Analysis And Diagnosis Of Extreme High Events On Intraseasonal Scale In Summer In Eastern China

Used the daily maximum temperature observation data of 189 stations in China and the reanalysis data of NCEP/DOE,ERA-interim from 1979 to 2017,by method of Butterworth band-pass filter,EOF,wavelet,Power spectrum analysis and composite analysis,features and causes of extreme hot events in Eastern China on intraseasonal time scale.Furthermore,extreme high temperature events related to the seasonal oscillation are classified and the low-frequency circulation characteristics of three different types of extreme high temperature events are discussed.Thermodynamic diagnosis was performed.The conclusion is as follows:(1)On a seasonal scale of 10 d,there are two areas with frequent high temperature days in north China and south China.The number of high temperature days occurred in early July and lasted until mid-august in the south of the Yangtze River;the period of high temperature days in north China is from mid-June to early July.The daily maximum temperature data of the national stations in summer half year minus the high temperature threshold value of the station results show that,there are seasonal variation signals in south of the Yangtze river and north China.In the first three spatial modes,south of the Yangtze river and north China of the first and third spatial modes are all high value regions,The distribution of the first mode is uniform nationwide and the distribution of the third mode is opposite to that of the northeast and the south of the Yangtze River;The second mode distribution is characterized by the reverse phase change between Xinjiang,western Tibet and other regions;The corresponding three time coefficients all present seasonal variation signals of about 10-25 d,and the first time coefficient also has obvious interannual signals.(2)The results of power spectrum analysis show that the daily maximum temperature in the Yangtze River and north China has a significant 10-25 d oscillation period.From 1979 to2017,a total of 79 extreme high temperature events occurred in the Yangtze River region,including 41 extreme high temperature events related to seasonal changes,and more than half of them were related to low-frequency fluctuations.A total of 49 extreme high temperature events occurred in north China,including 31 extreme high temperature events related toseasonal changes,and the vast majority of extreme high temperature events were related to low-frequency fluctuations.(3)The thermodynamic diagnosis results show that the convective change term,which contributes more to the temperature rise in the Yangtze River region,contributes greatly to the active phase 5.The direct calculation method of surface flux verifies the non-adiabatic heating term in the inverse algorithm.The results show that the surface short-wave radiation absorbs the solar radiation energy and is emitted in the form of long-wave radiation,latent heat flux and sensible heat flux.The results of thermal diagnosis in north China show that the non-adiabatic heating term,which contributes more to the temperature rise gradually,contributes greatly to the active phase 5.In north China,the phase distribution of surface flux is relatively complex,but the surface short-wave radiation is still the main energy absorbed by solar radiation in the heating process of phase 1 to phase 5.(4)Among the three types of circulation in the Yangtze River region,all present northwest and southeast wave columns on the upper troposphere at 200 hPa,but the middle latitude wave columns of the first and second circulation types are more obvious at the same time,while the third type does not present the same zonal wave train.In the process of warming phase evolution in the Yangtze River region,the upper troposphere is controlled by negative anomaly in 2-4 phases.At 850 hPa,the low-frequency wave series in the three circulation types are not obvious,but in the heating phase,the sky over the Yangtze river region is controlled by positive anomaly,that is,the circulation characteristics of convergence in the upper layer and divergence in the lower layer are formed,which is conducive to the phenomenon of temperature increase caused by the downdraft in the Yangtze river region.(5)Among the three types of circulation in north China,all present northwest and southeast wave trains on the upper troposphere at 200 hPa,but the middle latitude wave trains of the first and third circulation types are not obvious,and only the second type presents the zonal wave trains.The evolution of warming phase in north China is consistent with that in the Yangtze River,and the upper troposphere is controlled by negative anomaly in 2-4 phases.At 850 hPa,the low-frequency wave train in the three types of circulation is not obvious,but in the warming phase,the sky over north China is controlled by the positive anomaly,that is,the circulation characteristics of convergence in the upper layer and divergence in the lowerlayer are formed,which is conducive to the warming phenomenon caused by the downdraft in north China.