The WRF sensitivity test result shows that the central pressure change of "Noul" has a certain influence on the rainfall intensity and falling area,the central air pressure of the typhoon is strong,the precipitation in South China area is reduced and the precipitation area is scattered.The change of the intensity of the central pressure of the "Noul"to weaken the intensity of the west side of the subtropical high due to changing of dynamic and thermodynamic conditions is the main cause of influence on the rainfall in South China.