| In this paper,the influence of the stratospheric circulation on the flood opening process in the first flood season in south China is studied by means of statistical analysis and composite analysis,and the influence mechanism is analyzed,the effect of tropopause potential vorticity anomaly on precipitation is analyzed.Furthermore,the effect of tropopause on the development of vertical motion during the first flood season in south China is analyzed,and the physical mechanism of the effect of the upper troposphere and lower stratosphere circulation anomaly on the precipitation during the first flood season is discussed.Finally,the influence of stratospheric circulation in the early flood season of south China is analyzed statistically.The main conclusions reached are as follows:According to the start time of the south China pre-flood season determined by the "trial implementation of the operational regulations for the monitoring of the South China flood season",the climatic average date of the start of the pre-flood season in south China is obtained by using the precipitation data of the observation stations,the precipitation characteristics of the pre-flood season in south China are verified by using the CMAP precipitation data,and the tropospheric and stratospheric circulation before and after the start of the pre-flood season from 1979 to 2017 are analyzed.The first flood season in south China can be opened on March 3,1983,the latest can be opened on May 5,2015,the average opening date for April 6.During the flood season,the precipitation first appeared in the northwest of south China,and then expanded to the southeast.From the tropospheric perspective,by comparing the circulation patterns in the lower,middle and upper troposphere before and after the onset of the pre-flood season in south China,it is found that there are warm and humid air flows from the north side of the Western Pacific subtropical high in the lower troposphere of south China before and after the onset of the pre-flood season,in south China,the temperature and humidity in the lower layer have increased,meanwhile,the cold air in the middle troposphere has moved southward,and the cold and warm air flow have converged,and the convective activity in south China has gradually increased,thus leading to the early flood season.From the perspective of the stratosphere,before and after the beginning of the preflood season,there were positive temperature anomalies in the tropopause spreading eastward to the South China region,and the migration of the positive temperature anomalies caused the increase of the potential vorticity in the upper troposphere and the lower stratosphere in the south China region,the height of tropopause is decreased,which strengthens the anomaly of positive vorticity in troposphere,and is favorable for the occurrence and development of precipitation.Before the pre-flood season,the tropopause temperature in south China moved into the tropopause region,which caused the increase of the tropopause potential vorticity and the vorticity in the troposphere.Meanwhile,the Lower and middle tropospheric atmospheric static stability is weakened,which is favorable for precipitation,thus affecting the early flood season in south China.Through the diagnosis and analysis of the geostrophic equation on the vertical velocity during the first flood season in south China,it is found that the main items that affect the vertical velocity are the meridional transportation items of the disturbed air flow of the basic temperature,and the vertical variation term of the disturbance vorticity in the zonal direction of the basic airflow.The zonal transport of positive vorticity in the upper troposphere is favorable to the development of vertical motion and thus to the occurrence of heavy precipitation,while in the lower troposphere,the transportation of warm advection to south China affected the precipitation during the early flood season.The Rossby wave breakup in the Red Sea region before the flood season in southern China directly affected the circulation over the Tibetan plateau and brought the subtropical stratospheric high potential vorticity air into southern China.From the above,it can be seen that the vorticity of the upper troposphere is increased when the high potential vorticity air enters south China,and the vertical motion is maintained and enhanced further,which is beneficial to the upward development of the vertical motion.In the upper troposphere and lower troposphere,the potential vorticity anomaly mainly affects the vertical motion through the vorticity change,which is beneficial to the precipitation development during the pre-flood season in south China.There is a significant positive correlation between the date of the first flood season in south China and the westerly wind around the pole in March,and the strongest correlation with the stratospheric zonal wind over Barents Sea.On the contrary,the height of polar stratospheric potential is negatively correlated with the opening of flood season.It shows that the stronger the circumpolar westerly wind in March,the higher the polar potential height field,and the later the flood season begins in south China.Generally speaking,the opening time of the first flood season in south China is related to the changes of the stratospheric polar vortex.Based on the tropospheric circulation factors such as the subtropical high ridge line in the South China Sea in January,the subtropical high ridge line in North Africa in March,the 1000 hPa zonal wind in south China in March,and the polar vortex in the Arctic stratosphere in the same period,the starting date of the pre-flood season in south China is fitted by multi-variables,it is found that before adding the intensity of the polar vortex in the stratosphere in March,only the fitting curve of the tropospheric circulation factor is selected,and the correlation with the actual starting time series is 0.57,when the intensity factor of stratospheric polar vortex was added in March,the correlation reached 0.59,and the fitting effect was improved to some extent. |
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