Climatic Features Of Summertime Baroclinic Wave Packets Over Euroasia And Their Possible Impacts On Severe Precipitation Events In Southeast China

Using NECP/NCAR reanalysis daily,monthly,and climatology data along with 2479 weather stations precipitation data in China from National Meteorological Information Center during 1979 to 2016,and using power spectrum,one point lag correlation coefficent,wave activity-flux,information flow,we have analysed the relationship between precipitation in the middle and lower reaches of Yangtze River and baroclinic wave packets in the upper troposphere over Eurasia,and the relationship between precipitation in the Fujian-Jiangxi region and baroclinic wave packets.The result demonstrates that:During the Meiyu period,the wave numbers of impacting precipitation in the middle and lower reaches of Yangtze River focus on 5 to 9,and the dominate wave number is 7.The zonal wave length is from 40 to 72 longitudes,and the group speed of wave packets is about 19 longitudes one day.The high frequency baroclinic disturbances disperse the wave energy downstream into East Asia in Meiyu period.The disturbances appear in the Black sea,and then propagate southeastwards to middle and lower reaches of Yangtze River during 3 or 4 days.This wave energy propagation is well displayed by the baroclinic wave packets.The wave-activity fluxes also show that disturbance energy disperses from westerlies in middle-latitudes southeastwards into the middle and lower reaches of Yangtze River in Meiyu period.To further understand the relationship between the Rossby wave packets and severe precipitation in the middle and lower reaches of Yangtze River,we calculate the information flow,showing that there are indeed information flows between the baroclinic wave packets and precipitation changes in middle and lower reaches of Yangtze River in Meiyu period.These results provide clues to understanding and forecasting the heavy rainfall events in middle and lower reaches of Yangtze River.The wave packets on the middle and lower reaches of Yangtze River make precipitation be stable.During the heavy rainfall period in Fujian-Jingxi,wavenumbers is from 5 to 9 in Euroasia.Different from those in the middle and lower reaches of Yangtze River valley,the disturbances appear in the Balkhash lake,and then propagate southeastwards to middle and lower reaches of Fujian-Jiangxi region during 3 or 4 days.This wave energy propagation is well displayed by the baroclinic wave packets.The wave-activity fluxes also show that disturbance energy disperses from westerlies in middle-latitudes southeastwards into the Fujian-Jiangxi region in heavy rainfall period.To further understand the relationship between the Rossby wave packets and severe precipitation in the Fujian-Jiangxi region,we calculate the information flow,showing that there are indeed information flows between the baroclinic wave packets and precipitation changes in the Fujian-Jiangxi region in heavy rainfall period.Different from those in the middle and lower reaches of Yangtze River valley,wave packets on the Fujian-Jiangxi make the precipitation in this region be unstable,while the reason of making it more stable is the wave packets from the north of Balkhash lake to Siberia region and south of Balkhash lake to south of Meditterranean.Waves with Wavenumbers 5-6 are dominant in the mid-latitudes of northern hemisphere,while the wavenumber 6 is more frequent.Wavenumbers 4 and 7 also appear in a few years.Wave packets largely originate from western Europe to the north of the Alps,which corresponds to the upstream of Black Sea and Caspian Sea.Wave energy propagates downstream from this region.During the 38-year period,the horizontal wavenumbers 5-6 are dominant for baroclinic wave packets in Euroasia.The meridional width of the wave packets can be up to 17 latitudes and they can extend up to 120 longitudes along the zonal direction.The amplitudes of the wave packets grow larger and tilt northwestward with increasing height.Active areas of wave packets are mainly concentrated in the Alps,the Black Sea and the Caspian Sea.The wave packets follow three routes to propagate eastward,northeastward or southeastward.The eastward propagating wave packets can reach eastern China and the Upper Jansk Mountains of Siberia.In the summer,the zonal and meridional propagation velocities of the Rossby wave packet can reach 32.14m/s and 6.95m/s,respectively.Interannual variability of summertime precipitation in southern China are to varying degrees correlated with baroclinic wave activities in different regions of Euroasia.These results have great implications for in-depth understanding of characteristics of wave packets and their impacts on heavy precipitation anomalies in southern China.Such as Precipitation in the middle and lower reaches of Yangtze River valley is related to wave packet activities from northeastern Aral Sea to the west of Lake Balkhash,while precipitation in Fujian-Jingxi is closely associated with wave packet activities from the Norwegian Sea to the east of Ural Mountains,from Tianshan to Bohai and from lake Baikal to Outer Hinggan Range.