A Study On The Formation Mechanism Of A Thunderstorm In Chengdu At Night

This paper firstly uses the observation data of the China Meteorological Administration and the NCEP/NCAR reanalysis data to conduct a preliminary diagnosis of a continuous night thunderstorm weather process occurring in Chengdu and its surrounding areas(western Sichuan Basin)from July 15 to 17,2017;then use WRF to carry out high-resolution numerical simulation of the precipitation process.Based on the credibility of the simulation results,this research studied the formation mechanism of the night rain process;Finally through semi-topographic experiment,we investigate the impact of terrain on this night rain weather process.We mainly studied the formation and maintenance of this weather process,the transport of vapor and energy,and the effect of the valley circulation on the night rain process.We mainly got the following conclusions:(1)The precipitation during the nighttime weather has a strong precipitation intensity,strong locality and obvious night rain,and has obvious mesoscale characteristics in both time and space.(2)This weather process mainly occurs in the “North High South Low” circulation situation of 500 hPa.This circulation situation is conducive to transport the cold air in the north to the south.The basin is located near the low trough at the bottom of the low pressure.The air is continuously transported to the upper troposphere of the basin.The southerly airflow on the east side of the 850 hPa typhoon and the southerly airflow on the west side of the subtropical high are combined to form a conveyor belt,which transports a large amount of low-latitude water vapor and heat to the lower layer on the Sichuan Basin area.This process is conducive to the accumulation of low-level water vapor and energy over the rainstorm area to form Unstable area.The intersection of cold air and warm humid air over the basin creates convective instability,which in turn induces precipitation.(3)The water vapor in this heavy rain process mainly comes from the southerly airflow on the east side of the typhoon and the southerly airflow on the west side of the subtropical high,superimposed on each other through the south China Sea to the north of the water vapor transport belt,and the other one has an important contribution to summer precipitation in China.The southwest monsoon water vapor transport belt,due to the subtropical high-strength Shengxi extension at this time,the southwest monsoon water vapor transport belt was suppressed to the southwestern side of Sichuan,so it contributed little to the precipitation process.Due to the strengthening of the low-altitude wind speed at night,the nighttime water vapor is transported to the basin more than during the day,providing abundant water vapor for the night rain.(4)On July 15?17,the wind on the southeast side of the basin is slow,while the wind on the southeast side of the night basin is faster than that in the daytime,with daily variation characteristics,so the water vapor and heat transfer also have obvious daily variation characteristics.The low wind speed in the basin is conducive to the continuous supply of warm and humid air to the basin at night,which is conducive to the formation of nighttime precipitation.(5)The WRF model correctly shows the continuous night rain weather process,especially the contribution of the valley wind to the precipitation: since the western part of the basin is close to the plateau,the east slope of the Qinghai-Tibet Plateau is controlled by the “Valley Wind”.The bottom of the valley blows to the top of the mountain,forming a clockwise local circulation loop.The basin is located near the sinking branch on the right side of the local circulation circle.The sinking airflow is obvious and the weather is fine.At night,the eastern slope of the Qinghai-Tibet Plateau is controlled by the “mountain wind”.The wind blows from the top of the mountain to the bottom of the valley,forming a counterclockwise local circulation loop.The basin is located near the rising branch on the right side of the local circulation circle.The updraft is obvious,which is conducive to the development of convection and the diurnal variation of the southwest airflow at low altitude.It has the heat and water vapor conditions,which has produced the night rain phenomenon for three consecutive days.(6)The three helicities can better explain the relationship between the mechanism of nighttime precipitation formation and the valley wind.In the indication of precipitation,the indication of the rise of precipitation convergence is more significant than the degree of water vapor helicity,but the thermal helicity has no significant water vapor helicity for the divergence sinking zone indication of precipitation.(7)Through the semi-topographic sensitivity experiment,it is found that the terrain of the Qinghai-Tibet Plateau has a blocking effect on the low-level airflow.The presence of the plateau enables the lower-level airflow to converge in the basin,which is conducive to the precipitation movement and precipitation,and increases the precipitation of the basin.