Characteristics of the Regional Meteorological Drought Events

NO.3LI Yunjie,REN Fumin,LI Yiping,et al.381 Characteristics of the Regional Meteorological Drought Events

in Southwest China During1960–2010

LI Yunjie1,2(),REN Fumin2,3?(),LI Yiping4(),WANG Pengling5(),

and YAN Hongming6()

1College of Atmospheric Science,Nanjing University of Information Science&Technology,Nanjing210044

2State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,Beijing100081 3Jiangsu Collaborative Innovation Center for Climate Change,Nanjing210008

4Institute of Arid Meteorology,China Meteorological Administration,Lanzhou730020

5Beijing Climate Center,Beijing100081

6Yunnan Climate Center,Kunming650034

(Received October12,2013;in?nal form March27,2014)

ABSTRACT

An objective identi?cation technique for regional extreme events(OITREE)and the daily composite-drought index(CI)at101stations in Southwest China(including Sichuan,Yunnan,Guizhou,and Chongqing)

are used to detect regional meteorological drought events between1960and2010.Values of the parameters

of the OITREE method are determined.A total of87drought events are identi?ed,including9extreme

events.The2009–2010drought is the most serious in Southwest China during the past50years.The regional

meteorological drought events during1960–2010generally last for10–80days,with the longest being231

days.Droughts are more common from November to next April,and less common in the remaining months.

Droughts occur more often and with greater intensity in Yunnan and southern Sichuan than in other parts

of Southwest China.Strong(extreme and severe)regional meteorological drought events can be divided

into?ve types.The southern type has occurred most frequently,and Yunnan is the area most frequently

stricken by extreme and severe drought events.The regional meteorological drought events in Southwest

China have increased in both frequency and intensity over the study period,and the main reason appears to

be a signi?cant decrease in precipitation over this region,but a simultaneous increase in temperature also

contributes.

Key words:Southwest China,regional meteorological drought events,temporal characteristics,spatial distribution

Citation:Li Yunjie,Ren Fumin,Li Yiping,et al.,2014:Characteristics of the regional meteorological drought events in Southwest China during1960–2010.J.Meteor.Res.,28(3),381–392,doi:10.

1007/s13351-014-3144-1.

1.Introduction

Droughts are major meteorological events with long duration and wide-ranging e?ects(Zhang,2005). Increases in the frequency of droughts and disasters associated with droughts under global warming have caused great concern.Huang et al.(2006)analyzed decadal variations in the characteristics of droughts and?oods in China,and identi?ed an increase in the frequency of droughts in North China after the late 1970s.Some studies have suggested that the area af-fected by droughts has increased throughout China during the past half-century,although these increases are only statistically signi?cant in North,Northeast, and Northwest China(Zhang et al.,2003;Zhai and Zou,2005;Ma and Fu,2006;Zou and Zhang,2008; Yang et al.,2013).

Droughts in Southwest China have attracted gre-

Supported by the National Natural Science Foundation of China(41175075)and Climate Change Special Fund of the China Meteorological Administration(CCSF201333).

?Corresponding author:fmren@https://www.360docs.net/doc/d75460113.html,.

?The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg2014

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at concern.A number of studies have examined the characteristics of droughts in this region.Shi et al. (2002,2003)showed that the climate in Northwest China was predominantly warm and dry from the end of the little ice age until the1980s,but transi-tioned to warm and wet https://www.360docs.net/doc/d75460113.html,ing the stan-dardized precipitation index,Qi et al.(2006)found that droughts had become more frequent in Southwest China,the western part of Northwest China,and the northern part of North China over the last60years. Ma et al.(2006)analyzed the characteristics of cli-mate change in Southwest China.They identi?ed a signi?cant warming trend over the Yungui Plateau,as well as decreases in precipitation in the Sichuan basin and the hilly regions of Guizhou.Peng et al.(2009) reported that droughts in Yunnan are most intense between January and March.Two recent studies re-vealed that precipitation has decreased and tempera-ture has increased over Yunnan since the1950s(Cheng and Xie,2008;Liu et al.,2010).Liu et al.(2012)re-vealed that the frequency of droughts increased signif-icantly in the hilly regions of Yunnan,Guizhou,and Guangxi between1961and2009.Severe droughts have been particularly prevalent in Southwest China dur-ing the past10years,with a severe drought across Southwest China in the summer of2003,a rare early-spring drought in Yunnan in2005,an extreme summer drought in Sichuan and Chongqing in2006(Li et al., 2011),and a long period of extreme drought in South-west China from autumn of2009through spring of 2010.These trends have been especially pronounced since2000.

Almost all of the elements or drought indices used in the above studies have been calculated at individual points(observing sites or gridded values).Droughts are typically regional phenomena with distinct dura-tions and areas of impact.A few recent studies have focused on regional drought events.Andreadis et al. (2005)analyzed the characteristics of regional drought events in the United States during the20th century using monthly soil moisture data.Qian et al.(2011) studied regional drought events in China.Their re-sults indicated that droughts in South and Southwest China generally occur during winter,while droughts in northern China occur more frequently during spring. Ren et al.(2012)proposed an objective identi?ca-tion technique for regional extreme event(OITREE). This method has been used to identify a number of di?erent regional extreme events in China,such as droughts,heavy precipitation events,and tempera-ture extreme events.Cui(2010)analyzed regional meteorological drought events in China over the past 50years.Regional drought events occur most fre-quently and with greatest intensity in North China, but Southwest China is also seriously in?uenced by regional droughts(see Fig.1).Previous studies of meteorological droughts in Southwest China have focused mainly on individual drought events.This study employs the new method proposed by Ren et al.(2012)to study Southwest China regional meteo-rological drought events between1960and2010.

2.Data and method

2.1Data

According to the Grade-I China Meteorological Geographic Zoning(CMA Forecast and Mitigation Division,2006),Southwest China includes Yunnan, Sichuan,and Guizhou provinces,and Chongqing Mu-nicipality.

The daily composite-drought index(CI)com-puted by the National Climate Center are adopted in this study.The underlying measurements are taken at 101stations in Southwest China between1

January

Fig. 1.Distribution of the cumulative intensity of regional meteorological drought events in China during 1957–2008(from Cui,2010).

NO.3LI Yunjie,REN Fumin,LI Yiping,et al.383

1960and31December2010.Zhang et al.(2006)have described the method for calculating CI in detail.

2.2Method

The OITREE method(Ren et al.,2012)is applied in this study.The technique includes?ve steps.(1)A daily index is selected for each individual station;(2) the region is partitioned into daily abnormality belts;

(3)the temporal continuity of each event is veri?ed;

(4)a standard index for regional events is constructed; and(5)the extremity of each regional event is assessed.

Steps(2)and(3)in this process are the most im-portant.The partitioning of daily abnormality belts starts from a structural analysis of the daily distribu-tion of abnormality,which can then be separated into a set of distinct daily abnormality belts.The temporal continuity of each event is determined by analyzing the spatial overlap of the natural daily abnormality belts on adjacent dates.The index system(Table1),which is developed based on the features of regional events, includes?ve independent indices(extreme intensity, accumulated intensity,accumulated area,maximum impacted area,and duration)and one integrated index.

The values of the OITREE parameters are listed in Table2.These parameters are determined by ana-lyzing a set of Southwest China regional meteorologi-cal drought events,and can be summarized as follows. CI is the daily drought index for individual stations. This parameter is required for step(1)as described above.Step(2)requires four parameters.The thresh-old for abnormality is set to the threshold CI for mod-erate drought(–1.2),the threshold distance for de?n-ing neighboring sites is set to250km,the threshold

Table1.The index system for regional weather and climate events

Index

Grade I Grade II Grade III (for event)(for day k)(for station j)

Extreme intensity(I1)I1=Max

k=1,K (T1k)I1k=Max

i=1,J k

(T ki)I1|j=Max

k=1,K

(T kj)

Accumulated intensity(I2)I2=

K

k=1I2k I2k=

J k

i=1

(T ki?T ki|c)I2|j=

K

k=1

(T kj?T kj|c)

Accumulated impacted area(A s)A s=

K

k=1

A k A k=Area(S k)

Maximum impacted area(A m)A m=Area(

K

k=1

S k)

Lasting period(D)D=K

Integrated index(integrated Z=F(I1,I2,A s,A m,D)Z k=f(I1k,I2k,A k)

intensity)(Z)Scheme:?rst standardize Scheme:use the parameters

I1,I2,A s,A m,and D,then in the formulation of Z to

the weighted sum standardize I1k,I2k,and A k,

and then calculate the

weighted sum

Spatial location1)Distributions of station Distributions of the extent

extreme intensity(I1|j)and and geometric center of the

station accumulated intensity(I2|j)daily impacted area

2)maximum impacted area and

its geometric center,the

centers of gravity with weights

of(I1|j)and(I2|j)

Note:These indices are classi?ed into three grades.Grade I indices are calculated for the entire event,grade II indices provide daily extreme values,and grade III indices provide station extreme values.K is the duration of the event in days and J is the number of stations impacted by the event,while J k is the number of stations impacted on day k,S k is the spatial distribution of the impacted stations and Area(S k)is the area covered by S k.T ki is the value of the daily index for station i on day k,and T ki|c is the threshold value of that index.T kj and T kj|c are similar to T ki and T ki|c,but are calculated for each of the stations impacted by the event.I1,I2,A s,A m,and D are individual indices.After Ren et al.(2012).

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Table2.Values of the parameters of the OITREE method for identifying Southwest China regional meteorological drought events(adapted from Ren et al.(2012))

Parameter Code Meaning Value Daily index for individual stations CI The composite drought index CI

Threshold for the daily index CI0Abnormality occurs at individual–1.2

points(stations)when CI CI0

Distance threshold for neighbor d0All stations within d0of a point250km

stations(station)are de?ned as

neighbor stations

Threshold for the neighboring R0An abnormal station is de?ned0.3

abnormality ratio as the center of an abnormality

belt if its ratio is greater than R0

Threshold for number of M0If the number of abnormal stations11

abnormal stations in a belt in a belt is greater than or equals

to M0,the belt is an

abnormality belt.

Threshold for the number M gap Interruptions not longer0

of days for a gap in an event than M gap are allowed to

occur during an event

Direction indicator I direc If abnormality is de?ned as T T t,–1

I direc=1,otherwise I direc=–1

Coe?cients for calculating e1,e2,e3,e4,e5The?ve coe?cients in the formula–0.06,–0.32, the grade I integrated index Z=F(I1,I2,A s,A m,D)0.28,0.1,0.24

=e1I%1+e2I%2+e3A%s

+e4A%m+e5D%

Integrated intensity index and Z,Z0An event can be de?ned as a Z,0.3

threshold for de?ning a regional event if the integrated

regional event intensity index Z>Z0.

Thresholds for classifying Z1,Z2,Z3These thresholds are used to 3.63,1.72,0.7 regional events de?ne four intensity categories:

extreme(top10%,Z Z1),

severe(10%–30%,Z1>Z Z2),

moderate(30%–70%,Z2>Z Z3),

and slight(70%–100%,Z3>Z)

Note:I%1,I%2,A%s,A%m,and D%are standardized I1,I2,A s,A m,and D,respectively,and e1,e2,e3,e4,and e5are their coe?cients.

for the minimum number of sites in a daily drought belt is set to11,and the threshold ratio for overlap-ping sites in a daily drought belt is set to0.3.For the third step,the threshold for the number of days allowed for a gap in an event is set to zero,so that all events are continuous.

The fourth step requires a number of parameters. The direction indicator is set to–1because of the na-ture of CI.Ren et al.(2012)cautioned that the?ve weighting coe?cients used to calculate the integrated index Z must be determined objectively and carefully. Here,we use the following objective method to calcu-late these?ve coe?cients.First,the absolute values of the standardized drought indices are ranked in de-scending order.Second,the ratio of the sum of the top 10%percentile of values to the sum of all values in each series is calculated.Third,the?ve coe?cients are cal-culated by multiplying the direction indicator for each index and the ratio for each series divided by the sum of the?ve ratios.The?ve coe?cients obtained using this approach are–0.06,–0.32,0.28,0.10,and0.24, respectively.More than200events with durations of 10days or longer are identi?ed during1960–2010.

Step(5)involves a comparison of di?erent re-

NO.3LI Yunjie,REN Fumin,LI Yiping,et al.385

gional drought events.Figure2shows the frequency distribution of di?erent values of the integrated in-dex Z.Small values of Z(Z<0.3,corresponding to weak events with small impacted areas)occur with the greatest frequency.The threshold for de?ning a South-west China regional meteorological drought event is therefore set to0.3.A total of87Southwest China regional meteorological drought events are identi?ed. Following Ren et al.(2012),these87events are di-vided into extreme droughts(9events,about10%of the total),severe droughts(17events,about20%), moderate droughts(35events,about40%)and slight droughts(26events,about30%).The threshold val-ues of the integrated index Z for distinguishing these four classes of drought events are3.63,1.72,and0.70, respectively.

3.Examples of the identi?ed drought events

According to the integrated intensity,the three strongest drought events between1960and2010are identi?ed as follows.The?rst strongest event lasted for231days from September2009through April2010 and covered most of Southwest China.The second strongest event lasted for217days from November 1962through June1963.The third strongest event lasted for134days from January to June1969,and in?uenced all of Southwest China.

Previous studies have shown that these three ex-treme drought events were associated with serious economic losses(Ding,2008;Zhang et al.,2009). The methodology also identi?es other recent severe drought events,such as the serious autumn drought in Yunnan during2003(number15)and the extreme drought that occurred in Sichuan and Chongqing dur-ing2006(number14).

Figure3shows a variety of indices calculated for an extreme meteorological drought event that per-sisted from4September2009to22April2010.The maximum area impacted by this event is118.4×104 km2.Most regions of Yunnan,western Guizhou,and southern Sichuan are seriously a?ected(Fig.3a).The daily variations of integrated intensity,extreme in-tensity,accumulated intensity,and impacted area are largely consistent(Figs.3b–e).The drought event began in September2009and strengthened quickly during October before reaching its?rst peak in in-tensity around the mid November.The drought then entered a relatively weak phase until January2010,be-fore strengthening again and reaching a second peak in February and March2010.The drought eased and ended in April2010.These results are well consistent with the drought monitorings.

4.Analysis of results

4.1Temporal characteristics

Figure4shows frequency distributions of the du-ration and maximum impacted area of the87South-west China regional meteorological drought

events. Fig.2.Frequency(time)distribution of the integrated index Z for Southwest China drought events during1960–2010.

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Fig. 3.(a)The spatial distribution of accumulated in-tensity of the 2009–2010Southwest China regional extreme meteorological drought event and daily variations of (b)in-tegrated intensity,(c)extreme intensity,(d)accumulated intensity,and (e)impacted area of this event.Dotted lines indicate polynomial trends.

These events generally last for 10–80days,with a peak at 30–40days.The frequency of the events that last

longer than 80days is very small.Only three events last for more than 200days,with the longest being 231days.All of these long-duration events are also ex-treme drought events,ranking ?rst,second,and fourth out of the 87events.The impacted area of these events varies between 30×104and 130×104km 2,with a peak at 70×104–100×104km 2.

Regional drought events generally start between late autumn and early spring and end before mid sum-mer (?gure omitted).Figure 5shows seasonal varia-tions of the regional meteorological drought frequency.Droughts are most frequent during November–April,and relatively infrequent within May–October.We therefore de?ne November to April as the drought season and May to October as the non-drought season.

Figure 6shows interannual variations of South-west China regional meteorological drought event in-dices.The frequency of droughts has increased at a rate of 0.19per decade (statistically signi?cant at the 0.05level according to Student’s t test).The largest number of droughts occurs in 1994(5times),1992(4times),and 1988(4times).No droughts occur in 1965or 1967.Although the frequencies in 2010and 1963are both listed as 0,both years include the drought events that started the year before.The frequency statis-tics only account for the start time of drought events.Although accumulated integrated intensity (Fig.6b)and accumulated maximum impacted area (Fig.6c)both increase during 1960–2010,these increases are not statistically signi?cant.The top three values of accumulated integrated intensity occur in 2009,1978,and 1962.The top three values of accumulated max-imum impacted area occur in 1994,1992,and 1988.The top three years in terms of frequency are the same as those for accumulated maximum impacted area,but are completely di?erent from those for ac-cumulated integrated intensity.Severe drought events often last for a long time,so the frequency is low dur-ing the years within which they occur (such as 2009).Weak droughts generally last for shorter periods and are therefore more likely to repeat during a single year (such as 1994).

The interannual variations of precipitation and

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387

Fig. 4.Frequency distributions for(a)duration and(b)maximum impacted area of Southwest China regional meteo-rological drought events between1960and

2010.

Fig. 5.Seasonal variations of Southwest China regional meteorological drought event frequency between1960and 2010.

mean temperature between1960and2010(Fig.7) are analyzed to explore the reasons why regional me-teorological drought events have become stronger and more frequent in Southwest China.Annual precipita-tion has decreased signi?cantly at a rate of12.63mm per decade,while annual mean temperature has in-creased at a rate of0.15℃per decade.

The correlations between the time series of annual precipitation(Fig.7a)and the time series of drought frequency and accumulated integrated intensity(Figs. 6a and6b)are–0.44and–0.53,respectively.Both val-ues are statistically signi?cant at the0.01level based on Student’s t test.The correlations between the time series of mean temperature(Fig.7b)and the time se-ries of drought frequency and accumulated integrated intensity(Figs.6a and6b)are0.19and0.25,respec-tively.This result suggests that the main reason why droughts have become stronger and more frequent lies in the signi?cant decrease in precipitation,although the increase in temperature also contributes.Analy-sis of the accumulated integrated intensity on decadal timescales(?gure omitted)indicates that the intensity of droughts in the early21st century has been signi?-cantly stronger than the intensity of droughts in other periods.The average annual precipitation is smaller after2000than during other periods in the past50 years(Fig.7),while the temperature is much higher. These may be important factors in droughts becoming more serious in the?rst decade of the21st century.

Southwest China regional meteorological drought events vary on seasonal timescales,with drought sea-son between November and next April.Correlation co-e?cients have been calculated for the drought season to further examine the relationships between precipi-tation,temperature,and drought intensity(Table3). Accumulated integrated intensity is negatively corre-lated with precipitation during all analyzed time peri-ods,with statistically signi?cant relationships for an-nual precipitation,drought season precipitation,and September–April precipitation.The strongest corre-lation(–0.68)is for September–April precipitation. Accumulated integrated intensity is positively corre-lated with temperature during all analyzed time peri-ods,but none of the correlations is statistically signi?-cant.To summarize,the intensity of Southwest China regional meteorological drought events between1960 and2010is most closely related to precipitation during the September–April season.

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Fig.6.Interannual variations of the(a)frequency,(b)accumulated integrated intensity,and(c)accumulated maximum impacted area of Southwest China regional meteorological drought events between1960and2010.

Table3.Correlation coe?cients between the annual accumulated integrated intensity of Southwest China regional meteorological drought events and regional mean precipitation and temperature over Southwest China during di?-erent time periods

Integrated intensity Annual mean Drought season Drought and pre-drought Non-drought season (Nov–Apr)season(Sep–Apr)(May–Oct)

Precipitation–0.53–0.51–0.68–0.28 Temperature0.250.260.240.15

4.2Spatial distribution characteristics

Figure8shows spatial distributions of the fre-quency and accumulated intensity of Southwest China regional meteorological drought events between1960 and2010.Drought frequency is generally greater than 20across Southwest China.The highest frequencies occur in Yunnan and southern Sichuan,with peak val-ues of greater than70near the Yunnan–Sichuan bor-der.The spatial distribution of accumulated intensity is similar,with peak values in eastern-northern Yun-nan and southern Sichuan.The spatial distribution of drought in Southwest China over the past50years is characterized by two typical patterns of“strong south and weak north”and“strong west and weak east”. To summarize,Yunnan and southern Sichuan are the

NO.3LI Yunjie,REN Fumin,LI Yiping,et al.389

most heavily impacted areas by droughts in Southwest China.

Figure9shows the spatial distribution of trends in drought event frequency in Southwest China during 1960–2010.The frequency of droughts has increased across Southwest China,with signi?cant

increasing

Fig.7.Variations of(a)annual precipitation and(b)mean temperature over Southwest China between1960and2010. The dotted line represents the linear trend,and the short thick lines represent average values for each

decade.

Fig.8.Spatial distributions of(a)frequency(time)and(b)accumulated intensity of Southwest China regional meteorological drought events between1960and2010.

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Fig.9.Spatial distribution of linear trends in the annual frequency (per decade)of Southwest China regional me-teorological drought events between 1961and 2010.The shaded area indicates the trends that are statistically sig-ni?cant at the 0.05level based on Student’s t test.

trends in central and northern Yunnan and northern and western Guizhou.

Among the 87identi?ed drought events,26can be categorized as extreme and severe drought events (9extreme events and 17severe events).These 26events can then be divided into ?ve types based on the spatial distribution of their accumulated intensity:eastern type,southern type,western type,northern type,and whole-area type.Figure 10shows spatial distributions of the accumulated intensity for repre-sentative cases of the ?ve distribution types.East-ern type events mainly cover Guizhou and Chongqing (Fig.10a).Southern type events are concentrated in Yunnan (Fig.10b).Western type events are gen-erally located in western Sichuan and Yunnan (Fig.10c).Northern type events mainly occur in northern Sichuan and Chongqing (Fig.10d).Whole-area type events extend across Southwest China (Fig.10e).Fig-ure 11shows frequency of the ?ve-type extreme

and

Fig.10.Spatial distributions of accumulated intensity for representative cases of the ?ve distribution types of Southwest China regional meteorological drought events:(a)eastern type (19November 1988–2December 1989),(b)southern type (3December 1978–25June 1979),(c)western type (11October 1981–2February 1982),(d)northern type (11July–17September 2006),and (e)whole-area type (23January–5June 1969).

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Fig.11.Frequency of the extreme and severe Southwest China regional meteorological drought events in the ?ve distribution types.

severe droughts.Thirteen of the total 26events and 6of the 9extreme events are southern type events.The second most frequent type is the whole-area type (8,including the remaining 3extreme events).Two of the events are western type,two are northern type,and one is eastern type.Yunnan is the area most fre-quently impacted by the extreme and severe drought events in Southwest China.5.Conclusions and discussion

The results of the above analysis can be summa-rized as follows.

(1)The OITREE method shows a good capability for identifying Southwest China regional meteorologi-cal drought events.A total of 87events are identi?ed between 1960and 2010,including 9extreme events and 17severe events.The 2009–2010extreme drought is the most serious meteorological drought during the past 50years.

(2)Southwest China regional meteorological drought events generally last for 10–80days,with the longest being 231days.The maximum impacted area varies between 70×104and 100×104km 2.The drought season in Southwest China runs from November to next April and the non-drought season runs from May to October.

(3)Droughts are most frequent and most intense in Yunnan and southern Sichuan.Extreme and severe drought events can be divided into ?ve types based on their spatial distributions:southern type,west-ern type,northern type,eastern type,and whole-area type.The southern type occurs the most frequent.

Yunnan is the area most frequently stricken by ex-treme and severe drought events in Southwest China.

(4)Southwest China regional meteorological drought events have become more frequent and stronger over the past 50years.The main reason for these increases appears to be a signi?cant decrease in precipitation in Southwest China,although an in-crease in temperature also contributes.

This study has analyzed the characteristics of regional meteorological drought events in Southwest China.Further studies are needed to reveal the causes of and speci?c mechanisms that lead to droughts in Southwest China.

Acknowledgments.The language editor of this manuscript is Dr.Jonathon S.Wright.

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