Numerical Study of the E ff ect of Anthropogenic Aerosols on Spring Persistent R

DENG JiechunXU HaimingMA Hongyun and JIANG Zhihong

1Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters,Nanjing University of Information Science&Technology,Nanjing210044

2Key Laboratory of Meteorological Disasters of Ministry of Education,Nanjing University of Information Science&Technology,Nanjing210044

Numerical Study of the E ff ect of Anthropogenic Aerosols on Spring Persistent Rain over Eastern China

DENG Jiechun1,2XU Haiming1,2∗,MA Hongyun1,2, and JIANG Zhihong1,2

1Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters,Nanjing University of Information Science&Technology,Nanjing210044

2Key Laboratory of Meteorological Disasters of Ministry of Education,Nanjing University of Information Science&Technology,Nanjing210044

The e ff ect of anthropogenic aerosols on the spring persistent rain(SPR)over eastern China is investigated by using a high-resolution Community Atmosphere Model version 5.1(CAM5.1).The results show that the SPR starts later due to anthropogenic aerosols,with a shortened duration and reduced rainfall amount. A reduction in air temperature over the low latitudes in East Asia is linked to anthropogenic aerosols; so is a weakened southwesterly on the north side of the subtropical high.Meanwhile,air temperature increases signi fi cantly over the high latitudes.This north-south asymmetrical thermal e ff ect acts to reduce the meridional temperature gradient,weakening the upper-level westerly jet over East Asia and the vertical motion over southeastern China.As a result,the SPR is reduced and has a much shorter duration.The indirect e ff ect of anthropogenic aerosols also plays an important role in changing the SPR.Cloud droplet number concentration increases due to anthropogenic aerosols acting as cloud condensation nuclei,leading to a reduction in cloud e ff ective radius over eastern China and a reduced precipitation efficiency there.

CAM5.1 model,anthropogenic aerosols,East Asian climate,spring persistent rain

1.Introduction

The increase in surface air temperature induced by anthropogenic black carbon(BC)is signi fi cantly greater in the Northern Hemisphere than in the Southern Hemisphere,and a northward shift of the intertropical convergence zone(ITCZ)is thus predicted (Chung and Seinfeld,2005).However,some studies found that inter-hemispheric asymmetry cooling due to aerosols brings a signi fi cant reduction in precipitation to the north of the equator and an increase to the south,shifting the ITCZ southward(Ming and Ramaswamy,2009;Wang et al.,2013).China,located in East Asia,is a pronounced monsoon region. East Asian monsoons have great impacts on the regional climate change.Meanwhile,China has recently experienced a rapid economic development,and its emissions of anthropogenic aerosols have greatly increased with the expansion of urban areas along the east coast of China.These local aerosol emissions can play an important role in regional climate(Li Z.et al., 2010).

E ff ects of anthropogenic aerosols on East Asian monsoons have been studied and simulated.A decrease in surface temperature induced by aerosols can weaken both summer and winter monsoons(Liu et al., 2009;Deng et al.,2014a).Menon et al.(2002)showed that the solar radiation absorbed by BC could heat theair,leading to strengthened convective activities over southern China,which is in accordance with the“southern fl ood and northern drought” phenomenon in summer over China during the last 50 years.However, other studies suggested that aerosols do not contribute to the observed anomalous summer rainfall over East Asia,or even cause an opposite precipitation pattern (Gu et al.,2006;Wang et al.,2009a;Zhang H.et al., 2009;Zhang et al.,2012).Therefore,uncertainties still exist in simulating and estimating the impacts of aerosols on the summer precipitation over East Asia.

Previous studies mostly focused on East Asian summer or winter monsoons.There is a lack of research on the e ff ect of aerosols on atmospheric circulation during spring.Tian and Yasunari(1998)advanced the concept of spring persistent rain(SPR)and regarded the SPR as a climatic event.The SPR usually occurs in the area between the middle and lower reaches of the Yangtze River and the Nanling Mountains(25°–30°N)during March and April,which is a persistent and relatively steady pluvial period over eastern China before the onset of the South China Sea summer monsoon(SCSSM;Wan et al.,2008c).Wan et al.(2006,2008a)suggested that the formation of the SPR was the result of mechanical forcing and thermal forcing of the Tibetan Plateau(TP)and the location and intensity of the SPR were in fl uenced by the topography of the Nanling and Wuyi mountains. Moreover,the seasonal reverse of the zonal land-sea thermal contrast would precondition establishment of the East Asian subtropical summer monsoon(Qi et al.,2008),which may be crucial to the formation of the SPR as well(Li Chao et al.,2010;Zhang et al., 2011).Interdecadal variability of the SPR was studied by Jiang and Zhao(2012).They found a spring precipitation surplus over eastern China from the mid 1970s to the early 1980s,and a precipitation de fi cit after the early 1990s.

With less precipitation in the Northern Hemisphere before summer monsoon,wet deposition is at its minimum in spring,and as a result,aerosols will remain in the atmosphere for a longer time.Signi ficant thermal responses of atmosphere to the radiative forcing of aerosols may induce changes in general circulation to in fl uence the subsequent evolution of the East Asian summer monsoon(Lau and Kim,2006b). Using the Community Atmosphere Model version 3.0 (CAM3.0),Wang et al.(2009b)simulated the e ff ect of BC on summer monsoon over South Asia.They suggested that BC warmed the lower troposphere by strongly absorbing solar radiation in South Asia in late spring,leading to an advance of the rainy periods over the Bay of Bengal and its coast,together with the onset of the Indian summer monsoon.Using the same general circulation model,Hu et al.(2011)showed that the spring precipitation decreased in central to southern China but increased in northern China due to sulfate(SF)and BC over East Asia.It is noted that the response of atmosphere to the aerosols during spring is far from a simple pattern related to local direct aerosol forcing,and the structure and amplitude of such a regional response is determined by the balance of several factors,including solar heating,condensation and adiabatic heating,and turbulent di ff usion in the boundary layer(Kim et al.,2006).

The purpose of this study is to investigate the impact of anthropogenic aerosols on the SPR over eastern China using a high-resolution global climate model (CAM5.1)from the National Center for Atmospheric Research(NCAR).Section 2 describes the model,experimental design,and observational data used in this study.In Section 3,the corresponding results in spring responses of atmosphere to the radiative forcing of aerosols over eastern China are presented,and a possible mechanism is discussed.Finally,the conclusions and discussion are given in Section 4.

2.Model,experimental design,and data

2.1 General circulation model and aerosol module

An earlier version of the Community Atmosphere Model(CAM3.0)has been widely used to study the e ff ect of aerosols on the climate over East Asia(Liu et al.,2009;Wang et al.,2009a;Hu et al.,2011;Zhang et al.,2011).CAM5.1 is the latest version of the atmosphere model(Neale et al.,2010)in the Community Earth System Model version 1(CESM1)released in2011 by the NCAR.It could simulate the atmospheric circulation stand alone or be coupled with other models in the CESM1.Moreover,CAM5.1 provides a three-mode version of the modal aerosol module(MAM3)for long-term climate simulations,including the Atiken mode,accumulation mode,and coarse mode(Liu et al.,2012).The aerosols involved in each mode are internally mixed,which is closer to the existent state of aerosols in the observation.Therefore, the optical property of aerosols could be well described by MAM3 in CAM5.1,and the particle distribution and number concentration are accurately calculated for better simulation of the direct and indirect effects of aerosols.This aerosol module contains several main kinds of aerosols,such as SF,BC,organic carbon (OC),dust(Dst),and sea salt(SS).CAM5.1 used in this study has a high resolution of 0.9°×1.25°and a hybrid vertical coordinate with 30 levels,including a rigid lid at 3.643 hPa.

2.2 Experimental design

To investigate the e ff ect of anthropogenic aerosols on the SPR over eastern China,we perform two numerical experiments,i.e.,a control experiment (CTRL)with all anthropogenic aerosols forcing(for year 2000)and a sensitivity experiment(NOAERO). In NOAERO,anthropogenic aerosols(BC,SF,and OC)linked to human activities at the surface or in the upper air over eastern China(20°–45°N,100°–125°E) are all prescribed at the pre-industrial level(for year 1850),but natural aerosols(Dst and SS)produced by natural processes such as volcanic eruptions are unadjusted(Fig.1).The emission sources involved in both experiments are those used in the Intergovernmental Panel on Climate Change(IPCC) fi fth Assessment Report(AR5)(Lamarque et al.,2010).All greenhouse gases and interactive land-atmospheric forcing are the same in both simulations.Thus,the di ff erence between CTRL and NOAERO can be regarded as the impact of anthropogenic aerosols over eastern China on the East Asian climate.

In both experiments,sea surface temperature (SST)is prescribed with monthly mean SST from 1850 to 2010,which is a blended SST dataset with HadISST1 from the UK Met Office Hadley Center and OI SST from the National Oceanic and Atmospheric Administration(NOAA)(Hurrell et al.,2008).Consequently,the seasonal and interannual variations of the SST are present during the model integrations, and as a result the in fl uence of the intensity of East Asian monsoons on the spatial distribution of aerosols is considered;that is,the interaction between aerosolsand atmosphere is included in the experiments.Both simulations are integrated from January 1988 to December 2007,and the results from the last 15 years (1993–2007)are analyzed.Tian and Yasunari(1998) suggested that the duration of the SPR is in pentads 12–16(March to early May),while Wan et al.(2006) noted the SPR for the period of March and April.As the pre-rainy season in South China generally occurs in mid May,the period of March to April is chosen as the SPR period in this paper as in Wan et al.(2006).

Fig.1.Spatial distributions of initial anthropogenic aerosol emissions of(a,d)sulfate(108mol cm−2s−1),(b,e)black carbon(1010mol cm−2s−1),and(c,f)organic carbon(1010mol cm−2s−1)over East Asia for(a–c)CTRL and(d–f) NOAREO.

2.3 Data

The daily and monthly reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF),namely the ERA-Interim,is used to validate the simulated atmospheric circulation.The ERAInterim puts emphasis on improving its earlier reanalysis of ERA-40,including the representation of the hydrological cycle,the quality of the stratospheric circulation,and the consistency in time of the reanalyzed fi elds(Dee et al.,2011).We use the pentad precipitation dataset from the Climate Prediction Center Merged Analysis of Precipitation(CMAP;Xie and Arkin,1997)provided by the NOAA to compare with the simulated annual cycle of precipitation over eastern China.The CMAP dataset contains precipitation distributions with a full global coverage and improved data quality compared to individual data sources(Xie and Arkin,1997).

3.Results

3.1 Model validation

Numerous studies have simulated the features of aerosols and investigated the impacts of aerosols on regional and global climate by using CAM5.1(Gettelman et al.,2010;Hu and Liu,2013;Jiang et al.,2013). Liu et al.(2012)showed that CAM5.1 is able to qualitatively capture the observed geographical and temporal variation of aerosols,including aerosol mass,number concentration,size distribution,and aerosol optical properties.To examine the capability of CAM5.1 in simulating atmospheric circulation and precipitation fi elds over East Asia during the SPR period,Fig. 2 compares the simulated March–April mean circulation at 850 hPa and surface precipitation with observations.The simulated spatial distributions of 850-hPa wind and surface rainfall are consistent with the observations.In boreal spring,the southwesterly wind prevails over southern China(south of 30°N)with a heavy rainfall band(rainfall exceeding 3 mm day−1)extending from the north side of the subtropical high over the Paci fi c Ocean to the east of the Philippines.There is a fl ow on the southeast side of the TP due to the topography,forming an approximately cyclonic circulation pattern.Meanwhile,the northwesterly wind prevails in the mid and high latitudes of East Asia,which converges with the southwesterly wind over the middleand lower reaches of the Yangtze River.However,the simulated 1500-gpm isoheight at 850 hPa shifts northward in comparison with the observation,together with the SPR rain belt,leading to an underestimation of the spring rainfall over the southern coastal regions of China.Meanwhile,too much precipitation is simulated over the northwestern part of the Indo-China Peninsula and southwestern China,due to a westward shift of the subtropical high and an enhanced southwesterly in CTRL.Therefore,the spatial distribution and intensity of simulated precipitation over East Asia by CAM5.1 show some systematic errors,as seen in many other global climate models.In terms of the SPR,however,CAM5.1 still well captures the characteristics of atmospheric circulation and precipitation over eastern China.

Fig.2.March–April mean winds(vector;m s−1)and geopotential height(contour;gpm)at 850 hPa and surface precipitation(shading;mm day−1)from(a)CTRL and(b)observation.

3.2 Onset of the SPR

Figure 3 shows the latitude-time cross-section of precipitation along 110°–122.5°E.The SPR with precipitation exceeding 4 mm day−1appears in early March(pentad 15)when averaged from 1993 to 2007, and is located between 25°and 30°N over eastern China.The SPR indicates the seasonal transition of the East Asian circulation from winter to spring and the weakening of the East Asian winter monsoon system due to the shrunken land-sea thermal contrast, together with the occurrence of strong southwesterly wind and the water vapor convergence there.The rain belt remains until the end of April,but with a relatively weaker and interrupted heavy rainfall center (≥ 6 mm day−1).The precipitation over the northern SCS exceeds 4 mm day−1in pentad 26,declaring the onset of the SCSSM and the termination of the SPR.By contrast,the spring rainfall over 25°–30°N is slightly overestimated in CAM5.1,and a northwardshifted rain belt is present in the model,which is stationary(Fig.3b).It is clearly depicted that the spring rainfall begins in pentad 15 in CTRL with a magnitude continuously exceeding 4 mm day−1,which is in coincidence with the observation.

Based on precipitation and low-level circulation, Wan et al.(2008b)de fi ned the onset of the SPR as the fi rst pentad after pentad 7 that satis fi es the following criteria:1)area-averaged precipitation must be greater than 4 mm day−1over the SPR region(Region A:23°–30°N,110°–120°E);2)mean southwesterly velocity at 850 hPa must be greater than 4 m s−1over its upstream region(Region B:20°–25°N,110°–115°E);3)in the subsequent three pentads,at least one pentad satis fi es the above two criteria.Figure 4 shows the temporal evolutions of the aforementioned precipitation and southwesterly velocity at 850 hPa. As shown in Fig.4a,under the in fl uence of anthropogenic aerosols,a decrease in precipitation is obvious over Region A in pentads 14–15.Therefore,the onset time of precipitation exceeding 4 mm day−1in Region A is pentad 16 in CTRL,two pentads later than that in NOAERO.Similarly,the southwesterly velocity at 850 hPa is weakened over Region B during pentads13–15,showing that the onset of southwesterly velocity greater than 4 m s−1in CTRL is delayed by three pentads compared to that in NOAERO(Fig.4b).As a result,the SPR is established in pentads 16 and 14 in CTRL and NOAERO,respectively,meaning a delayed onset of the SPR due to anthropogenic aerosols over eastern China.Moreover,Deng et al.(2014b) found an advanced onset of the SCSSM induced by anthropogenic aerosols using CAM5.1,indicating that the SPR is established later but also ends earlier with a shortened duration.

Fig.3.Latitude-time cross-sections of precipitation along 110°–122.5°E from(a)CMAP and(b)CTRL(shading;mm day−1;contour ≥ 4 mm day−1).

Fig.4.Temporal evolutions of area-averaged(a)precipitation(mm day−1)over the SPR region(Region A)and(b) 850-hPa southwesterly velocity(m s−1)over its upstream region(Region B).Solid line is for CTRL,dashed line is for NOAERO,and bar chart is for CTRL–NOAERO.Shaded areas represent statistically signi fi cant changes at the 90% con fi dence level.

3.3 Precipitation and 850-hPa winds

Figure 5 shows the time series of March–April mean precipitation over Region A and 850-hPa southwesterly velocity over Region B.In most years,the SPR is reduced due to anthropogenic aerosols as a marked reduction in precipitation by 0.94 mm day−1and reduced southwesterly velocity by 0.92 m s−1.Figure 6a shows the mean changes of geopotential height and wind at 850 hPa in the SPR period.A positive geopotential height anomaly appears over southern China with a maximum greater than 5 gpm on the southeast side of the TP,while a negative anomaly appears over the western Paci fi c Ocean due to the weakening of the subtropical high.This positive and negative geopotential height anomalies form an east–west dipole pattern.Correspondingly,some anticyclonic anomaly fl ow shows up on the southeast side of the TP,which weakens the fl ow there.The northeasterly wind anomaly dominates southern China,the northern SCS,and the Indo-China Peninsula,while obvious northwesterly wind anomaly prevails north of the Yangtze River.Besides,there is westerly wind anomaly over the ocean east of the Philippines.As a result,the gradient of geopotential height is attenuated between the southeast side of the TP and the western Paci fi c Ocean under the e ff ect of anthropogenic aerosols,leading to weakened southwesterly on the north side of the subtropical high and reduced northward transport of water vapor from the tropics.Eventually,precipitation over southern China will be suppressed associated with the circulation change. To further verify the rainfall change,Fig.6b gives the March–April mean rainfall anomaly fi eld.A signi fi cantly suppressed rain belt is clearly seen over southeastern China,the East China Sea,and southern Japan,and precipitation drops obviously by more than 2 mm day−1over the middle and lower reaches of the Yangtze River.

3.4 Understanding the reduced SPR

Fig.5.Time series of March–April mean(a)precipitation(mm day−1)averaged over Region A and(b)850-hPa southwesterly velocity(m s−1)averaged over Region B.Solid line is for CTRL,dashed line is for NOAERO,and bar chart is for CTRL–NOAERO.Asterisk and double-asterisk represent statistically signi fi cant changes at the 90%and 95%con fi dence levels,respectively.

Fig.6.March–April mean di ff erences of(a)geopotential height and winds at 850 hPa and(b)precipitation di ff erence between CTRL and NOAERO.Only wind changes that are signi fi cant at the 90%con fi dence level are plotted.Contour intervals are 2 gpm or 0.5 mm day−1,with negative values given by dashed contours.Shaded areas represent statistically signi fi cant changes at the 90%con fi dence level.

Aerosols can in fl uence the radiative balance between land and atmosphere by scattering and absorbing solar radiation and long wave radiation,which is the direct e ff ect;so the thermal forcing of aerosols is regarded as an essential way to a ff ecting the climate. Figure 7a shows the vertical structure of air temperature di ff erence between CTRL and NOAERO averaged over eastern China.We can see that the air temperature decreases in the lower and upper troposphere south of 35°N with an amplitude of 0.4℃,but it increases at 200 hPa.However,anthropogenic aerosols heat the air below 250 hPa over the mid-high latitudes,with a warming amplitude greater than 1℃ in the mid-lower troposphere.Accordingly,the meridional temperature gradient averaged between 850 and 200 hPa is signi fi cantly reduced over southern China, but enhanced over the region 30°–50°N(solid line in Fig.7b).Therefore,this north–south asymmetrical thermal e ff ect reduces the mean meridional temperature gradient in spring over East Asia.The thermal wind relationship is as follows:

Fig.7.(a)Vertical cross-section of temperature di ff erences(℃)between CTRL and NOAERO along 110°–122.5°E, averaged for March and April and(b)March–April mean meridional tropospheric(850–200 hPa)temperature gradient di ff erences(solid line;10−6℃ m−1)and zonal wind di ff erences(dashed line;m s−1)between CTRL and NOAERO along 110°–122.5°E,together with climatological mean zonal wind in NOAERO(dash-dotted line;10 m s−1).Shaded areas and black dots represent statistically signi fi cant changes at the 90%con fi dence level.

To further reveal the relationship between circulation change and precipitation anomaly,Fig.8 shows the di ff erences of wind divergence and vertical meridional circulation.Because of the weakened southwesterly over southern China(Fig.6a),a positive divergence anomaly(solid line in Fig.8a)occurs at 850 hPa over 25°–35°N.Meanwhile,a signi fi cant negative anomaly(dashed line in Fig.8a)is found in the upper troposphere south of the Yangtze River due to the weaker and southward-shifted upper-level westerly jet. Both divergence anomaly centers in the lower and upper troposphere are near the axis of the westerly jet (30°N).This aerosol-induced circulation results in a signi fi cant anomalous downward motion over southern China through weakening the pumping below the axis of the westerly jet(Fig.8b),acting to suppress the spring precipitation there.However,the fact thatthe condensation latent heating decreases due to a weaker upward motion( fi gure omitted)will further decrease the meridional temperature gradient by cooling the air in the low latitudes,forming a positive feedback mechanism.Therefore,the response of air temperature to anthropogenic aerosols is determined by the radiative forcing of the aerosols and the positive feedback involved,and the thermal e ff ect of anthropogenic aerosols is very likely to cause a primary perturbation for the adjustments of air temperature and circulation.

Fig.8.March–April mean(a)divergence di ff erences(10−6s−1)at 850 hPa(solid line)and 200 hPa(dashed line) and(b)meridional circulation di ff erences(vectors;meridional wind in m s−1;vertical pressure velocity in 0.2 hPa s−1) between CTRL and NOAERO along 110°–122.5°E.Shaded areas and black dots represent statistically signi fi cant changes at the 90%con fi dence level.

In addition,aerosols can serve as cloud condensation nuclei(CCN),and thus change the cloud droplet e ff ective radius(Reff),giving rise to higher cloud albedo(Twomey,1997)and reducing precipitation effi ciency due to longer cloud lifetime(Albrecht,1989). This is the so-called indirect e ff ect.Numerous studies have veri fi ed the impact of anthropogenic aerosols on the micro-properties of clouds through in-cloud droplet number concentration(CDNC)(Han et al., 1994;Wetzel and Stowe,1999;Pwlowska and Brenguier,2000;Br´eon et al.,2002).After considering the e ff ect of anthropogenic aerosols,the aerosol optical depth increases markedly by 0.09 over the SPR region,together with CCN at supersaturation of 0.1% (CCN0.1)by 219.9 cm−3at 850 hPa(158.1%;Table 1).Figure 9 shows the changes of CDNC andReffat 850 hPa.With the increase in CCN0.1induced by anthropogenic aerosols,CDNC rises in southern China (Fig.9a),especially in the SPR region by a ratio of 48.6%.The reduction ofReffis clear in the SPR region (–13.5%)(Fig. 9b and Table 1),suppressing the coalescence of the cloud droplets and precipitation there.As a result,the indirect e ff ect of anthropogenic aerosols plays an important role in reducing the SPR. However,it is noted that the impact of aerosols on precipitation is a complex matter,and it is difficult tofi nd the exact relationship between CCN and rainfall. Nonlinearity between in-cloud microphysical parameters and surface rainfall remains uncertain.Furthermore,the cloud-aerosol interaction is also in fl uenced by the background circulation,such as atmospheric stability and wind shear(Yang et al.,2011).

Table 1.March–April mean absolute and percentage changes of aerosol optical depth(AOD)at 550-nm wavelength,cloud condensation nuclei number concentration at supersaturation of 0.1%(CCN0.1),incloud droplet number concentration(CDNC),and cloud droplet e ff ective radius(Reff)at 850 hPa due to anthropogenic aerosols averaged over the SPR region

Fig.9.March–April mean changes of(a)cloud droplet number concentration and(b)cloud droplet e ff ective radius at 850 hPa(CTRL minus NOAERO).Contour intervals are 10 cm−3and 0.2µm,respectively,with negative values given by dashed contours.Shaded areas represent statistically signi fi cant changes at the 90%con fi dence level.

In general,the thermal forcing associated with anthropogenic aerosols induces circulation changes over East Asia.On the other hand,the indirect e ff ect of anthropogenic aerosols has a great impact on clouds in southern China.Both e ff ects of the aerosol forcing suppress the SPR,which could be summarized as follows.Under the impact of anthropogenic aerosols, the decrease in air temperature leads to increased 850-hPa geopotential height in the low latitudes over East Asia,reducing the geopotential gradient between the south side of the TP and the western Paci fi c Ocean and weakening the southwesterly on the north side of the subtropical high in the lower troposphere,together with convergence in southern China.In comparison, a marked increase in air temperature is found in the mid-high latitudes,which reduces the meridional temperature gradient between the upper and lower troposphere.The upper-level westerly jet is weaker and shifts southward,giving rise to reduced convergence in the upper troposphere.This anomalous circulation leads to weakened upward motion and suppressed spring rainfall with a shortened duration.Moreover, in the SPR region,850-hPa CDNC increases due to anthropogenic aerosols acting as CCN,leading to a decrease inReff.As a result,the SPR is also suppressed by reduced coalescence of cloud droplets.

Hu and Liu(2013)noted that anthropogenic aerosols a ff ected the decadal change of late spring precipitation in South China during 1950–2000.The mechanism of aerosols a ff ecting spring rainfall by adjustment of circulation in East Asia is in coincidence with our results.Therefore,our results con fi rm the crucial role of anthropogenic aerosols in spring climate change in East Asia.However,the SPR usually occurs in early spring(March–mid April),with pre-rainy season in southern China in late spring(late April–May). Although both rainfall periods are considered spring precipitation in southern China,the features of precipitation and the intensity of circulation are di ff erent for these periods.Thus,further studies are needed to investigate the e ff ect of anthropogenic aerosols during di ff erent spring precipitation periods.

4.Conclusions and discussion

Based on a high-resolution model of CAM5.1,the impact of anthropogenic aerosols on the SPR over eastern China is investigated in this study.The results are as follows.

(1)The SPR is established two pentads later due to anthropogenic aerosols,together with a shortened duration and reduced precipitation amount.

(2)Under the in fl uence of anthropogenic aerosols, a signi fi cant decrease in air temperature leads to anincrease in 850-hPa geopotential height in the low latitudes over East Asia,reducing the geopotential gradient between the south side of the TP and the western Paci fi c Ocean and weakening the southwesterly on the north side of the subtropical high in the lower troposphere,together with convergence over southern China.In comparison,a marked increase in air temperature is found in the mid-high latitudes. This north-south asymmetrical thermal e ff ect acts to reduce the meridional temperature gradient so the upper-level westerly jet is weaker and shifts southward,giving rise to reduced convergence in the upper troposphere.As a result,the SPR is suppressed with a shortened duration.

(3)The indirect e ff ect of anthropogenic aerosols also plays an important role in reducing the SPR. CDNC in the lower troposphere is increased by aerosols acting as CCN in the SPR region,leading to an evident decrease inReff.As a result,the SPR is further suppressed due to reduced coalescence of cloud droplets.

In this study,we only explored the e ff ect of anthropogenic aerosols on the SPR over eastern China, though remote aerosols could also enhance the impact of local aerosols in the monsoon region(Cowan and Cai,2011).Furthermore,because of the SST response to aerosol forcing,the in fl uence of aerosols on precipitation presented in the air-sea coupled model di ff ers from that in the atmosphere model driven by prescribed SST(Chung et al.,2002;Wang et al.,2005; Lau and Kim,2006a).Our future studies will use a fully coupled model including the SST response to aerosol forcing to better understand the impact of anthropogenic aerosols on spring climate in East Asia.

Acknowledgments.We thank three anonymous reviewers for their valuable comments and suggestions.Thanks also go to Dr.Zuojun Yu for her careful review of the language and kindly help in improving the English.

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10.1007/s13351-014-3198-0.

(Received October 11,2013;in fi nal form March 13,2014)

Supported by the National Basic Research and Development(973)Program of China(2010CB428505),National Natural Science Foundation of China(41275094),Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD),and Jiangsu Provincial Qinglan Project.

∗Corresponding author:hxu@nuist.edu.cn.

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