The Persistent Heavy Rainfall over Southern China in June 2010: Evolution of Synoptic Systems and the Effects of the Tibetan Plateau Heating

LI Xuesong(李雪松),LUO Yali(罗亚丽),and GUAN Zhaoyong(管兆勇)

The Persistent Heavy Rainfall over Southern China in June 2010: Evolution of Synoptic Systems and the Effects of the Tibetan Plateau Heating

LI Xuesong1,2,3(李雪松),LUO Yali1∗(罗亚丽),and GUAN Zhaoyong2(管兆勇)

1 State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,Beijing 100081

2 Key Laboratory of Meteorological Disaster of Ministry of Education,School of Atmospheric Sciences, Nanjing University of Information Science&Technology,Nanjing 210044

3 Dalian Meteorological Bureau,Dalian 116001

This study investigates influencing weather systems for and the effect of Tibetan Plateau(TP)’s surface heating on the heavy rainfall over southern China in June 2010,focusing on the four persistent heavy rainfall events during 14-24 June 2010.The major weather systems include the South Asian high,midlatitude trough and ridge,western Pacific subtropical high in the middle troposphere,and shear lines and eastwardmoving vortices in the lower troposphere.An ensemble of convection-permitting simulations(CTL)is carried out with the WRF model for these rainfall events,which successfully reproduce the observed evolution of precipitation and weather systems.Another ensemble of simulations(SEN)with the surface albedo over the TP and its southern slope changed artificially to one,i.e.,the surface does not absorb any solar heating, otherwise it is identical to CTL,is also performed.Comparison between CTL and SEN suggests that the surface sensible heating of TP in CTL significantly affects the temperature distributions over the plateau and its surroundings,and the thermal wind adjustment consequently changes atmospheric circulations and properties of the synoptic systems,leading to intensified precipitation over southern China.Specifically, at 200 hPa,anticyclonic and cyclonic anomalies form over the western and eastern plateau,respectively, which enhances the southward cold air intrusion along the eastern TP and the divergence over southern China;at 500 hPa,the ridge over the northern plateau and the trough over eastern China are strengthened, the southwesterly flows along the northwestern side of the subtropical high are intensified,and the positive vorticity propagation from the plateau to its downstream is also enhanced significantly;at 850 hPa,the lowpressure vortices strongly develop and move eastward while the southwesterly low-level jet over southern China strengthens in CTL,leading to increased water vapor convergence and upward motion over the precipitation region.

persistent heavy rainfall over southern China,convection-permitting ensemble simulation, circulation and weather systems,Tibetan Plateau’s heating effect

1.Introduction

Covering about a quarter of the Chinese territory, the mean elevation of the Tibetan Plateau(TP)is more than 4000 m above sea level.Because of its special location,large size,and high elevation,the TP’s dynamic and heating effects can exert great influences on weather and climate in China,East Asia,and even throughout the globe(Ye and Gu,1955;Flohn,1957). The diabatic heating of the TP has a significant impact on the planetary-scale circulation and summer monsoon circulation(Hahnd and Manabe,1975;Huang,

1985;He et al.,1987;Kitoh,2004).The air over the TP sinking in winter and rising in summer acts like a huge“air pump”that affects surrounding and global atmospheric circulations.In summer,the plateau is a heat source on average compared to the atmosphere over the plateau(Ye and Gao,1979).The surface sensible heat flux,which is large near the northern and southern flanks of the TP,appears to be the major driving source.This“sensible heat pump”(Wu et al., 2006)regulates the development,advance,and retreat of the East Asian rainband(Xu et al.,2010).Research has also indicated that the lower sensible heating flux of TP in spring will delay the seasonal conversion of the land-sea thermal contrast in East Asia,and weaken the East Asian summer monsoon circulation.In contrast,a larger sensible heating flux in spring can lead to an earlier onset of the East Asian summer monsoon and strengthen the monsoon circulation(Duan et al., 2012).

Supported by the National(Key)Basic Research and Development(973)Program of China(2012CB417202),National Natural Science Foundation of China(41175049 and 41221064),Basic Research Funds of the Chinese Academy of Meteorological Sciences (2012Y001),and National Science and Technology Support Program of China(2012BAC22B03).

∗Corresponding author:yali@cams.cma.gov.cn.

©The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2014

The generation and development of the weather systems over the TP are significantly affected by the plateau’s surface thermal conditions.For example,the plateau’s surface sensible heating can reduce atmospheric stability,increasing the boundary perturbation (Wang,1987).This effect plays an important role in the generation of the low-pressure vorticity over the plateau and its southeastern margin near the Sichuan basin(Luo et al.,1991;Luo and Yang,1992;Chen et al.,1996;Li et al.,2002;Li and Liu,2006).By comparing adiabatic processes,diabatic processes,presence and absence of surface sensible heating and latent heating,respectively,in a series of numerical experiments,some scholars concluded that the formation of the plateau vortex is mainly caused by diabatic processes.Within the diabatic processes,they concluded that the plateau surface sensible heating effect is much more important than the surface latent heating(Luo and Yang,1992).Subsequent case studies also found that if there is no surface sensible heating effect,the plateau vortex will not form(Chen et al.,1996).Moreover,the strengthening and developing of the TP vortex after its formation is generally considered to be closely related to the release of latent heating,and the convectively generated latent heating feedback effect is more important than the large-scale latent heating(Chen et al.,1996).The surface sensible heating also plays an important role in the development of the plateau vortex,depending on the configuration of the vortex center and the sensible heating center.When the two centers are appropriately located,greater differences in temperature create a more conducive environment for the development of the vortex(Li et al., 2002).

In recent years,persistent heavy rainfall events have attracted increasing attention,and some progress has been made on the impact of TP on the persistent rainfall over southern China.Some researchers pointed out that the existence of the southwest wind center over the southeastern edge of the plateau is the main climatic cause of persistent precipitation in southeastern China(Wan and Wu,2007;Wan et al., 2009).In spring,TP is a weak heating source.TP heating-forced low-level cyclonic winds can strengthen the southwest flows over its southeastern margin, transporting warm air to South China.Meanwhile, the plateau’s bypassing effect strengthens the southward movement of northern cold air.The warm and cold air masses converge over South China,increasing spring precipitation thereby(Liang et al.,2005;Wan and Wu,2007;Wan et al.,2009).The plateau’s strong surface heating effect can maintain from spring to summer,turning the summer plateau into a strong atmospheric heating source(Wang et al.,2013).In summer, the TP’s heating effect is a main factor causing lowerlevel cyclonic circulation and higher-level anticyclonic circulation over the plateau and its surrounding areas(Wu and Liu,2000;Wu et al.,2002).The TP’s thermal forcing can strengthen the South Asian high, with its center moving northwestward(Liang et al., 2005).It can also lead to a Rossby wave train downstream.A cyclonic response over the northeastern plateau strengthens the lower-level northerly over the northern China,and an anticyclonic response over the western Pacific enhances the subtropical high and the lower-level southerly along its west.Warm and cold air flows converge over the Huaihe River basin,strengthening summer rainfall in the region.In summer,the TP’s sensible heating strengthens the warm center of

the plateau,which causes the eastward-moving warm advection to be enhanced,with upward motion and persistent precipitation in the Huaihe River basin also strengthened(Wang et al.,2013).In addition,changes of the East Asian circulation pattern and the abnormal distribution of clouds,which are induced by the abnormal thermal conditions of the TP surface,were main causes of the persistent rainfall over the Huaihe River in summer 1991(Zhang et al.,1995).

Previous studies of the TP focused mainly on the climate influence perspective,i.e.,more about the TP’s influence on the general circulation,monsoon, etc.,but less about the influence of TP from a synoptic perspective.There are few studies that look at the TP’s heating effects on the persistent heavy rainfall in southern China.Previous research has concentrated relatively more in spring,with fewer studies investigating summer patterns.More serious disasters are caused by heavy rainfall in summer and the physical mechanisms underlying the persistent heavy rainfall in southern China are still unclear.Therefore,it is necessary to analyze the TP’s impact on persistent heavy rainfall in southern China from the perspective of synoptic meteorology,to reveal the physical mechanisms of TP’s thermal forcing effects,and to identify the main influencing factors and pathways.

South China is a region with the longest rainy season,the highest maximum precipitation,and the most flood disasters.Persistent heavy rainfall often occurs in the rainy season,influencing economic development and people’s livelihood.In June 2010,South China and southern Jiangnan experienced persistent heavy rainfall that caused severe flooding,landslides,mudslides,and other disasters.This affected 1.432 million people in Fujian,Guangxi,Sichuan,Guangdong,and Jiangxi provinces,with a death toll of 42,missing people of 36,and collapsed houses of more than 6000.The direct economic losses were approximately 2.04 billion yuans(http://news.xinhuanet.com/politics/2010-06/16/c-12226604.htm).Kong(2010),Wang et al. (2011),and Yuan et al.(2012)described the largescale circulation patterns during the persistent heavy rainfall events in June 2010 in southern China.The higher-level South Asian high and westerly jet were conducive to the divergence over South China(Yuan et al.,2012).At middle and lower levels,the Ural blocking high and the Lake Baikal trough were stronger than normal and maintaining stable(Wang et al., 2011),with much fluctuation moving eastward in low latitudes.The subtropical high was stronger and located anomalously westward(Kong,2010).Along the northwest side of the subtropical high,the southwest monsoonal flows transported water vapor from the western Pacific,the Bay of Bengal,and the South China Sea to South China,and then converged with the dry and cold air from the mid and high latitudes (Wang et al.,2011;Yuan et al.,2012).He and Li (2013)carried out numerical experiments to investigate the TP’s topographic effects on persistent rainfall events in southern China in May 2010.Based on all these prior studies,we will further examine the mechanism of how the TP’s heating effects influence the persistent heavy rainfall in southern China in June 2010.

First,we compare and analyze the rainfall distribution and large-scale circulation in June 2008 and June 2010 to find similarities and differences in related persistent rainfall characteristics.Both 2008 and 2010 experienced the strongest persistent rainfall in southern China.Compared with June 2008,June 2010 is characterized by frequent occurrences of low-value systems in the middle and lower troposphere from the TP to the lower reaches of the Yangtze River(Figs.2c and 2d).Persistent precipitation over southern China is more likely to be influenced by the plateau’s heating effect,so we select June 2010 for further study.We analyze the evolution of weather systems in each precipitation event during 14-24 June 2010 in order to learn more about the relationship between the persistent heavy rainfall over southern China and the short-term changes of the main weather systems.For instance,we analyze the South Asian high,western Pacific subtropical high(WPSH),as well as the middle-and lowerlevel low systems.We then carry out two ensembles of high-resolution(4-km horizontal grid spacing)simulations,including a control-and a sensitivity-ensemble experiment,using the WRF model(Skamarock et al., 2008)for the four rainfall events.The control(CTL)

experiment reproduces the observed surface precipitation,atmospheric circulation,and the main features of the weather systems.The sensitivity(SEN)experiment is carried out with the surface albedo over the TP being changed artificially to one(Fig.1c);otherwise, it is identical to the CTL experiment.In the SEN experiment,the TP does not absorb solar radiation and the surface gradually cools,thus the plateau’s surface sensible heating effect on the atmosphere is eliminated. Finally,we compare the two ensemble simulations and discuss the influence of the TP’s heating effects on the persistent heavy rainfall in southern China in June 2010.

2.Data,model,and numerical experiments

2.1 Data

To reveal the distribution of precipitation,we use a 0.1°× 0.1°gridded hourly precipitation dataset (http://cdc.cma.gov.cn/dataSetDetailed.do)in June 2008 and June 2012. The data are generated by merging the Chinese automatic weather station rainfall records with CMORPH(Joyce et al.,2004).

The ERA-interim reanalysis from ECMWF with a horizontal resolution of 1.5°×1.5°(Dee et al.,2011) is averaged for June of 1981-2010.This is used to reveal the climatic circulation patterns in June.

We make use of the ERA-interim reanalysis data from 1800 UTC 12 to 0000 UTC 25 June 2010 at 6-h intervals(0000,0600,1200,and 1800 UTC)with a horizontal resolution of approximately 0.7°×0.7°and 37 vertical layers.This is used to analyze the circulation as well as weather systems and their evolution during the study period.The data are also used to provide the initial and lateral boundary conditions for the WRF model.

2.2 Model and experimental design

Recent studies show that numerical simulations (or forecasts)with 1-4-km horizontal resolutions can explicitly represent convective and mesoscale circulations,which is called“convection-permitting.”This method avoids the use of deep convective parameterization schemes and associated large uncertainties,thus improving the model’s precipitation simulation(Kain et al.,2008;Lean et al.,2008;Schwartz et al.,2009). Studies also show that the ensemble-mean precipitation from ensemble simulations(or forecasts)is much closer to observation than the results from individual members(Du et al.,1997;Chien and Jou,2004). Meanwhile,ensemble forecasts also show advantages to single deterministic forecasts for other variables such as wind(Grimit and Mass,2002),temperature, and humidity(Stensrud and Yussouf,2003).Therefore,we carry out a series of high-resolution(4 km) ensemble simulations for the persistent precipitation in June 2010 to study the heating effects of the TP.

In this study,we carry out two five-member convection-permitting ensemble simulations for the persistent heavy rains during 14-24 June 2010 in southern China using the WRF model.A set of control experiment(CTL)and sensitivity experiment(SEN) are carried out,respectively.All the simulations run with a single grid across the Eurasian continent,covering 15°-50°N,50°-125°E(Fig.1a).The east-west distance is up to 7600 km,the north to south is approximately 3550 km,the horizontal resolution is 4 km,and there are 1876×888 grid points and 40 vertical layers.The five members of each ensemble are produced with different initial fields,i.e.,starting from 1800 UTC 12 and 0000,0600,1200,and 1800 UTC 13 June 2010,respectively.All simulations terminate at 0000 UTC 25 June 2010.Outputs from 0000 UTC 14 to 0000 UTC 25 June 2010 are analyzed. The main plateau is defined as the region more than 3000 m above sea level over 25°-40°N,60°-110°E. The southern slope of the TP(SSP)is 500-3000 m above sea level from 70°to 98°E.In the SEN,the surface albedo over the TP and its SSP is changed artificially to one(Fig.1c).This means that the surface of the plateau and the SSP does not absorb solar heating,and thus the surface sensible heating effects on the atmosphere are removed.Through numerical simulation studies,Li et al.(2000)showed that when the surface albedo increases,the net radiation received by the ground is reduced,the ground temperature drops,the sensible heating of the bottom atmosphere is weakened,local convection and precip-

itation are suppressed and the corresponding condensation heating in the middle troposphere is also weakened.Boos and Kuang(2010)also artificially changed the surface albedo over the TP to one when studying the impact of the TP’s heating effects on the Asian monsoon.Physical parameterization schemes and model configuration details of the two simulations are shown in Table 1.All of the simulation results be-

low are the average of all members in an ensemble simulation.

Fig.1.(a)Simulation domain and topography.Surface albedo averaged during 14-24 June 2010 for the(b)control experiment(CTL)and(c)sensitivity experiment(SEN).The thick black line represents the 3-km topography contour while the thin black lines represent the Yellow River and the Yangtze River.

Table 1.Summary of the physical parameterization schemes used in the experiments

3.Rainfall and atmosphere circulation features in June of 2008 and 2010

Analyzing the daily average precipitation during June of 2006-2012 in southern China(20°-28°N, 110°-120°E)using CMORPH(figure omitted),we find that the maximum precipitation occurred in June of 2008 and 2010.In this section,we compare the rainfall and circulation characteristics in June of 2008 and 2010 to analyze the similarities and differences between the two years.

Comparing the distribution of cumulative precipitation(Figs.2a and 2b)in June of 2008 and 2010, we find that,in June 2008,heavy rainfall in southern China mainly occurred over the South China coasts as well as in Guangdong Province and Guangxi Region.In June 2010,several heavy rainfall centers appeared,mainly located in Guangdong,Guangxi,Fujian,Jiangxi,and Zhejiang.Compared with the situation in June 2008,the rainband in 2010 was northward and heavy rainfall centers were more dispersed.

The geopotential height and its climatological anomalies at 500 hPa in June 2008(Fig.2c)show that positive geopotential height anomalies extended northeastward from the northwestern TP to the Sea of Okhotsk.Negative anomalies covered eastern China, and the midlatitude trough and ridge were stronger. Cold and warm air activities were also stronger and they converged in the Yangtze River basin and South China,which was conducive to persistent heavy rainfall. In June 2010(Fig. 2d),negative anomalies of 500-hPa geopotential anomaly extended east-westward from the TP to the Yangtze River basin,suggesting frequent low system activities.The low-latitude positive anomalies covered from the Arabian Sea to the western Pacific,illustrating that the western Pacific subtropical high(WPSH)was stronger and oriented westward.The 588-dagpm geopotential height contours extended westward to about 105°E and the southwest flows along the northwest side of the sub-

tropical high were transporting moisture to South China.The cold and warm air masses converged in the Yangtze River basin and South China.Compared with the case in June 2008 and the climate mean condition,the WPSH was much stronger and much more westward in June 2010.

Fig.2.Comparison between(a,b)accumulated precipitation(color shading;mm;blue lines denote 1-and 3-km terrain contours)and(c,d)500-hPa geopotential height(black solid line;dagpm)and its climatic anomalies(color shading; dagpm;thick purple lines denote the 3.5-km terrain contour)during(a,c)June 2008 and(b,d)June 2010.

Comparing the anomalies in divergence and wind fields at 850 hPa between June 2008 and June 2010 (figure omitted),we find that the similarities between the two years lie in an anomalous convergence zone that extended from south of the Yangtze River to South China,the stronger southwest flows along the South China coast,and the control of anticyclonic circulation anomalies over the South China Sea.The difference is that in June 2008 the abnormal wind convergence zone near the South China coast was mainly located in Guangdong and Guangxi,whereas in 2010 it was relatively more to the north,corresponding to the differences in location of precipitation.

4.Rainfall distribution and weather system evolution during 14-24 June 2010

To further clarify the relationship between the persistent heavy rainfall over southern China in June and main short-term changes of the weather systems such as the WPSH,we choose four strongest precipitation events during 14-24 June 2010 for a further analysis.

During 14-24 June 2010,four cumulative precipitation centers were found located in the northern Arabian Sea,the northern part of the Bay of Bengal,the SSP,and southern China(Fig.3a).The overall rain band in southern China was east to west oriented with a patchy distribution,and the maximum rainfall centers were mainly located on the borders of Fujian,Zhejiang,and Jiangxi provinces.According to the rainfall distribution and atmosphere circulation pattern, we divide 14-24 June into four sub-periods:14-15, 16-18,19-21,and 22-24 June.We then analyze the spatial and temporal distributions of rainfall and the main weather systems during these sub-periods.

During 14-15 June(Fig. 4a),southern China experienced a heavy precipitation event,showing a northeast-southwest zonal distribution.The precipitation was more than 100 mm in eastern Guangxi, northern Guangdong,and Fujian.During 16-18 June (Fig.4b),heavy precipitation occurred in Jiangnan and South China again,showing a patchy distribution,and there were two main heavy precipitation centers(＞100 mm).One of the centers was located in northern Guangxi,and the other in Jiangxi and Fujian.During 19-21 June(Fig.4c),two heavy rainfall bands appeared in southern China.They were located on the south side of the middle reaches of the Yangtze River(Jiangxi and Hunan)and from the borders of Fujian,Zhejiang,and Jiangxi provinces to the north of Guangxi,respectively.The largest precipitation(＞ 200 mm)appeared on the borders of Jiangxi,Fujian, and Zhejiang provinces.During 22-24 June(Fig.4d), Jiangnan and South China experienced the fourth precipitation process.The heavy precipitation center(＞100 mm)appeared in northern Fujian,Jiangxi,and central Hunan.

Comparing the four events,we find that heavy precipitation was all located in southern China with a zonal or patchy distribution.The difference is that in the first event(Fig.4a)the precipitation occurred relatively south,while in the second,third,and fourth events(Figs.4b-d)the precipitation mainly occurred in Jiangnan and northern South China with the main precipitation center located on the borders of Fujian, Zhejiang,and Jiangxi provinces.

During the four events,the associated weather systems at 200 hPa were the South Asian high and the accompanying high pressure ridge. The South Asian high appeared over the southern TP while the high pressure ridge controlled main TP and eastern China(figure omitted).During the first three precipitation events,the South Asian high gradually intensified,expanded and shifted towards the TP.In the fourth event,the South Asian high substantially weakened and shrinked,and the precipitation gradually dissipated.During the four events,strong divergence mainly occurred over South China,TP,and the Bay of Bengal.However,in the first event,strong divergence mainly occurred over South China coastal areas,while in the second,third,and fourth events,

divergence was found over Jiangnan and northern South China,indicating that the upper-level divergence corresponds well with the location of the surface heavy rainfall area during each event.

Fig.3.Accumulated precipitation(mm)during 14-24 June 2010 from(a)observation(OBS),(b)control experiment (CTL),and(c)sensitivity experiment(SEN);(d)difference in the accumulated precipitation between the control and sensitivity experiments(CTL-SEN);(e)significance test result,where the blue/red shading represents significant values at the 95%confidence level.Blue lines denote the 1-and 3-km terrain contours;black line denotes the coastline,the Yangtze River,and the Yellow River.

The main weather systems at 500 hPa were the mid-and high-latitude trough and ridge,the WPSH, as well as the eastward-moving shallow troughs in the straight westerlies over South China.In the first three events(Figs.5a-c),the mid-and high-latitude trough and ridge strengthened and the WPSH gradually extended westward and northward.In the fourth event (Fig.5d),the trough and ridge moved eastward,the WPSH retreated eastward and southward,and the precipitation gradually stopped.

At 700 and 850 hPa,the main weather systems were the WPSH,the deep trough stretching southward from the North China vortex,shear lines near the Yangtze River,and eastward-moving vortices along the shear line.In the first,third,and fourth events (Figs.5e,5g,5h),southwest vortices were generated around the east of the plateau and moved eastward along the shear line(the vortices were originated from the Sichuan basin and its surrounding areas),and large water vapor flux appeared south of the shear line,or over the vortex centers,or on the right side ahead of the vortex centers’moving direction.In the second process(Fig.5f),South China was located in front of the deep trough that stretched southward from the

North China vortex,and large water vapor flux occurred to the southeast of the trough tip.

Fig.4.Accumulated precipitation(mm)during each rainfall event during 14-24 June 2010 from(a-d)the observation (OBS)and(e-h)the control experiment(CTL).The panels from top to bottom show the first,second,third,and fourth precipitation events.Blue lines denote the 1-and 3-km terrain contours;black lines denote the coastline,the Yangtze River,and the Yellow River.

Previous studies show that the intensity of the South Asian high,the generation and development of TP vortices are closely related to the TP’s heating conditions(Li et al.,2002;Wu et al.,2002). Recently,Li and Duan(2011)found that the diabatic heating over the TP modulates the behavior of the South Asian high to a considerable degree.When the diabatic heating is strong,the upper-level anticyclone is usually located over the western TP to the Iranian Plateau,corresponding to a westward South Asian

high,namely,there occurs the“Iranian high mode.”In contrast,when the heating is weak,the South Asian high is eastward,namely,there appears the“Tibetan high mode.”To further study the impacts of the TP’s heating effect on the weather systems and the persistent rainfall in southern China in June 2010,the following numerical experiments are carried out.

Fig.5.(a-d)500-hPa geopotential height(black solid lines;dagpm),temperature(red dotted lines;℃),wind barbs, and relative vorticity(gray shading;10-5s-1)averaged during each precipitation event during 14-24 June 2010.Blue solid lines denote the 3-km topography contour.(e-h)850-hPa geopotential height(black solid curves;dagpm),wind (vectors),and water vapor flux(blue shading;m g kg-1s-1)at selected times of each precipitation event:(e)1800 UTC 14 June,(f)0000 UTC 17 June,(g)1800 UTC 19 June,and(h)0000 UTC 24 June.Dark black lines denote shear line and green lines denote the Yellow River and the Yangtze River.

5.The numerical simulations

Comparing the simulated and observed cumulative rainfall distributions during 14-24 June(Figs.3a and 3b),we find that the precipitation area,scope,and intensity in the ensemble mean are consistent with the observation.The simulation successfully reproduces the heavy rainfall centers in the northern Arabian Sea, northern Bay of Bengal,the SSP,and southern China. The main discrepancies between the simulation and observation are that the simulated maximum rainfall in southern China,southeast of the TP,and northern Bay of Bengal are stronger,heavy rainfall(＞500 mm) area is larger and more southeastward,and the precipitation over the Indian Peninsula is weaker than the observations.For the four precipitation events over southern China,it is obvious that the simulation reproduces the observed rainfall distribution,with only slight differences in the intensity or location of the rainfall centers(Fig.4).

For the upper-level atmosphere circulations(Figs. 6a and 6b),the simulation successfully reproduces the time-averaged circulation and divergence at 200 hPa. The location and intensity of the South Asian high are consistent with the observation,but the upperlevel divergence over southern China,particularly over the South China coast,are stronger than the observation.This corresponds to the stronger rainfall over the South China coast in the simulation(Figs.3a and 3b).Comparing the simulated and observed circulation and relative vorticity at 500 hPa(Figs.6c and 6d),we find that the simulation reproduces the ridge over the northern TP,the southern trough near the Bay of Bengal,and the WPSH,including their location and intensity.The simulation also reproduces the distribution characteristics of the relative vorticity,including the positive vorticity zone that stretched eastward from the TP to the mid-lower reaches of Yangtze River.The observed relative vorticity averaged over 25°-35°N at 500 hPa(Fig.7a)clearly shows that positive vorticity from the eastern TP propagated eastward four times during 14-25 June and arrived at

the East China Sea 2-3 days later,corresponding to the simulated generation and development of low-level vortices(Fig.7b).

Fig.6.(a,b)Geopotential height(black solid lines;dagpm),temperature(red dotted line;℃),wind barbs,and divergence(gray shading;10-5s-1)at 200 hPa averaged during 14-24 June 2010,and(c,d)geopotential height(black solid curves;dagpm),wind barbs,and relative vorticity(gray shading;10-5s-1)at 500 hPa averaged during 14-24 June 2010 from(a-c)ERA-interim and(b,d)the control experiment(CTL).Blue solid lines denote the 3-km topography contour,gray lines denote coastal lines,and green lines denote the Yellow River and the Yangtze River.

Fig. 7. Relative vorticity(10-5s-1)at 500 hPa averaged over 25°-35°N from(a)ERA-interim,(b)CTL,and (c)SEN.

The fourth event(22-24 June)had the strongest vortices,and is now further analyzed.At 0600 UTC 23 June(Fig.8a),the vortices had already formed near the Sichuan basin.At 1800 UTC 23 June(Fig. 8b),the vortices moved out of the basin and developed eastward.By 1800 UTC 24 June(Fig.8d),the vortices had significantly enhanced with their centers located south of the Yangtze River,and heavy rainfall occurred to the south of the vortices’center.Comparing the simulated(Figs.8e-h)and observed(Figs. 8a-d)circulation at 700 hPa in the fourth event,it is obvious that the CTL experiment successfully reproduces the generation and development of the TP vortices.

Similar to Figs.5e-h,Figs.9a-d show the atmospheric circulation and water vapor flux at 850 hPa for selected times of each precipitation event in CTL and SEN.The ensemble simulations reproduce the observed main weather systems at 850 hPa,i.e.,the WPSH,the southern trough near the Bay of Bengal (Figs.9c and 9d),the deep trough stretching southward from the North China vortex(Fig.9b),the shear lines near the Yangtze River,and the eastward-moving vortices along the shear line(Figs.9a,9c,9d).However,from the second to the fourth events,the simulated strong wind is located on the south and east side of the observations;correspondingly,the maximum water vapor flux is also more southward and eastward.This is likely to cause the simulated heavy rainfall to occur to the south and east of the observation in southern China(Figs.4b-d vs.Figs.4f-h). Overall,the CTL successfully reproduces the accumulated rainfall during the entire period and in each precipitation event.The CTL also reproduces the temporal evolution of the circulation and weather systems as well as distributions of variables such as water vapor flux at 850 hPa,relative vorticity at 500 hPa,and wind divergence at 200 hPa.Thus,by comparing the results from CTL and SEN,we can discuss the influence of the TP’s heating effects on the persistent heavy rainfall in southern China during June 2010.

6.Comparison between the control and sensitivity experiments

Fig.8.700-hPa geopotential height(black solid curves;dagpm),wind barbs,and water vapor flux(blue shading;m g kg-1s-1)during the fourth precipitation event at(a,e)0600 UTC 23 June,(b,f)1800 UTC 23 June,(c,g)0600 UTC 24 June,and(d,h)1800 UTC 24 June 2010 from(a-d)ERA-interim and(e-h)CTL.Gray lines denote coastal lines, the Yellow River,and the Yangtze River.

There is a significant difference in the distribution of accumulated rainfall between CTL and SEN (Figs.3b and 3c).In CTL(Fig.3b),the heavy rainfall (＞500 mm)area is obviously larger,with also stronger centers than that in SEN(Fig.3c).The difference in precipitation(Fig.3d)and related significance test (Fig.3e)shows as well that the rainfall in the heavy precipitation center is stronger in CTL.

Comparing the averaged surface sensible heat flux in the two experiments during 14-24 June(figure omitted),we find that except for the western TP,the sensible heat flux is largely positive with its heating center over the central TP in CTL.However,in SEN, most of the plateau displays negative sensible heat

flux.The difference between the two experiments(Fig. 10)shows that in CTL,the TP’s heating effect significantly increases the surface sensible heat flux(by approximately 40-80 W m-2),but the sensible heat flux decreases(by approximately 1-20 W m-2)in eastern China.Changes in the surface sensible heat flux over the TP and its surrounding areas may have changed the atmospheric temperature,further altered the upper-and low-level circulations through thermal wind adjustment,and changed the water vapor flux, convergence and divergence as well as the precipitation in southern China.

Fig.9.As in Figs.5e-h,but for(a-d)CTL and(e-h)SEN.

Fig. 10. Difference of surface sensible heat flux between the control and sensitivity experiments(CTL-SEN; W m-2)averaged during 14-24 June 2010.Purple lines denote the 1-and 3-km topography contours;black lines denote the coastline,the Yangtze River,and the Yellow River.

At 500 hPa,the difference of temperature averaged during 14-24 June 2010(Fig.11a)shows that the TP’s heating effect increases the atmosphere temperature over the TP and its surrounding areas.The maximum temperature center is located near the central TP or slightly west.According to the theory of thermal wind,the temperature anomaly distribution will strengthen the upper-level anticyclone and the lowlevel cyclone.The difference between CTL and SEN is consistent with the above theory.

The difference in geopotential height and wind at 200 hPa(Fig.11b)shows that in CTL the northerly significantly strengthens over the central and eastern TP,with an anticyclonic circulation over its west(the center located in northwestern TP)and a cyclonic circulation over its east.Moreover,in CTL,the geopotential height over the TP,Bay of Bengal,and East China coast is comparatively higher,but is lower in the region between the east of the plateau and the east coast. In CTL,the South Asian high at 200 hPa is much stronger,larger,and more westward,and the corresponding high pressure ridge and trough in eastern China are also intensified.The northerly in front of the ridge is strengthened,leading to stronger cold air intrusion southward.Similarly,the thermal wind induced by the TP’s temperature increase will also lead to enhanced low-level cyclonic wind around the surrounding areas of the TP.The differences in geopotential height and wind at 850 hPa(Fig.12a) show that in CTL the easterly to the north of the TP and the southwesterly to its south are both strengthened,which forms a cyclonic circulation.Moreover, in CTL,the geopotential height over the mid-lower reaches of Yangtze River and to its south is significantly lower than in SEN and is also accompanied by a cyclonic circulation anomaly that corresponds to stronger southwest flows over the South China coast (conducive to water vapor transport to South China). The northerly over the middle Yangtze River is also enhanced(i.e.,dry and cold air southward intrusion enhanced).Meanwhile,in CTL,the column integrated moisture convergence above South China and Jiangnan coast increases(Fig.12b),consistent with the enhanced precipitation in the heavy rainfall area.

In addition,the difference of relative vorticity at 500 hPa(Fig.11a)shows that in CTL the positive relative vorticity zone from the central TP to the midlower Yangtze River is significantly stronger than in SEN.The time-longitude distribution of the relative

vorticity averaged between 25°and 35°N at 500 hPa (Figs.7b and 7c)clearly shows that during 14-25 June the intensity of the eastward-moving positive vorticity in CTL gradually grows stronger than that in SEN with time.

Fig.11.Differences between the control and sensitivity experiments(CTL-SEN)averaged for 14-24 June 2010 of(a) temperature(solid red lines denote positive values,and solid blue lines denote negative values;℃)and relative vorticity (color shading;10-5s-1)at 500 hPa,(b)geopotential height(shading;dagpm)and wind(barbs)at 200 hPa.Purple lines denote the 3-km topography contour;gray lines denote the coastline,the Yangtze River,and the Yellow River.

Fig.12.Differences between the control and sensitivity experiments(CTL-SEN)averaged during 14-24 June 2010 of(a)geopotential height(shading;dagpm)and wind(barbs)at 850 hPa,(b)column water vapor flux convergence (shading;10-6kg m-2s-1)and temperature at 850 hPa(red solid lines denote positive values and blue solid lines denote negative values;℃).Gray lines denote the 3-km topography contour,coastline,Yangtze River,Yellow River,and Huai River.

By comparing the wind,geopotential height,and water vapor flux at 850 hPa between the two experiments at selected times of the four events(Fig.9),we find that in the first and second events(Figs.9a,9b vs. 9e,9f),the two experiments are relatively consistent, but in the third and fourth events(Figs.9c,9d vs. 9g,9h),there is a significant difference.In CTL,lowlevel vortices develop intensely while there is almost no vortex formation in SEN.Consistent with previous studies,this study confirms that the TP’s heating effect is in favor of the generation,development,and eastward-movement of the vortices over the TP and its downstream.In addition,this study confirms that the feedback of latent heat can intensify the vortices and provide a favorable lifting mechanism for the vertical movement of the air downstream.

To illustrate the influence of the TP’s heating effect on the horizontal and vertical circulation,we further analyze the vertical cross-section at 25°N,115°E where the heavy rainfall center is located.The vertical circulations at 25°N in CTL and SEN(Figs. 13a and 13b)are almost the same.The maximum southwest wind is located over 110°-120°E,i.e.,the heavy rainfall area,with strong upward motion.However,in CTL,the southerlies(Fig.13d)and westerlies (Fig.13c)as well as the upward motion are stronger than in SEN.It is obvious that the TP’s heating effect in the mid troposphere leads to the geostrophic wind changing with height on the south side of TP (Fig.13c).The low-level westerlies and upper-level easterlies are strengthened,which is consistent with the wind difference at 850 hPa(Fig.12a)and 200 hPa(Fig.11b)on the south side of the TP.The differences in the vertical circulation,north wind,and temperature(Fig.14c)between the two experiments show that in CTL the warm moist southerly south of 25°N is relatively larger.In addition,the cold and dry northerly to the north and the upward motion over the heavy rainfall area are much stronger in CTL,corresponding to the more intense rainfall in the precipitation center.Overall,the TP’s heating effect leads to significant enhancement of the southwesterlies over Southeast China,and results in strengthening of the northerlies from higher latitudes along the eastern TP and the upward motion over southern China,which enhances precipitation over southern China.

Fig.13.(a,b)Meridional wind speed(shading;m s-1)and circulation(vectors)along 25°N averaged for 14-24 June 2010 in(a)CTL and(b)SEN.(c)Differences in the zonal wind(CTL-SEN;shading;m s-1)and the vertical circulation (vectors).(d)Differences in the meridional wind(CTL-SEN;shading;m s-1)and temperature(red and black solid lines denote positive and negative values,respectively;℃).

Previous studies show that the TP’s heating effect in summer is important to the low-level cyclonic circulation and upper-level anticyclonic circulation over the TP and its surrounding areas(Wu and Liu,2000; Wu et al.,2002).When the rainfall in South China is abnormally large,anticyclonic and cyclonic circulation anomalies usually appear over the northwest of the TP and over the eastern TP to eastern China,respectively(Yang,2011).In this study,the changes in circulation are consistent with previous studies.Moreover,we carry out an ensemble of high-resolution numerical simulations and compare the precipitation processes in two sets of experiments to identity the role of the TP’s heating effect.We find that the lowlevel vortices start to change significantly 5 to 6 days from the beginning of the simulations.In the third and fourth precipitation events in CTL,vortices are generated near the Sichuan basin and develop eastward, but in SEN almost no vortices are found.This illustrates that the TP’s heating effect favors the generation and development of vortices.We clearly demonstrate the main weather systems of each precipitation event and their evolution during the persistent rainfall from a synoptic perspective,especially the generation and eastward movement of the vortices.This reveals the influence of the TP’s heating effect,which is different from the previous climate studies.

7.Conclusions and discussion

In this study,we first compare June 2008 and June 2010,the two strongest persistent rainfall years in southern China in recent years.We analyze the similarities and differences in rainfall distribution and large-scale circulations related to the persistent rainfall.Further focus is placed on four heavy precipitation events during 14-24 June 2010,through analyzing the rainfall distribution and the evolution of main weather systems in each event from the synoptic perspective.Then,we carry out an ensemble of fivemember convection-permitting simulations using the WRF model.The CTL experiment reproduces the observed surface precipitation,atmospheric circulation, and the main features of the weather systems.An ensemble sensitivity experiment(also with five members) is carried out with the surface albedo over the TP

and its south slope artificially changed to one to investigate the influence of the TP’s heating effect on the persistent heavy rainfall in southern China during June 2010.Main results are summarized as follows.

Fig.14.(a,b)Meridional wind speed(shading;m s-1)and vertical circulation(vectors)along 115°E averaged for 14-24 June 2010 in(a)CTL and(b)SEN.(c)Differences in the meridional wind(CTL-SEN;shading;m s-1),vertical circulation(vectors),and temperature(red and black solid lines denote positive and negative values,respectively;℃).

(1)During June 2008,the north of the TP and eastern China were in control of the ridge and trough at 500 hPa,respectively.The trough and ridge were stronger than normal with intensified cold and warm air activities.The heaviest rainfall was mainly concentrated in Guangdong Province and Guangxi Region in South China.June 2010 was characterized by frequent occurrences of low-value systems in the middle and low troposphere from the TP to the lower reaches of Yangtze River.Compared with June 2008,June 2010 had a significantly intensified western Pacific subtropical high which led to more northward located low-level anomalous wind convergence and rainbands,with the heaviest rainfall centers in Guangdong,Guangxi,Fujian,Jiangxi,and Zhejiang.

(2)According to the rainfall distribution and the evolution of the atmosphere circulation,we divide 14-24 June into four episodes:14-15,16-18,19-21, and 22-24 June 2010.The accumulated precipitation of each of the four episodes displayed a ribbon or patchy distribution.In the first event,the precipitation was mainly located in South China,while in the second,third,and fourth events the precipitation mainly occurred in Jiangnan and northern South China.In the four events,water vapor came from the South China Sea and the Bay of Bengal,and the positive relative vorticity propagated eastward from the eastern TP to the East China Sea.These characteristics corresponded to the frequent occurrences of the eastward-moving shallow troughs at 500 hPa, the shear lines,and the eastward-moving vortices in the lower troposphere. During the first three precipitation episodes,the South Asian high gradually intensified,expanding and shifting towards the TP;at 500 hPa,the mid-and high-latitude trough and ridge were strengthened and the WPSH remained strong; in the mid-and lower troposphere,the warm and cold air flows converged over the mid-lower Yangtze River

and in northern South China.In the fourth event,the South Asian high substantially weakened and shrank, the mid-level trough and ridge moved eastward,the WPSH retreated southeastward,and the precipitation gradually dissipated.

(3)An ensemble of five-member convectionpermitting simulations(CTL)with high resolution(4 km)is carried out.The CTL successfully reproduces the accumulated rainfall during the entire period and in each precipitation episode.The area and intensity of the precipitation over South China are consistent with the observations,except that in CTL the simulated heavy rainfall center was slightly southeastward. The CTL also reproduces the upper-,mid-,and lowlevel circulations and the relevant weather systems, including the location,intensity,and evolution of the South Asian high,the mid-and high-latitude trough and ridge,the WPSH,the low-level shear line and vortices.The CTL also reproduces the observed distributions of variables such as water vapor flux,relative vorticity,and wind divergence.

(4)The TP’s heating effect(i.e.,strong surface sensible heating)changes the temperature distributions of the atmosphere over the plateau and its surroundings. The thermal wind adjustment consequently changes atmospheric circulations and properties of synoptic systems from the lower to upper troposphere.At 200 hPa,northerlies over the central and eastern TP are strengthened,with anticyclonic and cyclonic anomalies formed over the western and eastern plateau,respectively;the South Asian high intensifies and is located relatively westward. At 500 hPa,the positive vorticity propagations from the plateau to the downstream are enhanced significantly,as well as the ridge and trough.The cyclonic wind anomalies over the middle of the Yangtze River significantly intensify the southwesterly flows along the northwest side of the subtropical high.At 850 hPa,the low-pressure vortices strongly develop and move eastward. The southwesterly low-level jet to the south of the vortices is intensified,transporting warm and moist air toward South China.Meanwhile, the southward cold air intrusion from higher latitudes is also strengthened,cold and warm air convergence is enhanced,and thus the moisture convergence and upward motion in South China are enhanced.These changes increase the precipitation in South China.

It should be noted that this study focuses on the persistent heavy rainfall in southern China during June 2010.In June of 2005,2008,and several years of the 1990s,persistent heavy rainfall also occurred. All of the events need to be analyzed and discussed individually to determine whether our conclusions are of universal significance.Moreover,the results of our study show that the influence of the TP’s heating effect on persistent heavy rainfall in southern China are mediated via changes in the upper-and low-level atmosphere circulation,main weather systems,and the generation and development of the low-level vortices over the east of the plateau(near the Sichuan basin).The generation and development mechanisms of the low-level vortices could be further investigated. In addition,the heating effects of the main body and the southern slope of the TP(SSP)are investigated together in this study.More sensitivity experiments could be designed to investigate the heating effects over different sub-regions of the TP,such as the main body of TP,the SSP,the north slope,the western and eastern TP,on the persistent heavy rainfall in southern China,and the associated physical mechanisms as well.

Acknowledgments.The authors thank Dr. Zheng Yongjun at the Chinese Academy of Meteorological Sciences(CAMS)and Dr.Rao Jian at the Institute of Atmospheric Physics,Chinese Academy of Sciences for help running the WRF model.The ERA-interim data were downloaded from the ECMWF website(http://data-portal.ecmwf.int/).

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(Received January 16,2014;in final form April 28,2014)