Diurnal variations of precipitation during the warm season over China

INTERNATIONAL JOURNAL OF CLIMATOLOGYInt. J. Climatol. (2008)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/joc.1758

Diurnal variations of precipitation during the warm seasonover China

Shuiqing Yin,a,b Deliang Chenb,c* and Yun Xiea

a School of Geography, Beijing Normal University, Key Laboratory of Environmental Change and Natural Disaster, Ministry of Education,Beijing, China

b Department of Earth Sciences, University of Gothenburg, Gothenburg, Swedenc Beijing Climate Center, China Meteorological Administration, Beijing, China

ABSTRACT: The spatial distribution and seasonal change of diurnal variations of precipitation during the warm season(May–September) over China were analysed using hourly data from 62 rain gauges during 1954–2001. Nocturnalprecipitation predominates for the stations over the Tibetan Plateau as well as in south-west and north-west China. Forstations to the east of about 110 °E, daytime precipitation predominates in the northern and southern parts, whereas nocturnalprecipitation predominates in the middle and lower reaches of the Yangtze River and the North China Plain. Further, themagnitude of diurnal variations first decreases and then increases from the south towards the north. Two stations locatedclose to the Bohai Bay manifest morning maxima. The significant diurnal variations in vertical circulation (stronger duringthe day and weaker at night), the meso-local-scale mountain-valley winds, and the nighttime cooling of cloud tops couldbe the processes behind the nocturnal rain in the western part of China (to the west of about 110 °E). Daytime precipitationin the eastern part of China (to the east of about 110 °E) could be attributed to the moist convection connected with solarheating. The morning peaks for stations in the middle and lower reaches of the Yangtze River and the North China Plainand the magnitude changing pattern in the eastern part seem to be connected with the advance and retreat of the EastAsian summer monsoon. The morning maxima at coastal stations may be mainly attributed to the local land–sea breeze.The seasonal changes of diurnal variations for the western part of China are relatively small, with nighttime precipitationmaxima remaining fairly constant during the warm season. This part is not directly influenced by the monsoon. For mostof the stations in the eastern part, morning peaks tend to be strengthened/weakened along with the East Asian monsoon’sadvancing/retreating. Copyright 2008 Royal Meteorological Society

KEY WORDS precipitation; diurnal variations; China; warm season; the East Asian summer monsoon

Received 10 July 2007; Revised 18 March 2008; Accepted 21 July 2008

1. Introduction

Diurnal variations of precipitation are an important aspectof regional climate because precipitation occurring reg-ularly during a particular time of the day is connectedwith the regional and local atmospheric circulation and/ordynamics. This is why diurnal variations simulated by aclimate model are often checked with observation (Chenet al., 1996; Collier and Bowman, 2004; Knievel et al.,2004). On the other hand, verified climate models canenhance the understanding of mechanisms that causediurnal variations (Dai et al., 1999; Mapes et al., 2003;Zhang, 2003; Dai and Trenberth, 2004; Li et al., 2004;Liang et al., 2004; Woolnough et al., 2004). There is evi-dence that precipitation diurnal variations have exhibitedlong-term changes. For example, a 220-year precipitationrecord for Seoul, Korea was divided into two wet peri-ods and one dry period. The wet periods showed similarcharacteristics of diurnal cycle with a peak near 0600

* Correspondence to: Deliang Chen, Department of Earth Sciences,University of Gothenburg, 40530 Gothenburg, Sweden.E-mail: [email protected]

LST and a broad evening minimum around 1900–2400LST during most of the years, but the dry period does notmanifest the morning peak clearly and showed many dif-ferent characteristics from two wet periods (Jung et al.,2001). The diurnal precipitation patterns at PotsdamStation during the last 30 years of observation period(1971–2000) were obviously different from that of thefirst 30 years (1893–1922). Both precipitation sums andevents decreased for the period 1971–2000, while theintensities remained nearly constant (Werner and Gersten-garbe, 2004). The researches both from the United States(Dai, 1999) and Japan (Fujibe et al., 2006) indicated thatthe changes in the diurnal variation patterns existed asregional and seasonal variations. How the precipitationdiurnal variation will change in the future is also ofgreat interest. For example, a time-slice global warm-ing experiment was performed by Kitoh and Arakawa(2005) and the reduced amplitudes of rainfall diurnalvariation were found both in land and ocean over BorneoIsland and the Java Sea at the end of the 21st century. Inareas where diurnal variations are significant, stochasticmodels used for disaggregation of daily precipitation

Copyright 2008 Royal Meteorological Society

S. YIN ET AL.

cannot ignore the diurnal trends (Morrissey and Krajew-ski, 1993; Glasbey et al., 1995; Katz and Parlange, 1995;Gyasi–Agyei, 2001) because, for example, the diurnalvariations of precipitation frequency and intensity havelarge effects on surface hydrological processes such asrunoff and evaporation.

Many researchers, using observation data, soundingdata, and satellite photos, have shown that convectiveprecipitation has a preference for afternoons in warm,tropical, continental climates because of the local surfaceheating maxima (Wallace, 1975; Dai, 2001; Sorooshianet al., 2002; Bowman et al., 2005; Hirose and Nakamura,2005; Hong et al., 2005; Janowiak et al., 2005). Thepattern of such afternoon maxima may show spatialvariation worldwide because of the interaction of land andsea breezes near the coasts and the effects of topography.Seasonal change may also play a role because theconvection can vary through the year with changes inthe degree of radiant heating and variation in the vapourcontent of the air (Oki and Musiake, 1994; Ward andRobinson, 2000; Angelis et al., 2004; Sato and Kimura,2005).

On a worldwide scale, diurnal variations of varioustypes of precipitation and thunderstorms were studied by3-hourly weather data from 15 000 stations around theglobe (Dai, 2001). It was indicated that atmospheric rel-ative humidity contributes to the morning maximum ofdrizzle and non-showery precipitation and that solar heat-ing on the ground produces a late-afternoon maximum ofshowery precipitation and thunderstorms over most landareas. The strong continental diurnal cycle induces a diur-nal cycle of opposite phase in low-level convergence overlarge nearby oceanic areas that favours a morning maxi-mum of maritime showery precipitation.

In Asia, diurnal variations of precipitation amount dur-ing the summer monsoon season (May–August) overeastern China, Japan, and Korea were detected and itwas found that this region dominated by the monsoonmanifests a morning maximum with an afternoon maxi-mum as well, while the surrounding regions showed nosuch marked patterns (Ramage, 1952). However, datafrom only about 20 stations in eastern China were col-lected and 8 of them were with observations of lessthan 5 years, which calls for more widely distributedstations with longer observation series to be used tofurther analyse the pattern of precipitation diurnal varia-tions. Zhang and Lin (1985) analysed 5–8 years of hourlydata over China and showed that daytime precipitationpredominates in the inland area, whereas nocturnal pre-cipitation predominates in coastal and small-island areasas well as in valleys, especially in the Sichuan Basin.Lin (1995) further emphasized that topography has agreat influence on the diurnal variations of precipitation.Daytime precipitation predominates in areas with a con-vex shape, whereas nocturnal precipitation maxima occurin concave topography. Ye and Gao (1979) specificallydemonstrated that for most areas in the Tibetan Plateauand its surrounding areas nocturnal precipitation is typ-ical. The diurnal variations of precipitation during the

monsoon season in the Tibetan Plateau were also stud-ied by Liu et al. (2002). The relationships between pre-cipitation and thermodynamic variables, such as con-vective available potential energy (CAPE) and liftingcondensation level (LCL), were also investigated in theirstudy by the use of soundings, precipitation gauges,and radar data in the summer of 1998. It was foundthat the diurnal variations of precipitation, CAPE andintegrated water vapour content have similar patternswith peak values appearing at night. The precipitationreaches its maximum when the maximum CAPE andintegrated water vapour content and the minimum LCLappear.

Recently, 3 years (from 2002 to 2004) of surfaceobservation and satellite data (Zhao et al., 2005) and14 years (from 1991 to 2004) of hourly observation data(Yu et al., 2007) were used to detect the spatial patternof diurnal variations of summer (June–August or JJA)precipitation over China. Although these studies providesome insight into the dynamics of the diurnal variationsin China, they are limited by their span of coverage interms of both the seasons and the years considered. Afurther analysis, using longer-time observations coveringmonths beyond the summer season, would be useful toexplore seasonal variation and to establish climatologicalcharacteristics of the diurnal variations.

The climate in China is deeply influenced by themonsoons and the Tibetan Plateau (Ye and Gao, 1979;Ding and Murakami, 1994), and many climate mod-els cannot realistically simulate the complex character-istics of precipitation dynamics in China (Zhao et al.,2000; Kang, 2004; Hu and Ding, 2006). One aspect ofprecipitation dynamics is diurnal variations; however,the climatology of diurnal variations in precipitationand its seasonal change have not been fully studiedusing long-time, sub-daily data series over China. Thisstudy aims at establishing a climatology of the sea-sonal evolution of the diurnal variations in the warmseason in relation to the migration of the summer mon-soon in China. A systematic description of the dynamicswould help in improving our understanding of the rele-vant processes that create diurnal variations and wouldprovide a useful observation basis for climate modelverification.

2. Data and methods

Hourly precipitation data from 62 observation stationsall over China were collected and quality-controlled bythe National Meteorological Information Center (NMIC)of China Meteorology Administration (CMA, Table Iand Figure 1). This study focuses on precipitationdata from the warm season (May–September) during1954 and 2001. Most precipitation is concentrated inMay–September, comprising about 40–98% of the wholeyear’s precipitation for most parts of China. The missingdata during the study period for all the stations is lessthan 15%. The maximum and mean rate of missing dataduring the study period for all stations is 12.2 and 3.8%,

Copyright 2008 Royal Meteorological Society Int. J. Climatol. (2008)DOI: 10.1002/joc

DIURNAL VARIATIONS OF PRECIPITATION IN CHINA

Table I. The explained variances of the diurnal and semidiurnal cycles. The data are from China Meteorology Administration.e1 and e2 are the percentage of the variances explained by the diurnal cycle (S1) and semidiurnal cycle (S2) respectively.

No. Stationno.

Station name Latitude(°N)

Longitude(°E)

Height(m)

e1

(%)e2

(%)

1 58321 Anhui Hefei 31.87 117.23 27.9 13.7 692 54511 Beijing 39.8 116.47 31.3 64.9 26.43 57516 Chongqing Shapingba 29.58 106.47 259.1 91.1 4.34 58847 Fujian Fuzhou 26.08 119.28 84 76.1 17.35 52889 Gansu Lanzhou 36.05 103.88 1517.2 72.8 20.46 53915 Gansu Pingliang 35.55 106.67 1346.6 43.8 35.77 57006 Gansu Tianshui 34.58 105.75 1141.7 86.9 7.68 59287 Guangdong Guangzhou 23.17 113.33 41 73.4 22.19 59316 Guangdong Shantou 23.4 116.68 2.9 75.4 17.2

10 59211 Guangxi Baise 23.9 106.6 173.5 75.2 17.911 57957 Guangxi Guilin 25.32 110.3 164.4 72 2212 59431 Guangxi Nanning 22.63 108.22 121.6 67.4 26.413 57707 Guizhou Bijie 27.3 105.28 1510.6 88 5.614 57816 Guizhou Guiyang 26.58 106.73 1223.8 85.4 8.815 59758 Hainan Haikou 20.03 110.35 13.9 69 23.616 53698 Hebei Shijiazhuang 38.03 114.42 81 48.9 3917 50953 Heilongjiang Ha’erbin 45.75 126.77 142.3 17 69.618 50557 Heilongjiang Nenjiang 49.17 125.23 242.2 21.9 58.619 57297 Henan Xinyang 32.13 114.05 114.5 3.5 85.520 57083 Henan Zhengzhou 34.72 113.65 110.4 77.6 8.521 57494 Hubei Wuhan 30.62 114.13 23.1 53.7 33.322 57461 Hubei Yichang 30.7 111.3 133.1 24.6 65.423 57679 Hunan Changsha 28.22 112.92 68 44.2 43.524 57745 Hunan Zhijiang 27.45 109.68 272.2 80.7 13.925 58238 Jiangsu Nanjing 32 118.8 7.1 51.9 38.326 57993 Jiangxi Ganzhou 25.85 114.95 123.8 68.4 26.627 58606 Jiangxi Nanchang 28.6 115.92 46.9 7.5 84.428 54374 Jilin Linjiang 41.8 126.92 332.7 47.8 40.729 54324 Liaoning Chaoyang 41.55 120.45 169.9 36.8 54.930 54662 Liaoning Dalian 38.9 121.63 91.5 88.6 531 54497 Liaoning Dandong 40.05 124.33 13.8 69 21.832 54342 Liaoning Shenyang 41.73 123.45 44.7 8.4 80.933 54292 Liaoning Yanji 42.88 129.47 176.8 54.1 37.734 53391 Neimeng Huade 41.9 114 1482.7 48 42.335 53463 Neimeng Huhehaote 40.82 111.68 1063 31.3 60.736 54026 Neimeng Zhalu 44.57 120.9 265 46.6 26.337 53817 Ningxia Guyuan 36 106.27 1753 42.4 38.338 53614 Ningxia Yinchuan 38.48 106.22 1111.4 48.3 23.239 52866 Qinghai Xining 36.72 101.75 2295.2 90 4.540 57127 Shaanxi Hanzhong 33.07 107.03 509.5 93.1 141 57036 Shaanxi Xi’an 34.3 108.93 397.5 50.4 3742 53845 Shaanxi Yan’an 36.6 109.5 958.5 17.8 69.543 53646 Shaanxi Yulin 38.23 109.7 1057.5 26.1 64.344 54823 Shandong Jinan 36.6 117.05 170.3 32.9 58.445 54843 Shandong Weifang 36.75 119.18 22.2 12.7 78.446 58367 Shanghai Longhua 31.4 121.48 6 10.6 72.647 53487 Shanxi Datong 40.1 113.33 1067.2 21.5 71.548 53853 Shanxi Jixian 36.7 110.95 1052.7 7 82.549 53772 Shanxi Taiyuan 37.78 112.55 778.3 44.7 47.550 53663 Shanxi Wuzhai 38.92 111.82 1401 5.8 86.251 53664 Shanxi Xingxian 38.47 111.13 1012.6 15.2 77.752 53975 Shanxi Yangcheng 35.48 112.4 659.5 69.7 24.653 53673 Shanxi Yuanping 38.73 112.72 828.2 44.6 48.154 53959 Shanxi Yuncheng 35.03 111.02 376 30.5 6055 56294 Sichuan Chengdu 30.67 104.02 506.1 95 0.656 56571 Sichuan Xichang 27.9 102.27 1590.9 94.7 0.957 51463 Xinjiang Wulumuqi 43.78 87.65 935 89.6 4.2


S. YIN ET AL.

Table I. (Continued ).

No. Stationno.

Station name Latitude(°N)

Longitude(°E)

Height(m)

e1

(%)e2

(%)

58 51431 Xinjiang Yining 43.95 81.33 662.5 87.4 6.659 55591 Xizang Lasa 29.67 91.13 3648.7 95.4 060 56778 Yunnan Kunming 25.02 102.68 1892.4 24.5 68.361 56739 Yunnan Tengchong 25.02 98.5 1654.6 80 14.962 58659 Zhejiang Wenzhou 27.78 120.65 39.2 70.9 22.4

Figure 1. Stations with hourly precipitation data used in this study. Different symbols are used to show the spatial distribution of the classifieddiurnal variations groups. The diurnal cycle-predominating are indicated by white triangles; the semidiurnal cycle-predominating group by white

squares; and the diurnal and semidiurnal cycle comparable group by white circles.

respectively. The missing data were simply neglected inthe calculation of the statistics analysed in the follow-ing sections, which means that the missing data weretaken as the same as the long-term means based on theexisting data. The measurements were made according toBeijing time for all the stations. To facilitate comparisonof diurnal cycles among all the stations, Beijing time isconverted to Local Solar Time (LST) and the data arelinearly interpolated to hourly values according to LST.

It is well known that there is a large spatial variationin the ratio of the May–September precipitation to theannual precipitation over China and it would be inter-esting to include other months in the analysis. However,most part of China is affected by summer monsoon andone of the aims of this study is to identify features in thediurnal variations associated with the march of the sum-mer monsoon, and the choice of the season studied isjustified. In addition, the data loss in cold season (Octo-ber–April) is crucial for the stations in the northern part

due to the occurrence of snow. To balance the number ofstations included and the length of the season analysed,we chose to study the diurnal variations of precipitationduring the warm season from May to September.

The diurnal variations of precipitation are mainly char-acterised by the frequency of precipitation occurrenceand the precipitation amount. The frequency of occur-rence is defined as the number of hours reported withprecipitation (≥0.1 mm/h) divided by the total observa-tion hours (Equations (1) and (2)); the total precipitationamount is the sum of the hourly precipitation amounts(Equation (3)). To facilitate the comparisons among allthe stations, the hourly occurrence and amount data arenormalized by dividing them by the 24-h means (Equa-tions (4) and (5)).

fn =

D∑d=1

cdn

D(1)



cdn ={

0, pdn < 0.1 mm/h1, pdn ≥ 0.1 mm/h

(2)

an =D∑

d=1

pdn (3)

(NF )n = 24fn

24∑n=1

fn

(4)

(NA)n = 24an

24∑n=1

an

(5)

where n is the LST expressed in hours, D is thenumber of observation days, fn is the frequency ofprecipitation occurrence, cdn is a counter; an is thetotal precipitation amount, pdn is the hourly precipitationamount, NF is the normalized precipitation frequency,and NA is the normalized precipitation amount. FollowingWallace (1975), Dai (2001) and Angelis et al. (2004), thehourly normalized frequency and amount data at eachstation were subjected to a harmonic analysis to obtainamplitudes and phases of the diurnal and semidiurnalcycles as follows:

y(n) = y + S1(n) + S2(n) + residual (6)

Sk(n) = Ck × cos(2kπn

24− σk) (7)

ek = 0.5Ck2

yvar(8)

Fk = 0.5C2k × (24 − 2 − 1)

(yvar − C2k ) × 2

(9)

where k = 1, 2 (for diurnal and semidiurnal cycles),y is the daily mean value, and S1 and S2 are thefirst and second harmonics (the diurnal and semidiurnalharmonics), respectively (Equation (6)). The residual inEquation (6) contains the higher-order harmonics of thedaily variations. Ck is the amplitude and σk is the phasefor the kth harmonic (Equation (7)); yvar is defined as thevariance of the 24-h frequency or amount values; and ek

is the percentage of the total daily variance explained bythe kth harmonic, which is used to reflect how muchof the diurnal variations can be explained by theseharmonics (Equation (8)). The F -tests, by comparingFk defined in Equation (9) with the inverse of the F

distribution function with 2 and 21 degrees of freedomat significance level 0.90, were used to determine thesignificance of the harmonics (Benedetto, 1996). Theharmonic analysis allows the diurnal and semidiurnalcycles at all stations to be quantified and compared.

The diurnal variations described by the first two har-monics, i.e. the diurnal and semidiurnal cycles, are thenstudied. To reflect the concentration degree of precipita-tion distribution in a day, the magnitude of variationsare defined as the maximum simulated frequency by

the first two harmonics (y) minus the minimum y foreach station. The LST when maximum y appears can beclassified into two groups: the maximum that appearsduring the daytime (0800–2000 LST) and the maxi-mum that appears during the nocturnal time (2000–0800LST). Before the harmonic analysis is applied, thehourly data are smoothed by a three-point filer withweights of 0.25, 0.5, and 0.25 for each month andeach station to reduce the influence of random varia-tions when seasonal changes of diurnal variations areanalysed.

3. Results and discussions

A comparison between diurnal variations of precipi-tation occurrence frequency and precipitation amountshows that they are quite similar. However, some minordifferences do exist and can be described as follows:(1) the magnitudes of the diurnal variations of amount areslightly higher for most stations; (2) the afternoon max-ima for diurnal variations of amount are usually higherdue to intensive convective precipitation; and (3) thecurves of diurnal variations for amount are less smooth,implying that there is more randomness in the diurnalvariations of amount. These differences are illustrated inFigure 2, which gives a few examples. In the remainderof this paper, only the diurnal variations of precipitationoccurrence frequency are discussed.

3.1. The significance of the diurnal and semidiurnalcycles

For the warm season, the percentage of the variancesexplained by the diurnal cycle (S1) and semidiurnal cycle(S2) are shown in Table I and Figure 3. The F -tests (90%significance level) show that all of them can be wellexplained by the first two harmonics (no harmonic exceptS1 and S2 is significant); this implies that the higherharmonics (k ≥ 3) are not significant contributors to thevariances. The explained variances for the first two har-monics range from 72 to 96% (e1 + e2, Table I). Fifteenstations belong to the diurnal cycle-predominating group(only the first harmonic S1 is significant); another ninestations belong to the semidiurnal cycle-predominatinggroup (only the second harmonic S2 is significant); andthe rest can be classified into the diurnal and semidiurnalcycle comparable group (S1 and S2 are both significant).The spatial distributions for these classifications can beseen in Figure 1.

3.2. The amplitudes of the diurnal and semidiurnalcycles

Figure 4(a) and (b) shows the amplitudes of the diurnaland semidiurnal cycles, respectively. The amplitude ofdiurnal cycle ranges from about 2 to 80% of the dailymean. The stations in the Tibetan Plateau and south-westChina (the region between the east of the Tibetan Plateauand the west of 105–110 °E, including the Sichuan Basinand its southward area) show a very significant diurnal


S. YIN ET AL.

Figure 2. Diurnal variations of normalized precipitation amount (solid curve) and normalized precipitation occurrence frequency (dashed curve)at six example stations.

cycle with amplitudes over 40% of the daily means. Thestation Xizang Lasa (No. 59 in Table I and Figure 1) inthe Plateau exhibited strongest diurnal cycle signal withamplitude equalling 84% of the daily means. StationsXinjiang Wulumuqi (No. 57 in Table I and Figure 1),Xinjiang Yining (No. 58 in Table I and Figure 1) innorth-west China (the region to the west of 105–110 °Eand to the north of the Tibetan Plateau and the SichuanBasin) and the stations in the south-east (the regionto the east of 105–110 °E and to the south of middleand lower reaches of the Yangtze River) also showa significant diurnal cycle with amplitude over 20%of the daily mean. The amplitudes of the stations inthe middle (including the North China Plain and themiddle and lower reaches of the Yangtze River) andnorth-east China (the region to the east of 105–110 °Eand to the north of North China Plain) are less than20%, except for the station Liaoning Dalian (No. 30in Table I and Figure 1) which is near the Bohai Bay(Figure 4(a)).

The amplitudes of semidiurnal cycles range from 1 to30% of the daily mean, and are considerably weaker thanthe diurnal cycle for most stations. In the north-east, theamplitudes of semidiurnal cycles are comparable to thoseof the diurnal cycles for many stations. In the south-east,the stations also show significant semidiurnal cycles withamplitudes over 10% of the daily means. The stations inthe south-west and north-west have smaller amplitudesof semidiurnal cycles (Figure 4(b)).

3.3. The phase of the diurnal and semidiurnal cycles

The phase of the S1 can be expressed by the LSTwhen the diurnal harmonic peaks. Figure 5(a) showsthat at stations in north-west China and the TibetanPlateau the diurnal cycles of mean frequency tend topeak in the evening and early morning (0800–0200LST), whereas at stations in south-west China andthe Sichuan Basin the diurnal cycle tend to manifestmorning peaks (0200–0800 LST). At stations to the



Figure 3. Observed normalized mean frequency of precipitation occurrence (dots) together with the fitted diurnal (long-dashed curve) andsemidiurnal (short-dashed curve) harmonics at six stations averaged over 1954–2001. The solid curve is the sum of the two harmonics. Theexplain-var is the sum of the percentage of variances explained by the first two harmonics. The two stations in the top belong to the diurnalcycle-predominating group; the two in the middle belong to the semidiurnal cycle-predominating group; and the two at the bottom belong to the

diurnal and semidiurnal cycle comparable group.

east of 110 °E, the S1 peaks in the daytime (0800–2000LST) in the northern and southern parts, whereas itpeaks in the nocturnal time (2000–0800 LST) in themiddle and lower reaches of the Yangtze River and theNorth China Plain.

The phase of the S2 can be expressed by the LSTwhen the first peak appears for the semidiurnal harmonic,because the second peak of the semidiurnal harmonicappears 12 h later. Figure 5(b) shows that most stationspeak in the period 0300–0600 LST, except severalstations in the north-west (which peak in the period0600–0900 LST) and south-west (which peak in theperiod 0000–0300 LST).

3.4. Classification of the normalized diurnal variations

As can be seen in Figure 6, the maximum frequency ofprecipitation occurrence appears in the nocturnal time(2000–0800 LST) for the stations in the Tibetan Plateau,in the south-west and in eastern part of north-westChina, and the magnitude is relatively large, especiallyfor stations over the Tibetan Plateau. In the stations tothe east of about 110 °E, the precipitation occurrencefrequency peaks in the daytime (0800–2000 LST) inthe northern and southern parts, whereas it peaks in thenocturnal time (2000–0800 LST) in the middle and lowerreaches of the Yangtze River and the North China Plain. It


S. YIN ET AL.

(a)

(b)

Figure 4. (a) Amplitudes of the diurnal harmonic (S1) of mean precipitation occurrence frequency for the warm season averaged over 1954–2001.The range is from 0.02 to 0.84. (b) Amplitudes of the semidiurnal harmonic (S2) of mean precipitation occurrence frequency for the warm season

averaged over 1954–2001. The range is from 0.01 to 0.30.

also can be seen that the magnitude of diurnal variationsfirst decreases and then increases from south towardsnorth for the eastern part of China (Figure 6).

The general spatial pattern of diurnal variations overChina is by and large in agreement with previousstudies (Zhao et al., 2005; Yu et al., 2007), althoughour result is not as detailed as theirs, owing to theuneven distribution of the stations. Furthermore, ourresults show that the occurrence frequency peaks in the

early morning (about 0500 LST) for stations LiaoningDalian and Liaoning Dandong (No. 31 in Table I andFigure 1), located close to the Bohai Bay (Figure 6).Zhang and Lin (1985) reported that stations over thecoastal and small-island areas, including Liaoning Dalian,showed morning maxima in precipitation amount. Amidnight to early morning maximum in the frequencyof precipitation occurrence in the coastal areas wasalso reported in other places (Oki and Musiake, 1994;



(a)

(b)

Figure 5. (a) Phases of the diurnal harmonic (S1) of mean precipitation occurrence frequency for the warm season averaged over 1954–2001.The LST when the diurnal harmonic peaks. (b) Phases of the semidiurnal harmonic (S2) of mean precipitation occurrence frequency for the

warm season averaged over 1954–2001. The LST when the first peak appears for the semidiurnal harmonic.

Dai, 2001; Ohsawa et al., 2001). Morning maxima forstations Liaoning Dalian and Liaoning Dandong maybe mainly attributed to diurnal land–sea breezes causedby the land–ocean thermal contrast. During the summerafternoons and evenings, air can rise over the land andsubsidence can occur over the ocean as a result of thewarmer air over the land than the nearby ocean (see,for example, Borne et al., 1998). From midnight to early

morning, the land–ocean circulation reverses because theair over the ocean becomes warmer than that over theland (Dai, 2001). Besides, the radiative cooling at cloudtops in the night destabilizes the boundary layer andpromotes convection over the ocean. Thus, early morningprecipitation maxima exist over the ocean areas. Due tothe influence of the nearby ocean, some stations in thecoastal regions also show morning maxima.


S. YIN ET AL.

Figure 6. The magnitude of the diurnal variations and the location of the LST when the diurnal variations peak as indicated by the first twoharmonics. The black circles are stations with maximum precipitation occurrence frequency appearing in the nocturnal time (2000-0800 LST)and the white circles are in the daytime (0800-2000 LST). The magnitude was graded into four levels. These data are for warm season averaged

over 1954–2001.

The nocturnal precipitation maxima for stations overthe Tibetan Plateau and its surrounding areas are simi-lar to that reported by Ye and Gao (1979) and Liu et al.(2002). The significant diurnal variations in vertical circu-lation (stronger in the daytime and weaker at night) couldbe the most important contributor to the nocturnal max-ima. As is known, the mass of the atmospheric columnover the Tibetan Plateau is much lighter than that of otherregions, which results in significant circulation variationover the Tibetan Plateau caused by the diurnal change ofsolar radiation heating (Liu et al., 2006). Kuo and Qian(1981) showed that when the solar radiation graduallyincreases in the daytime, the positive vorticity of lower-level atmosphere and the negative vorticity of high-levelatmosphere over the Tibetan Plateau also increase.

All of these factors lead to significant circulation andweather diurnal changes over the Tibetan Plateau andits surrounding areas. According to Ye and Gao (1979),the convective activities over the plateau have significantdiurnal variations, with a peak from 1600 LST to evening,and is relatively weak in the early morning. Their analysisof cumulonimbus frequency also showed that convectiveclouds over the Tibetan Plateau are strengthened during1400–2000 LST and weakened during 0200–0800 LST.Reverse diurnal variations of convective clouds exist inthe surrounding areas of the plateau (Ye and Gao, 1979).These may explain the typical midnight maxima (around0200 LST) and noon minima (around 1200–1400 LST)over the Tibetan Plateau, and the early morning max-ima (around 0400–0600 LST) in its surrounding areas,including stations in eastern part of north-west China

[Ningxia Yinchuan (No. 38 in Table I and Figure 1),Gansu Lanzhou (No. 5 in Table I and Figure 1), ShaanxiHanzhong (No. 40 in Table I and Figure 1), Gansu Tian-shui (No. 7 in Table I and Figure 1), and Shaanxi Yulin(No. 43 in Table I and Figure 1)] and the stations in thewestern part of south-west China.

Besides the large-scale (the whole plateau-scale) moun-tain–valley circulation, the meso-scale (e.g. TianshanMountain, Qilian Mountain) and small-scale moun-tain–valley wind could abet the occurrence of nocturnalrain (Kousky, 1980; Astling, 1984; Ohsawa et al., 2001;Sato and Kimura, 2005). The more effective heating inthe valley produces a pressure drop in the valley relativeto the surroundings, which in turn produces an upvalleywind, which begins in late mornings (usually 1–4 h aftersunrise). The divergence-driven subsidence at the foot ofthe mountains restrains the development of cloud and pre-cipitation in the valley during the daytime. The radiativecooling of the hillslope cools the surface air after sun-set. The subsidence of cool air due to gravity along theslope (downvalley wind) creates convergence and risingmotion in the middle of the valley, which is favourablefor the generation of precipitation.

The diurnal variations of winds due to local moun-tain–valley topography in the north-west area of Chinawere addressed by Bai and Xu (1998). Taking Wulumuqiin Xinjiang as an example, the frequency of south andsouth-west wind is more than 51% in the nocturnal time,while north-west wind predominates in the daytime. Dengand Zhang (1984) attributed the nocturnal precipitation inWulumuqi to the convergence of two cold flows: one is



from the Gobi and desert area north-west of Wulumuqi;the other is from the northern slope of Tianshan Moun-tain. However, as we can see in Figure 6, there are stillsome stations located in north-west China showing day-time precipitation maxima (for example, Shaanxi Xi’an(No. 41 in Table I and Figure 1), Gansu Pingliang (No.6 in Table I and Figure 1), and Ningxia Guyuan (No. 37in Table I and Figure 1)), which might be influenced byinteraction between the local topography and dominatingwind conditions (e.g. Johansson and Chen, 2003).

The nocturnal precipitation in the Sichuan Basin andthe south-west part of China is well established and wasdiscussed by Lu (1942) and Li et al. (1990). Besidesthe difference of diurnal variations of vertical circu-lation between the Tibetan Plateau and its surround-ing areas, and the meso-scale and small-scale moun-tain–valley wind in the mountainous south-west China,the source of this nocturnal rain could also be high humid-ity and convective clouds that originate in the south-west. The Sichuan Basin in summer is characterised by

(a)

(b)

Figure 7. Same as Figure 6 but (a) for May (b) for June (c) for July (d) for August (e) for September.


S. YIN ET AL.

(c)

(d)

Figure 7. (Continued).

moist air and cloudy sky. The afternoon convection isrelatively difficult to develop due to the shading effectof clouds and in the evening the temperature of thecloud bottoms increases because of longwave radiationfrom the earth surface, and the temperature of cloud topsdecreases because of longwave cooling. The temperaturedifferences between the bottoms and tops of the cloudscreate unstable stratification, and the rising motions ofthe warm and humid air can trigger the occurrenceof rain.

Large synoptic systems form easily over the TibetanPlateau and its surrounding areas because of the topogra-phy effect of the plateau. About 67% of convective cloudsoccur in the south-west area of 25–35°N, 100–105 °E(Xu, 2001). These convective clouds show evident diur-nal variations: they start in the afternoon and earlyevening and then propagate eastward and disappear dur-ing the following afternoon. All these processes may playa role in the diurnal variations of precipitation in theSichuan Basin and the south-west part of China.



(e)

Figure 7. (Continued).

While daytime precipitation (peak in the afternoon) inthe eastern part of China can be attributed to the moistconvection connected with solar heating, the morningpeaks for stations in the middle and lower reaches ofthe Yangtze River and the North China Plain and themagnitude changing pattern in eastern part seem to beconnected with the advance and retreat of the summermonsoon. The next subsection deals specifically with therole of the monsoon.

3.5. Seasonal change of the diurnal variations

Hann (1901) suggested that there is a shift towardsmorning maxima in precipitation occurrence during thewet season over parts of the monsoon areas of the tropics.Many other previous studies also showed that there isa tendency towards predominant morning peaks duringthe warm rainy season of the Asian summer monsoonregions (Ramage, 1952; Fujibe, 1988; Oki and Musiake,1994; Jung et al., 2001; Barros and Lang, 2003; Bhattand Nakamura, 2006). As is well known, the climatein China is strongly influenced by the monsoon (Dingand Murakami, 1994). To detect whether there is alsoa morning peak dominating in the warm season of themonsoon areas in China, we analysed the monthly changeof the diurnal variations.

When it refers to monthly characteristics of diurnalvariations, the percentage of variances explained by firsttwo harmonics ranges from 13 to 96%. The F -tests (90%significance level) show that 96% of the stations’ monthlydata can be well explained by these two harmonics (only11 out of 310 could not).

For most of the stations to the east of about 110 °E,there are two peaks (early morning and afternoon) of

precipitation frequency for each month, and the morningpeak tends to strengthen and weaken along with the EastAsian monsoon’s advance and retreat, except for the twostations near the Bohai Bay (with negligible changeswith season) (Figure 7). The seasonal advance and retreatof the summer monsoonal airflow and monsoon rainbelt in East Asia behave in a stepwise manner (Ding,2004). When the East Asian summer monsoon advancesnorthward, it undergoes three standing stages (SouthChina and northern South China Sea from mid-May toearly June; 25–30°N from mid-June to mid-July; and40–45°N during the last 10 days of July to mid-August),and two stages of abrupt northward shifts (the first10 days of June and around mid-July). In early or mid-August the rainy season of North China comes to end,with the major monsoon rain belt disappearing. From theend of August to early September the monsoon rain beltmoves back to South China again.

The monthly change of diurnal variations connectedwith the migration of the monsoon is reflected in thespatial pattern of diurnal variations for the warm seasonin the eastern part of China (to the east of about 110 °E).Note that the warm season in this study is defined as Mayto September. The stations in the middle and the lowerreaches of the Yangtze River and the North China Plainare exposed to the monsoon rain for the longest time,which lead to larger peaks appearing in the early morningin this area and the magnitude first decreasing and thenincreasing from south towards north in the eastern partof China, for the whole warm season.

The thermodynamic profiles obtained from radiosondessoundings in central Nepal support the observed nocturnalmaxima in precipitation during the monsoon, with total


S. YIN ET AL.

Figure 8. The diurnal variations of the frequency of precipitation occurrence for eight stations in south-west China. The short-dashed curve isfor May; the long-dashed curve is for June; the solid curve is for July; the solid curve with stars is for August; and the solid curve with circles

is for September.

column moisture and instability maximized just beforeprecipitation peaks (Barros and Lang, 2003). The signifi-cant increase in moisture due to the south-easteren windsin the mature monsoon season around the southern partof the Himalayas seems to produce favourable conditions

for midnight/early morning rain. The downvalley wind atmidnight probably triggers moist convection. In addition,the radiative cooling at the cloud tops may enhance theconvection (Bhatt and Nakamura, 2006). The moist airmass in the eastern part of China mainly comes from



the South China Sea during the monsoon season (Ding,2004). In the Meiyu zone the air is very humid, with ahigh specific humidity belt observed at low atmospherelevels (Ding and Chan, 2005). The Meiyu-Baiu frontalzone associated with intense convective precipitation isnot characterised by strong convective instability but,rather, by nearly moist neutral stratification. The after-noon convection is relatively difficult to develop due tothe shading effect of clouds, whereas during nighttime theradiative cooling of the cloud tops helps in destabilizingthe warm, humid air to cause rain.

In comparison with the eastern part of China, theseasonal changes of the diurnal variations for thewestern part are relatively small, with nighttime precip-itation maxima remaining fairly constant from May toSeptember (Figure 7(a)–(e)). The magnitudes of diurnalvariations in July and August are smaller than in theother 3 months. It can be seen that the afternoon sub-peakbegins to rise in July and August for the stations in thesouth-west (Figure 8). For some stations, the afternoonpeaks are even a little higher than the early morning peaksin July and August. The afternoon maxima are most likelyattributed to the moist convection. More solar radiationheating in July and August induces stronger afternoonmoist convection, resulting in higher afternoon sub-peaksthan in the other months.

4. Conclusion and outlook

The spatial distribution and seasonal change of diurnalvariations of precipitation in the warm season over Chinawere analysed using 48-year hourly data from 62 raingauges during 1954–2001. During this period, 72–96%of the total diurnal variances are explained by the first twoharmonics describing the diurnal and semidiurnal cycles.Three groups with distinctive diurnal variations havebeen identified: 15 stations belonging to the first groupreport a predominating diurnal cycle; 9 stations in thesecond group show a predominating semidiurnal cycle;and the rest belong to a mixed diurnal and semidiurnalcycle group. The amplitude of the diurnal cycles rangesfrom about 2% to more than 80% of the daily mean,and that of semidiurnal cycles from 1 to 30% of thedaily mean. Nocturnal precipitation predominates forthe stations over the Tibetan Plateau area and south-west China, with the amplitude of the diurnal cyclesaveraging more than 40% of the daily mean. In theeastern part of north-west China the average amplitudeof the diurnal cycles occupies more than 20% of thedaily mean. For the stations to the east of about 110 °E,daytime precipitation predominates in the northern andsouthern parts of China, whereas nocturnal precipitationpredominates in the middle and lower reaches of theYangtze River and the North China Plain. The magnitudeof diurnal variations first decreases and then increasesfrom south towards north. The occurrence frequencypeaks in the early morning for two stations located closeto the Bohai Bay.

The spatial patterns of diurnal variations are gener-ally a result of the interaction of local topography andatmospheric processes, with convection as a key process.The significant diurnal variations in the vertical atmo-spheric circulation (stronger in the daytime and weakerat night), the meso-scale and small-scale mountain-valleywind, and nighttime cooling of cloud tops could be thepossible reasons for the nocturnal rain in the western partof China (to the west of about 110 °E). Daytime precip-itation for the eastern part of China (to the east of about110 °E) may be attributed to the moist convection con-nected with the solar heating. The morning peaks forstations in the middle and lower reaches of the YangtzeRiver and the North China Plain seem to be connectedwith the advancing and retreating of the East Asian mon-soon. The morning maxima for coastal stations may bemainly attributed to the diurnal circulation caused byland–ocean contrast.

The seasonal changes of the diurnal variations for thewestern part of China (to the west of about 110 °E)are relatively small, with nighttime precipitation max-ima remaining fairly constant from May to September.For most of the stations to the east of about 110 °E,there are two peaks for precipitation frequency for eachmonth, and the morning peaks tend to be strength-ened/weakened along with the East Asian monsoon’sadvancing/retreating except for the two stations near theBohai Bay (having minor changes with season).

Since the diurnal variations of precipitation are deter-mined by large-scale, dynamic backgrounds and localconditions (Jia et al., 2006; Wetterhall et al., 2006), fur-ther analysis should relate the identified diurnal variationsto large-scale synoptic conditions, which may lead to abetter understanding of mechanisms of precipitation diur-nal variations.

Acknowledgements

We thank the anonymous reviewers for their contribu-tions and helpful detailed comments. This research issupported by grants from the Swedish Foundation forInternational Cooperation in Research and High Educa-tion, the Swedish Agency for International DevelopmentCooperation, and the National Key Basic Research Spe-cial Foundation Project (2007CB407203).

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Diurnal variations of precipitation during the warm season over China

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