Seasonal Prediction of June Rainfall over South China: Model Assessment and Statistical Downscaling

Kun-Hui YE,Chi-Yung TAM,Wen ZHOU,and Soo-Jin SOHN

1Guy Carpenter Asia-Paci fi c Climate Impact Centre,City University of Hong Kong,Hong Kong,China

2School of Energy and Environment,City University of Hong Kong,Hong Kong,China

3Climate Prediction Team,APEC Climate Center,Busan,Republic of Korea

Seasonal Prediction of June Rainfall over South China: Model Assessment and Statistical Downscaling

Kun-Hui YE?1,Chi-Yung TAM??1,2,Wen ZHOU1,2,and Soo-Jin SOHN3

1Guy Carpenter Asia-Paci fi c Climate Impact Centre,City University of Hong Kong,Hong Kong,China

2School of Energy and Environment,City University of Hong Kong,Hong Kong,China

3Climate Prediction Team,APEC Climate Center,Busan,Republic of Korea

The performances of various dynamical models from the Asia-Paci fi c Economic Cooperation(APEC)Climate Center (APCC)multi-model ensemble(MME)in predicting station-scale rainfall in South China(SC)in June were evaluated.It was found that the MME mean of model hindcasts can skillfully predict the June rainfall anomaly averaged over the SC domain. This could be related tothe MME’s abilityin capturing the observed linkages between SC rainfall and atmospheric large-scale circulation anomalies in the Indo-Paci fi c region.Further assessment of station-scale June rainfall prediction based on direct model output(DMO)over 97 stations in SC revealed that the MME mean outperforms each individual model.However,poor prediction abilities in some in-land and southeastern SC stations are apparent in the MME mean and in a number of models. In order to improve the performance at those stations with poor DMO prediction skill,a station-based statistical downscaling scheme was constructed and applied to the individual and MME mean hindcast runs.For several models,this scheme can outperform DMO at more than 30 stations,because it can tap into the abilities of the models in capturing the anomalous Indo-Paci fi c circulation to which SC rainfall is considerably sensitive.Therefore,enhanced rainfall prediction abilities in these models should make them more useful for disaster preparedness and mitigation purposes.

June South China rainfall,multi-model ensemble prediction,statistical downscaling,bias correction

1. Introduction

Over South China(SC),the June monthly mean rainfall and its interannual variability are the largest in magnitude amongthosethroughoutthe calendaryear(Yuanet al.,2012). Infact,theoutbreakoftheSouthChinaSeasummermonsoon (SCSSM)occursclimatologicallyinthefourthpentadofMay (Murakami and Matsumoto,1994;Wu and Wang,2000;Gao et al.,2001;Zhou et al.,2005),and its associated rainband migrates northward in mid-to-late June.After the SCSSM onset,precipitation over SC increases dramatically,marking the beginning of the seasonal march of the summer monsoon rainband(Ding,2004).Accurate seasonal forecasts of the June SC rainfall would be invaluable for the water management sector,as well as for mitigation and disaster preparedness in the region.

On the interannual timescale,there are a number of circulation systems that can greatly in fl uence the SC summertime rainfall.The western North Paci fi c subtropical high (WNPSH),in particular,can strongly in fl uence the SC summer rainfall through its east–west displacement and strength (Chang et al.,2000;Yang and Sun,2005).In fact,SST anomalies in both the South China Sea(SCS)(Zhou and Chan,2007)and the Indian Ocean(Yuan et al.,2008;Xie et al.,2009)are able to affect SC through such a link between the WNPSH and SC rainfall.An out-of-phase relationship between the rainfall of regions surrounding the SCS and Indian summer monsoon(ISM)rainfall has also been proposed (Kripalani and Kulkarni,2001).In addition,extratropical atmospheric activities can affect SC rainfall.Zhang and Tao (1998)pointed out that the persistent blocking high over the Sea of Okhotsk could give rise to above-normalprecipitation in summer over East Asia.Also,the arrival of midlatitude fronts may be linked to the onset of the SCSSM(Chang and Chen,1995;Tong et al.,2009).Yuan et al.(2012)concluded that extratropical forcing induced by the west Siberian low was an important contributor to the excessive June rainfall in 2010 over SC.Finally,equatorial eastern Paci fi c SST is a prominent climatic factor that in fl uences the SC rainfall.For example,SC may suffer from droughts in the developing stage of ENSO,whereas fl oods are likely to occur over SC when the latter decays(Huang and Wu,1989).Chang et al. (2000)found that summer rainfall during the period 1951–77 was stronger than that during the period 1978–96 in the southeasterncoastalareaofChina,duetoastrongerandmore expansive mean western Paci fi c subtropical ridge caused by highermean equatorialeastern Paci fi c SST overthe courseof the latter period.

General circulation models(GCMs)are widely used for seasonal forecasting,but still lack suf fi cient ability in reproducing climate at the local scale(Stockdale et al.,1998).In the Asian summermonsoonregion,for instance,state-of-theartcoupledmodelsgenerallyhavedif fi cultiesin capturingthe mean precipitation,even with a one-month lead time(Wang et al.,2007,2008,2009;Kug et al.,2008;Lee et al.,2010). Nevertheless,in terms of predicting the East Asian and western North Paci fi c monsoon,models tend to perform better when forecasting after the mature phase of ENSO(Wang et al.,2009;Liangetal.,2009;Chowdaryet al.,2010;Leeet al., 2011a;Lee et al.,2011b).In recent years,the multi-model ensemble(MME)method(Krishnamurti et al.,1999,2000; Doblas-Reyeset al.,2000)has beenadoptedby several major operationalcenters for seasonal climate prediction(Palmer et al.,2004;Lee et al.,2009).In the present reported study, we used the hindcast runs from various climate models participating in the Asia-Paci fi c Economic Cooperation(APEC) Climate Center(APCC)MME to assess their performance in predicting SC rainfall.In addition,we established a stationbased statistical downscaling scheme for the SC region in an attempt to enhance the rainfall prediction abilities of some of the models.Statistical downscaling,the fundamental principle of which is to derive local climate anomalies from the large-scale circulation,is widely regarded as a technique that canimprovemodelpredictions(e.g.,Karletal.,1990;Wigley et al.,1990;WilbyandWigley,1997;Kanget al.,2007;Sohn et al.,2013a).Indeed,a pattern-based statistical downscaling approach for SC summer rainfall prediction has previously been tested by Tung et al.(2013).Liu et al.(2011)also used statistical downscaling for summer rainfall prediction,but in a smallerdomaininthe easternpartofSC.Severalotherstudies have also attempted to improve model prediction by statistical methods(Wang and Fan,2009;Fan et al.,2008,2012; Liu and Fan,2014).

The remainder of the paper is organized as follows:Section 2 provides a description of the models,datasets,and the statistical downscaling method.The results of the relationship between June rainfall and the large-scale circulation,the models’abilities to reproduce the June SC rainfall,and the performance of the downscaling prediction are presented in section 3.Finally,a brief discussion and summary of the key fi ndings is provided in section 4.

2. Data and methods

2.1.Observations and model hindcast data

This study used the daily meanprecipitationrecordsfrom 740 stations over the SC region(18°–27°N,105°–120°E). The geographical location of SC within China is shown in Fig.1c.Inspection of the observational data revealed that, during the 1983–2003 period,97 stations gave complete records of daily rainfall in June.Precipitation values from these 97 stations were considered in our analysis.The spatial distribution of these 97 stations is shown in Fig.1d. In addition,500 hPa geopotential height(Z500),850 hPa wind,and the mean sea level pressure(SLP)from the National Centers for Environmental Prediction–Department of Energy(NCEP–DOE)Atmospheric Model Intercomparison Project(AMIP-II)reanalysis(Kanamitsu et al.,2002)were also used.

The APCC MME hindcast considered in this study comprises the seasonal prediction experiments from 11 climate models.They are provided by various operational centers and research institutes from the APEC economies including Canada,China,Korea,Taiwan,and the United States. Experiments of both the Seasonal Model Intercomparison Project/Historical ForecastProject(SMIP/HFP)andthe Coupled Model Intercomparison Project(CMIP)types are involved,with the common hindcast period of 1983 to 2003. Table 1 provides details of these model experiments.All model predictions targeted in June were initiated in May of the same year,and the simulation results were interpolated onto the same 2.5°×2.5°grid.In addition to outputs from individual models,their MME average(which is the simple average of their products)was also considered in this study. In order to compare with observations,the models’rainfall outputs were spatially interpolated using bilinear interpolation onto the station locations in SC.Finally,as this study focuses on the interannualvariabilityof the SC rainfall,longterm trends in both the observational and model data were removed prior to the analysis.

2.2.Station-based statistical downscaling

Besides using model precipitation to predict the SC rainfall,a statistical downscaling method with model SLP and Z500 values as predictors was also tested for predicting the June rainfall at station locations.The construction of the statistical prediction scheme is outlined as follows.First,correlation maps between station rainfall and SLP,as well as those with Z500,for each individual station location were inspected,in order to unveil any linkages between the Indo-Paci fi c large-scale circulation and SC rainfall.Results indicated that there were robust connections between rainfall variations observed at individual stations and SLP or Z500 signals from models over the ISM and western north Paci fi c regions(see also section 3.1).Adopting the model output statistics(MOS;Wilks,1995)approach,model Z500 and SLP were then chosen as predictors for the observed station rainfall,based on the following equation:

Table 1.Description of the hindcast experiments considered in this study.

wherePiis the June rainfall anomaly at stationi,SLPAAand Z500ABrepresent the anomalous SLP and Z500 averaged over rectangular domains A and B located in the ISM region and western north Paci fi c,respectively.Note that the above represents an empirical relationship between stationscale rainfall and the large-scale circulation,and coef fi cientsaiandbi(the values of which depend on the station being considered)are determined from model hindcast and station rainfall data by multiple linear regression.The exact location of domain A(B)can be found by fi rst setting a“movingwindow”with 50°of longitudeand 30°of latitude over the tropical Indian Ocean/Indian subcontinent(western north Paci fi c) area.Thelocationofthewindowis thenadjustedsothatthere are a maximum number of stations in SC with rainfall significantly correlated to the SLP(Z500) fi eld over at least 1/3 of the grid points within domain A(domain B).Finally,the longitudinal and latitudinal extents of domains A and B arefi ne-tuned,so as to capture the most numberof stations in SC linked to either SLP or Z500 variations.Using this method, about 25%and 20%of the SC stations were identi fi ed with rainfall linked to SLP and Z500 variability over the Indo-Paci fi c,respectively.This schemeis processedat eachstation for every model,as well as for the MME average.For instance,domain A was found to be(20°N–10°S,60°–110°E), while domain B covered(5°–25°N,130°–180°E)when the MME mean SLP and Z500 were used.In this study,the statistical downscaling prediction was validated using a leaveone-outcross-validationframework.Such a frameworkseeks to mimic the actual forecast situation when data in the target periodare not known;it is commonlyused to validate predictions based on statistical schemes(e.g.,Sohn et al.,2013b).

3. Results

3.1.Relationship between SC rainfall and large-scale circulation

BeforeanalyzingtherelationshipbetweenSC rainfalland large-scale circulation,the June climatological conditions from reanalysis data and model simulations were compared. Figures 1a and b show the reanalysis-based and MME mean values of Z500 and 850 hPa wind over Asia and the western Paci fi cinJune.ItcanbeseenthattheMME850hPawindcirculation(greenarrowsin Fig.1a)overtheIndo-Paci fi cregion compares well with reanalysis data(black arrows).Note that the position of the monsoon trough extending from northern Indochina into the SCS/Philippines Sea was also reproduced in the MME hindcasts.In addition,the WNPSH was also captured well by the MME hindcast(green contours in Fig. 1b),albeit with a slightly eastward shifted position compared with reanalysis data(black contours).The observed accumulatedrainfallin JuneoverSC is also shown(Fig.1d).Overall, strong spatial variation is exhibited;monthly mean precipitation can reach the value of 500 mm in some southern coastal stations,while those over inland locations receive only 200 mm or less rainfall in the same month.

To examine how SC rainfall is related to the large-scale circulation over the Indo-Paci fi c region,Z500,SLP and the 850hPawindswere regressedontotheprecipitationaveraged over SC in June.Figure 2 shows the regression coef fi cients computed based on observations as well as the MME average.From the reanalysis-based Z500 regression map,positive anomalies can be found centered over the Philippines to the SCS,and also in the western North Paci fi c(WNP)(Fig. 2a).CollocatedwiththeformerZ500featureis ananomalous anticyclone in the lower levels(Fig.2c).The above indicates a close relationship between SC precipitation variation and WNPSH activity.In particular,the low-level wind branch accompanyingan anomalous high over the Philippines/WNP leads to strongersoutherliesfromthe SCS.The latter is likely to transport more water vapor into SC,resulting in abovenormal rainfall in the region.In addition to those in the subtropical western North Paci fi c,signi fi cant climate signals are also found in the northern Indian Ocean and at some extra-tropical sites.From the SLP and 850 hPa wind regression maps,there is a broad-scale SLP negative anomaly over the IndianOcean with alongshorecyclonicwind off Sumatra and the Somali coast(see Fig.2c).Such a circulation feature might re fl ect the in fl uence of Indian Ocean dipole activity on SC rainfall in boreal summer(Guan and Yamagata,2003). Finally,there appears to be a wave train in the 500 hPa height fi eld from the subtropics extending into northeast Asia/the north Paci fi c region(see Fig.2a).On the other hand,we cannot discern anysigni fi cant signals at moreupstream locations over continental Eurasia.

In the model environment,an analogous relationship betweenthelarge-scalecirculationandSCrainfallis alsofound. The Z500 regression map for the MME average shows that increased SC rainfall is associated with a stronger WNPSH over the far WNP to the SCS,with correlation even stronger than that in observations(see Fig.2b).Interestingly,the MME produces very strong correlation between SC rainfall and Z500 in the tropics;this could be related to the multiensemble and multi-model averaging process,which tends to increase the signal-to-noise ratio in the dataset.There is also anomalous anticyclonic fl ow at 850 hPa and positive SLP in the vicinity and to its east,again bearing much semblance to observations(Fig.2d).Inspection of the hindcasts fromindividualmodelsrevealedthat such a WNPSH(orlowlevel anticyclone)–SC rainfall relationship is reproduced by models includingCWB AGCM,GDAPS,MSC-GM2,NCEP CFS,PNU CGCM andPOAMA( fi guresnotshown).Inaddition,low-level easterly and northeasterly wind perturbations are seen over India and the Arabian Sea to Somalia,respectively.Related to this is a broad-scale positive SLP anomaly spanning the Indian subcontinent to Saudi Arabia.This suggests that the ISM tends to be weaker(stronger)when the SC mean rainfall is enhanced(suppressed)in the MME average.A positive meridional SLP gradient in the northern Indian Ocean is also found in the environments of the CWB AGCM,GDAPS,NCEP CFS,PNU CGCM and POAMA models when they produce above-normalSC rainfall( fi gures not shown).Finally,there are signi fi cant circulation signals over the North Paci fi c in the MME mean regression map, which are even more robust than their observational counterparts.

We next compared the year-to-year SC rainfall variation from observations and the MME mean of hindcasts.The observed and MME mean rainfall averaged over SC in June, normalized by their respective standard deviations,are given in Fig.3.The two time series were found to be highly correlated,with a correlation coef fi cient of 0.73.In other words, the MME average is capable of capturing the SC rainfall variation in June.For individual models,the correlation between their predicted SC rainfall and observations ranged from-0.49 to 0.67.For models that yielded a correlation coef fi cient of 0.3 or above,most of them(CWB AGCM, GDAPS,NCEP CFS,PNU CGCM and POAMA)reproduced the aforementioned anomalous WNPSH and SLP gradient over the Indian subcontinent associated with SC rainfall changes.This suggests that successfully capturing the WNPSH and ISM–SC regionalclimate linkages could be important for predicting SC precipitation.

3.2.Station-scale prediction based on direct model outputs and statistical downscaling

Inordertoassess theperformanceofdirectmodeloutputs (DMO)in capturing the station-scale rainfall,hindcast precipitation data were fi rst spatially interpolatedonto 97 station locations.Temporal correlation between the observed and DMO-derived station-scale rainfall was then computed.The results are shown in Fig.4.It can be seen that GCPS and NCEP CFS are the most skillful in reproducing the SC rainfall,with correlation reaching 0.6 or more at some stations. On the other hand,for BCC CGCM and MSC SEF,there are only a few stations with correlation coef fi cients greater than 0.2.The June rainfall variation in eastern SC seems to be relatively well captured by GCPS,MSC-GM3,NIMR AGCM, and POAMA,whereas CWB AGCM,GDAPS,NCEP CFS and PNU CGCM are more skillful in southern coastal locations.Overall,in the more in-land western to northwestern part of SC,the abilities of the models tend to be lower.Finally,the correlation between the observed and MME mean DMO precipitation at station locations was also computed. Compared to individual models,the MME average gives the best overall performance in predicting the SC station rainfall(as evidenced by the MME mean giving the most number of stations with correlation larger than 0.2;see Fig.4l). The MME method can reduceforecast uncertaintiesby better sampling of errors in the circulation–precipitation relations in different models.Thus,in this study,we have been able to demonstrate that even simple averaging of model products can lead to an overall improvement in SC rainfall prediction.

It is evident that DMO prediction can be skillful at some stations in SC,but for other stations realistic forecasts could not be achieved.We applied the statistical downscaling technique for the purpose of improving station rainfall prediction by individual models.As shown previously,SC rainfall in June is closely related to WNPSH activity,as well as SLP variations over the ISM region(see section 3.1).We have also seen that such linkages are also reproducedin the model environment.Z500 and SLP simulations over the WNP and Indian Ocean/Indian subcontinent regions,respectively,were therefore chosen as predictors for statistical downscaling. Notice that the regions over which the Z500 and SLP variables are averaged can differ from model to model(see section 2.2 for details of the downscaling method).The exact domains used for spatial averaging are provided in Table 2.

Figure 5 shows the difference between statistical predic-tion–observation correlation and DMO–observation correlation,for each individual model as well as their MME mean. There are 50 stations over which the downscaling scheme can improve the rainfall perdition for BCC CGCM,with improvementof the correlationbygreaterthan 0.6at some locations.Most of these stations are found in southern locations ofSC nearthe PearlRiver Delta andtoits west in Guangdong (solid circles in Fig.5a),where the correlation between observations and DMO is about-0.2 to-0.3.The number of stations in SC at which statistical downscaling outperforms DMO in predicting the June rainfall is also provided in Fig. 6.For GDAPS,MSC GM2 and the MME average,the statistical scheme is able to improve the rainfall prediction only at a handful of stations.It can be seen that downscaling can improve predictions where the DMO performance is poor(see Fig.4 for the skill of DMO).Statistical downscaling leads to improvementat about 30 stations for NCEP,many of them located in southeastern coastal SC(see Fig.5h).Statistically downscaled rainfall prediction outperforms DMO for NIMR AGCM at more than 40 stations,which are located mainly in the southern part of the domain(see Fig.5i).

Table 2.Domains over which the model-predicted SLP and Z500 were averaged for statistical downscaling.See text for details.

To furtherexaminewhydownscalingcan greatly improve the skill of the two models,large-scale circulation variables were regressed onto the observed June rainfall averaged over those locations with prediction improvements.It was found that above-normal precipitation over the Pearl River Delta (corresponding to stations where downscaling leads to improvement for BCC CGCM;see Fig.5a)is positively correlated with BCC CGCM SLP over the Indian Ocean/Indian subcontinent region( fi gures not shown).An analogous relationship was also found between the observed meridional SLP gradient over the Indian Ocean/Indian subcontinent region and the rainfall variations over the SC(see section 3.1). Ontheotherhand,theregressionmapforBCC CGCM shows a positiveSLP anomalyover the Indiansub-continentright in domain A(i.e.the region over which model SLP values were used for statistical downscaling;see Table 2).Thus,there is an inverse relationship between observed rainfall at these select stations and the strength of the ISM in the BCC CGCM model.Such a relation is exploited by the station-based statistical downscalingschemeforlocalSC precipitationprediction,leading to improvements in June rainfall prediction.As for the NIMR AGCM,anomalousZ500in the SCS extending into the WNP is linked to the observed rainfall variations in the corresponding station locations( fi gures not shown).Apparently,thestation-basedstatistical downscalingschemecan make use of such a linkage to successfully enhance the prediction skill over these selected stations.

4. Discussion and summary

The performance of 11 dynamical seasonal prediction systems as well as their MME mean in predicting the June SC rainfall was assessed.It was found that the MME mean is skillful in capturing the rainfall variation averaged over the SC domain.The relationship between SC rainfall and the atmospheric large-scale circulation over the Indo-Paci fi c region in the reanalysis data and model environment was also examined.Observations showed that enhanced rainfall over SC is accompanied by stronger WNPSH activity and possibly weaker ISM in June.Such SC rainfall–WNPSH and ISM linkageswerealsoseenin theMMEmeanaswell as thehindcast runs from a number of models.The MME mean(and some of the individual models)might bene fi t from this and therefore show good skill in forecasting the June SC rainfall.

The performances of individual models and the MME in terms of their June rainfall predictions at 97 SC stations were also assessed.Overall,the MME mean also outperforms all individual models in capturing the station SC rainfall variability.The better performance of the MME average may be related to the better sampling of model uncertainties in the MMEmethod.Somemodels(e.g.NCEPCFS andGCPS)are also considerably more skillful than others in their stationscale rainfall predictions.On the other hand,most individual models and the MME mean demonstrate very little skill at some locations,such as in-land western SC and eastern coastal SC.Finally,a few individual models such as BCC CGCM and MSC SEF possess almost no ability to predict rainfall at many stations in SC.

We also carried out station-based statistical downscaling, in an attempt to improve station rainfall prediction.Model circulation patterns were directly linked to the observed station rainfall in a statistical scheme,enabling climate signals from the simulated large-scale circulation to be utilized for station-scale prediction.In particular,SLP and Z500 averaged over the ISM and WNP regions,respectively,were chosen as predictorsfor downscalingprediction.Cross-validated prediction results were then compared to those based on DMO.It was found that statistical downscaling can lead to more skillful forecasts compared to DMO at more than 30 station locations for seven models(out of a total of 12 hindcast runs,including the MME mean).Improvementsbrought about by statistical downscaling for BCC CGCM and NIMR AGCM were particularly noticeable.Further inspection of their simulations revealed that rainfall variations at some SC locations were robustly linked to the Indo-Paci fi c circulation changes in these two models.In addition,we also compared the performance of DMO and statistical downscaling for selected years.For June 1999,the statistical scheme can even outperform DMO in fi ve different models regionally,to such an extent that the scheme can correct a bias in the MME over the southern coast of SC.Overall,the statistical scheme can apparently tap into the abilities of models to capture the large-scale circulation,thereby providing reasonably skillful station-scale rainfall predictions for these models.

To summarize,a number of dynamical models examined in this study,and especially the MME mean,can successfully predict the June rainfall averaged over SC.The MME average also gives reasonably accurate predictions at the station scale,except for some locations in SC.The performance of individual models in capturing station-scale rainfall is much more variable,but the skill of some models can be improved by employingstatistical downscaling.Reliable predictionsofthe hydrological variations in this region would be useful for the water management sector,as well as for disaster mitigation and adaptation.The approaches to making such predictions highlighted in the present study could prove useful in these respects.

Acknowledgements.The authors thank those institutes participating in the APCC multi-model ensemble operational system for providing the hindcast experiment data.This study was supported by the City University of Hong Kong(Grant No.9360126).

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:Ye,K.-H.,C.-Y.Tam,W.Zhou,and S.-J.Sohn,2015:Seasonal prediction of June rainfall over South China:Model assessment and statistical downscaling.Adv.Atmos.Sci.,32(5),680–689,

10.1007/s00376-014-4047-x.

(Received 18 March 2014;revised 8 September 2014;accepted 8 October 2014)

?Current address:Institute of Space and Earth Information Science, the Chinese University of Hong Kong,Hong Kong

?Corresponding author:Chi-Yung TAM Email:Francis.Tam@cuhk.edu.hk

?Current address:Earth System Science Programme,The Chinese University of Hong Kong,Shatin,N.T.,Hong Kong