The Relationship Between Summer Regional Extreme Precipitation Events In Eastern China And Baroclinic Rossby Wave Packet Activity In The Upper Troposphere

Using the daily NCEP/NCAR reanalysis,datasets of daily basic meteorological elements from China national meteorological stations,the CPC global daily precipitation grid dataset,and the CMA tropical cyclone best track dataset from the Shanghai Typhoon Research Institute,this paper analyzes the variation characteristics of regional extreme precipitation events and the climatic characteristics of regional precipitation in eastern China in summer and its relationship with baroclinic Rossby wave activity in the upper troposphere over Eurasian.The main conclusions are as follows:(1)The relationship between summer precipitation and Rossby wave activity in the middle and lower reaches of the Yangtze River(MLRYR)is different in multi-year mean and special years.In daily climatology,the typical wavenumbers for meridional wind disturbances and lowfrequency meridional winds in the upper troposphere at mid-latitudes are 4-6 waves,while highfrequency meridional winds are 7-9 waves.The mobility wave train and Rossby wave packets can still be detected in the subtropical westerly jet stream.At this time,the downstream dispersion characteristics of the high-frequency fluctuations are obvious,and compared with the northern wave packets,the southern wave packets have a more pronounced influence on the high-frequency precipitation in MLRYR.On the other hand,the climate state and lowfrequency fluctuations show quasi-stationary,which suggests that low-frequency or even quasistationary forcing plays an important role in daily climatology.When the Meiyu period in 2020 is chosen as a particular year,the wavenumbers of high-frequency(2-14 day)meridional winds at 300 h Pa over the mid-latitude ranges from 5 to 7.The high-frequency fluctuations originate near Lake Baikal.It travels from northwest to southeast along the westerly jet stream to MLRYR,bringing the eddy energy required for unusually heavy precipitation in the downstream region.(2)Taking the Yangtze-Huai Rivers(YHR)region and Huang-Huai Rivers(HHR)region as examples,regional daily-precipitation extreme events(RDPEs)in the eastern China region are closely related to the wave packet activity.Using the percentile threshold method,RDPEs are screened and analyzed,and it is found that when RDPEs occur in YHR region and HHR region,most of the fluctuations in the upper tropospheric originate near the Caspian Sea or the Black Sea and take about 4 days to travel to the downstream areas.When RDPEs occur over YHR region,the eddy enstrophy in the upper troposphere reaches its maximum on day-1 and rapidly decreases from then on,while it reaches its maximum on day0 in the middle and lower troposphere.The advection transport term of the time-mean airflow to the disturbance vorticity and the horizontal divergence term in the disturbance airflow are the major contributors to the change of the eddy enstrophy over YHR region.The eddy kinetic energy over HHR region decreases rapidly in the upper troposphere and strengthening rapidly in the middle and lower troposphere from day-1 to day0.The main causes of the variation of the eddy kinetic energy are the kinetic energy production term,the advection transport term,and the barotropic conversion term.Therefore,the enhancement and maintenance of the eddy enstrophy and the eddy kinetic energy associated with wave packet activity over the region play an important role in the development of RDPEs.(3)During the period of extreme heavy precipitation in North China from 18-22 July in2016,the wave disturbance energy in the lower troposphere mainly propagates in the meridional direction while in the upper troposphere it propagates in the zonal direction.The influence of wave disturbance energy in the lower troposphere on North China is more obvious than that in the upper troposphere.The enhancement and maintenance of the eddy kinetic energy in North China are mainly due to the combined effects of the divergence of non-geostrophic eddy geopotential flux,the baroclinic conversion term and the residual term.Besides,the change of eddy heat flux supports the barotropic and baroclinic conversion,the change of eddy momentum flux is favorable to the enhancement of the eddy kinetic energy,and the variation trend of eddy kinetic energy and eddy flux is consistent with that of precipitation.The above results have deepened the understanding of the causes of extreme precipitation events in eastern China and could provide clues for effective predictions.