The Persistent Heavy Rainfall In The Southern China In The Fall Of 2015

There's a persistent heavy rainfall(PHR)event occurring in Southern China in the fall 2015 which is rare in history.With the NCEP/NCAR daily reanalysis data and the TRMM satellite precipitation data,this work analyzed the large-scale circulation situation during this anomalous PHR event and the roles of low-frequency oscillations in the building of stable circulation for the PHR event.The PHR event influenced an extremely broad area from the boundary of Guangxi and Hunan to the boundary of Jiangxi,Zhejiang and Fujian.The whole event consisted of five significant sub-processes precipitation.The 3rd sub-process of precipitation occurring from 10 November to 13 November was a typical rainstorm process.It lasted 4 days with a single-day rainfall more than 90 mm near the center of the heavy rain area.During the period of this heavy precipitation event,the favorable and stable circulation situation has the following characteristics:1)The South Asia high and the westerly trough above East Asia are anomalous more west compared to year-on-year average in the upper troposphere.The southwesterly in the front of the trough and the northwesterly in the northeast of the South Asia high make up the significant divergence in upper levels above the Southern China.The upper-level jet core in the East Asia located at front left of Southern China,which favored for the large-scale upward motion in Southern China via the geostrophical forcing.2)An anomalous warm and deep ridge situated near the 15°E in the middle-high-latitude in the Northern Hemisphere,and the zonal propagating Rossby waves is active in the middle latitude in the middle-to-upper troposphere.Under the effect of the northwest winds prevailing in the front of the ridge,the high-latitude cold air was adevected into the mid-latitude region and then intrude into the Southern China along with the propagation of Rossby waves.3)The Northwest Pacific subtropical high was stronger than the climatological average in the same period of the year in the lower-to-middle troposphere,which was beneficial to the transport the water vapor from the South China Sea and the Bay of Bengal to the Southern China.4)Distinct front and low-level jet situated in Southern China in the low levels.It is found that the persistent heavy rainfall event was resulted from the interaction of multi-scale perturbations.The common effect of different scale disturbances made the heavy rainfall and weak precipitation intermittently occur in the south of China under the condition of stable circulation,and there appeared five heavy precipitation processes sequentially.In the PHR event,the low frequency oscillations including 10-20-day,20-50-day and 50-day-above scale are very active.Among them,the low-frequency oscillation of 50-day-above makes obvious convergence and divergence within the equatorial area.It makes a strong divergence situation in lower-level area of Indian Ocean,and has a trend to spread to the east.In the whole event,this kind of low-frequency oscillation which makes convection controls Malaysia Indonesia and nearby,so these areas present an obvios divergence situation on the upper-level 200hPa,this also will leads to a stronger divergence situation in lower level of west Pacific subtropical high.The stronger WHSP will intensify the south flow which contain moisture,this is benficial for the rain.And on the upper-level,because of the strong convection in Indian Ocean,there is an obvious divergence situation which lead to an anticycle circulation in upper-level of the north Indian Ocean.This anticycle makes the South Asia high move to the west and intensified,and this will lead to the divergence situation in the upper-level of the precipitation region.So,the 50-day-more low-frequency oscillation is favorable to the precipitation event both in lower level and upper level.As to the low-frequency oscillation of 10-20-day scale,it affects the 1st and 3rd precipitation mainly in the upper-level.In the 200hPa level,there are 2 wave trains,one is from north of 30°N and propagate to the southeast,the other is in the south of 30°N and propagate to the northeast,they both propagate to East Asia.During the period of 1st and 3rd precipitation,the north one leads to a cyclonic circulation to the northeast of the precipitation region,the south one leads to a anticyclonic circulation to the southwest of the precipitation region.In this case,the precipitation region locates between these 2 opposite system,this makes a divergence situation in the upper level of the precipitation region.It is beneficial for the precipitation.In the matter of the 20-50-day low-frequency oscillation,it has a significant propagation characteristic in the 500hPa middle level of middle-high latitudes.This low-frequency oscillation makes an anticycle in the west of Lake Baikal before this PHR events.And this anticycle propagates to the southeast in th middle latitude,and arrives at the Bohai Gulf before the 2nd precipitation.At the same time,there is an anticyclone in 500hPa level above the west Pacific,these two anticyclone will merge,and then it appears a stronger anticyclone.So the south flow in the precipitation region will be intensified by the steering current of the anticyclone,it is beneficial for the transportation of the moisture and making the precipitation.This anticyclone's centre stays at Bohai Gulf until the end of 4th precipitation,and then continues to propagate to the east later.