Diagnosis Of Three Extreme Weather Processes In Northeast China From July To August 2017

The two processes of strong precipitation occurred in Yongji County,Jilin Province on July 13-14 and 19-21,2017,a process of large-scale cooling occurred in Northeast China on August 27-31,2017,the three processes had all passed the threshold test of extreme precipitation or low temperature events.In order to study the formation mechanism of the three processes and achieve the purpose of disaster prevention and mitigation,based on ERA-Interim 0.5°×0.5° 6 hourly reanalyzed data,the conventionally observed data,FY-2E,FY-2F satellite data,the three processes were diagnostic analyzed,and based on the numerical forecast products on T639 method,a process of strong precipitation in Jilin Province on July 13-14,2017 was tested,the main conclusions show that:(1)The precipitation in Yongji County on July 13,14 and 20 all exceeded the threshold of extreme precipitation event,it can be considered that the two strong precipitation in Yongji County on July 13-14 and 19-21 were both the extreme precipitation events.The number of stations below the threshold of extreme low temperature event was higher in Northeast China on August 28-30,and the number of stations on the 29 th was the highest,it can be considered that most regions of Northeast China experienced extreme low temperature events around August 29.(2)The favorable configuration of the northeast cold vortex and the western Pacific subtropical high was the background of large-scale circulation of the two processes,the first event was influenced by the strong convergence of the northwest current and the southwest low-altitude jet stream,the second event was influenced by the shear line of the northeast current and the southwest low-altitude jet stream.In the first event,the low-altitude jet stream was stronger,the water vapor was more abundant,the high-altitude jet stream had stronger suction effect on rainstorm area.The period of maximum precipitation at Yongji County in the first event was influenced by the warm and wet air in the lower layer enhanced the effect on the dry and cold air in the middle layer,in the second event,it was influenced by the dry and cold air in the middle layer down into the lower layer.The short-term strong precipitation in Yongji County were influenced by severe convective cloud cluster with TBB?-50°C or-60°C in the first process,and in the second process,the early TBB?-50°C,then the convection gradually weakened.(3)The northeast cold vortex was the main influence system of the process of rain,snow and cooling.The dry,cold airflow and the warm,wet airflow were intertwined in the center of the cold vortex to form dipoles,the invasion of dry and cold air was an important factor in the formation of cold vortex and the maintenance of cold structure.The cold air moved southward and meeted the warm air,they formed a high-altitude cold front zone,and gradually moved southward.the sharp drop of the low-level temperature was an important cause of snowfall.The water vapor was transported counterclockwise along the cold vortex circulation,and along with the cold vortex east.The precipitation associated with the cold vortex depended on the structural variation characteristics of the wind field of the cold vortex,when the updraft was strong,the precipitation was also strong.(4)The T639 method had certain reference value for the forecast of the precipitation process in Jilin Province,on the whole,as the period of validity increased,the accuracy decreased.