Clim Dyn (****) **:**** ****

DOI **.****/s*****-*08-0478-3

Diurnal variation of surface wind over central eastern China

Rucong Yu Jian Li Haoming Chen

Received: 27 June 2008 / Accepted: 6 October 2008 / Published online: 25 October 2008

Springer-Verlag 2008

Abstract Hourly wind observations from 452 meteoro- cycle of the tropospheric low-level wind. The wind speed

logical stations are used to document the diurnal cycle of over these stations is highest in pre-dawn and lowest in the

the surface wind over the central eastern China afternoon. The wind anomaly rotates clockwise from late

(100 122 E, 20 42.5 N). Both the surface wind speed night to late afternoon, and shows signi cant seasonal

and the wind direction show large diurnal variation with variation as in uenced by the annual cycle of the monsoon

pronounced topographic effects. At most stations, the sur- system. The contribution of the diurnal surface wind to the

face wind speed reaches the maximum in the afternoon and diurnal feature of precipitation is brie y discussed.

the minimum in early-morning. This diurnal phase shows

Keywords Surface wind Diurnal cycle

small seasonal variation, whereas the diurnal amplitude

varies signi cantly in different seasons. The diurnal Seasonal variation

amplitude of the surface wind speed reaches maximum in

spring over the northern and southwestern China and in

summer over the southern China. The diurnal cycle of the 1 Introduction

wind direction is more complicated. Over the coastal

(mountain) regions, the diurnal wind direction is greatly Observational and modeling studies have demonstrated

in uenced by the land sea (mountain valley) breezes with that diurnal processes forced by the daily cycle of incoming

large (small) seasonal variation. Over the northern plain solar radiation at the top-of-atmosphere occur in many

region, the wind direction exhibits small diurnal variation atmospheric quantities, including precipitation, winds,

but with remarkable seasonal rotation. The surface wind surface pressure, cloudiness, and radiation uxes (Dai and

over the stations located on the top of mountains shows Deser 1999; Dai and Wang 1999; Dai 2001; Krishnamurti

distinct diurnal variation, which represents the diurnal and Kishtawal 2000; Wang et al. 2004; Lin et al. 2000).

The inhomogeneous surface forces, related with complex

land sea and mountain valley distributions, result in the

R. Yu J. Li robust regional aspects of diurnal cycle in varied atmo-

LaSW, Chinese Academy of Meteorological Sciences,

spheric processes (Nesbitt and Zipser 2003; Yang and

China Meteorological Administration, Beijing, China

e-mail: abpj91@r.postjobfree.com; abpj91@r.postjobfree.com Slingo 2001).

Because of more synthetic integrated signals involved in

R. Yu

e-mail: abpj91@r.postjobfree.com precipitation processes and its close relation with human

being, previous studies involving diurnal cycle have

H. Chen

focused primarily on describing the diurnal variation of

LASG, Institute of Atmospheric Physics,

precipitation. The mainland of China, occupying the largest

Chinese Academy of Sciences, Beijing, China

e-mail: abpj91@r.postjobfree.com continental area over East Asia monsoon region, sur-

rounded by the complex land sea and characterized by

H. Chen

irregular mountain valley distributions, exhibits large

Graduate School of the Chinese Academy of Sciences,

diurnal variation with considerable regional features in

Beijing, China

123

1090 R. Yu et al.: Diurnal variation of surface wind over china

and minimum temperature is also used to present the

summer precipitation (Yu et al. 2007a). However, little is

amplitude of the diurnal variation of surface air

known about the forced local circulation corresponding to

temperature.

the regional differences of diurnal precipitation variation

over China. Diurnally forced land sea and terrain slope

differential heating can initiate varied local circulations,

which support organized cloud clusters that are character- 3 Diurnal variation of the surface wind speed

ized by a pronounced diurnal cycle (Williams and Houze

Figure 1a, b shows the spatial distribution of the time

1987). For example, over coastal regions, the development

when the maximum (minimum) surface wind speed

of convection could be dominated by the land sea breeze

occurs by arrows on a circular 24-h dial clock. The annual

(Saito et al. 2001), whereas over complex terrain regions,

surface wind speed exhibits a nearly uniform afternoon

the development of convection could be dominated by

maximum and an early-morning minimum. The diurnal

the mountain valley breeze (Reiter and Tang 1984). As the

wind speed peaks between 1300 and 1700 LST at 424

diurnal cycle in low-level convergence largely controls the

stations (93.8% out of 452 stations). And most (339)

diurnal timing in summer precipitation (Dai and Deser

stations show the lowest wind speed in the predawn and

1999), the studies on diurnal variation of surface wind are

early morning (0300 0700 LST). It is understandable that

potentially important to understand the diurnal behavior of

during daytime, as a response to the surface solar heating,

precipitation. Moreover, the diurnal variation of low-level

the downward vertical turbulent transport of momentum

circulation provides information on the physical processes

reaches its strongest in the afternoon. During night time,

acting above the surface, which should be represented

as a response to the nocturnal cooling in the boundary

accurately in weather and climate models. In this study, the

layer, the eddy viscosity is reduced and less momentum is

diurnal surface wind variation is investigated by using

transported to the lower level. The surface wind slows

the quality-controlled hourly surface wind records over the

down gradually due to the surface friction. The regional

central eastern China. We introduce the data in Sect. 2, and

averaged diurnal variation of the surface wind speed over

then present the results in Sects. 3, 4, 5. A summary and a

the central eastern China is plotted by a solid line with

brief discussion are given in the last section.

lled squares in Fig. 1c. The averaged wind speed

2 Data description

The quality-controlled hourly data of surface wind speed

and direction were obtained from the National Meteoro-

logical Information Center (NMIC) of China

Meteorological Administration (CMA). It contains records

from 683 stations in the national climatic reference net-

work and national weather surface network of China during

1991 2007. Hourly wind speed and direction were auto-

matically recorded by anemorumbograph and the data were

collected and quality-controlled by NMIC. The quality

control consists of two steps: an extreme check and a

consistency check. During the extreme check, any record

with invalid wind direction code or invalid wind speed

value (not within the range of 0 75 m/s) was rejected. For

the records passing the extreme check, the consistency

check was carried out. If the hourly wind speed exceeded

the maximum of the diurnal wind speed, the record was

labeled as suspicious data. Most of the inconsistencies of

Fig. 1 a The unit vectors denote the local solar time when the annual

wind data were caused by the instrumental fault.

mean diurnal wind speed peaks. The Yangtze River (south) and

In this study, data from 452 stations are analyzed. There Yellow River (north) are outlined by gray lines. The location of seven

are two criterions for the selection of stations. Firstly, they mountain stations are marked by circles. b is the same as (a) except

the unit vectors showing the local solar time (LST) of the minimum

should locate in central eastern China (100 122 E, 20

wind speed. c Diurnal variation of the annual mean wind speed

42.5 N). Secondly, they should have more than 900 days

averaged over central eastern China (100 122 E, 20 42.5 N) (line

without missing values or suspicious values during the with solid squares, left Y-axis) and that averaged over seven mountain

period of 1991 2007. Station-observed daily maximum stations (line with solid circles, right Y-axis)

123

R. Yu et al.: Diurnal variation of surface wind over china 1091

exhibits a diurnal cycle which peaks at 1500 LST and

reaches the minimum at 0500 LST, with the amplitude

about 1.2 m/s.

Carefully examining Fig. 1a and b, we nd that there are

several stations where the arrows point to the almost

reversed direction to that of most other stations. By iden-

tifying each of these stations, it s found that some of them

are located on the edge of the Sichuan Basin (103 108 E,

28 32 N) and the other seven stations are located at the

top of mountains (marked by open circles in Fig. 1a, b).

For the seven mountain stations, the averaged height of

them is 1,669.1 m and all of them are above 1,200 m. The

averaged diurnal cycle of surface wind speed for the

mountain stations is shown by a solid line with lled circles

in Fig. 1c. The reversed diurnal phase of surface wind

speed is found between the mountain stations and the rela-

tive low-altitude stations. At the mountain stations, the

maximum (minimum) wind speed occurs in pre-dawn Fig. 2 The unit vectors denote the month when the amplitude of the

(afternoon). This stark contrast shown in Fig. 1c is con- diurnal variation of surface wind reaches the annual maximum.Three

distinct regions are labeled

sistent with the previous study by Crawford and Hudson

(1973). Through analyzing 1 year of wind data from a

television tower, they stated that the speed at lower

(higher) levels is lowest near midnight (midday) and

highest in the afternoon (midnight). The onset and cessa-

tion of convective mixing in the boundary layer, which is

closely associated with the diurnal cycle of solar heating,

can partly explain the relation between the two lines in

Fig. 1c. The diurnal variation of wind at mountain stations

will be discussed in a separate section.

Seasonal variation in the diurnal phase of the surface

wind speed is small. In all seasons the averaged surface

wind speed peaks around 1400 1500 LST and reaches the

Fig. 3 The annual cycle of the amplitude (m/s) of the diurnal

minimum between late night and early morning (Figure variation of surface wind over three distinct regions, the northern

omitted). In contrast, the amplitude of diurnal variation of China (solid line with open squares), the southern China (dash line

with triangles) and the southwestern China (gray line, right Y-axis)

the surface wind speed varies signi cantly from month to

month. Arrows in Fig. 2 denote the month in which the

different mechanisms governing the annual cycle of wind-

largest diurnal amplitude occurs. In different regions, the

speed diurnal variation. As shown in Fig. 4, both the

amplitude peaks in different months. Over the northern

diurnal temperature difference and the daily maximum

China, the amplitude in most of the stations reaches the

wind speed peak in April (March) over North China (the

maximum in April. In the southern China, a majority of

southwestern plateau region). Over these two regions, the

vectors point to July. Over the plateau area of the south-

diurnal temperature variation and the background wind

western China, the largest amplitude appears in early

speed might regulate the amplitude of the diurnal variation

spring. Figure 3 provides more details on the seasonal

of the wind speed. In contrast, when the wind speed over

variation of the diurnal amplitude in these three typical

the southern China exhibits the maximum diurnal variation

regions. During the rst half of a year, the seasonal vari-

in July, neither the diurnal temperature range nor the wind

ation of the diurnal amplitude in the southwestern (gray

speed reaches the annal peak. This indicates that there is

line) and the northern China (solid line with open squares)

another factor which plays a more important role. In July,

show similar pattern, with the annual phase in the south-

the temperature contrast between the southern contiguous

western China leading by 1 month. In summer, the

China and the adjacent sea reaches a maximum. The warm

amplitude in both the northern China and the southwestern

land and cool ocean lead to strong sea breezes, which

China reaches a valley, while the amplitude in the southern

might contribute to the largest diurnal wind speed ampli-

China (solid line with triangles) presents peak value. The

tude in a year.

remarkable regional features indicate that there are

123

1092 R. Yu et al.: Diurnal variation of surface wind over china

For the region west of 114 E, a large portion of it has

elevation higher than 500 m. Moreover, the land surface

conditions are complex, which can partly explain the

spatial inhomogeneities of the surface wind directions

shown in Fig. 5. Due to different surface heating and

cooling over mountains and valleys, the surface wind

always presents as upslope wind toward mountain at 1500

LST and downslope wind toward valley at 0500 LST. The

mountain valley breezes driven by orography thermal

contrasts are shown in Fig. 6. The diurnal variation of

wind direction is large and is subject to the local topog-

raphy. The local thermal gradient originating from

Fig. 4 a The unit vectors denote the month when the amplitude of the mountain valley distributions determines the diurnal var-

diurnal variation of surface air temperature reaches the annual

iation of the wind directions. In contrast, the in uence of

maximum. b The unit vectors denote the month when the surface

the seasonally varying large-scale circulations is small. No

wind speed reaches the annual maximum. Three distinct regions are

labeled remarkable systemic seasonal changes are found in the

mountain valley breezes.

4 Diurnal variation of the surface wind direction Over the northern plain region, the wind direction dif-

ference between 0500 and 1500 LST is small and the

Compared with the wind speed, the diurnal variation of the dominant wind direction changes seasonally. To exhibit the

surface wind direction plays a more important role in wind direction speci cally, Fig. 7 shows the wind vectors

modulating synoptic events and local climate. Figure 5a d by scatters. In spring, most stations show southerly wind at

show the spatial distribution of the surface wind at 1500 both of 0500 and 1500 LST. During summer, the south-

LST (red arrows) and 0500 LST (blue arrows) in each easterly wind dominates the northern plain region.

season. These two times are selected because they can Especially in summer afternoon, the southeasterly blows

signify two extreme phases of the diurnal thermal condi- over 88.5% stations on the plain. The wind turns into

tions and the maximum and minimum wind speed is northerly in autumn, with the northerly wind observed at

detected at these two times respectively. both 0500 and 1500 LST at more than 70% stations. During

The diurnal variation of the surface wind direction is winter, the wind blows toward southeast, which is reversed

signi cantly in uenced by the land sea and mountain with the wind in summer. As shown in Fig. 7, the wind

valley thermal contrasts. As seen in Fig. 5, the diurnal direction rotates anticlockwise along with the shifts of

variation of the surface wind direction presents remarkable seasons and large wind direction changes exist between

regional features and can be grouped into three categories: summer and autumn and between winter and spring. The

the southeastern coastal region, the western mountain seasonal rotation of the prevailing wind implies that the

region and the northern plain region. Firstly, the surface surface wind over the northern plain area is signi cantly

wind at the southeastern coastal stations is examined. In in uenced by the large-scale tropospheric wind, which

summer (JJA), southerly wind is found at 1500 LST. The also changes seasonally as a member of the monsoon

wind directions vary along the coast, nearly keeping per- system.

pendicular to the coastal line and pointing to the inland

region. This strong sea breeze is determined mainly by the

thermal contrast between warm land and cool ocean in the 5 Diurnal variation of the wind at mountain stations

summer afternoon. At 0500 LST, the wind speed is con-

siderably reduced and the northerly wind is found in some In Fig. 1, different diurnal cycle of surface wind has been

coastal stations. During autumn (SON) and winter (DJF), found between mountain stations and low-altitude stations.

surface winds at both of the two times show strong Located at high elevation, mountain stations can grasp

northeasterly component. The angle between blue and red certain features of the lower troposphere, which differ with

arrows separates the land and sea breezes. In spring the condition in the bottom of the boundary layer. Thus, the

(MAM), there are southeasterly winds in the afternoon and diurnal wind variation at seven mountain stations is spe-

northeasterly winds in the early morning. From cold season ci cally studied. The diurnal oscillation of surface wind

to summer, the sea breeze presents signi cant seasonal vectors at mountain stations is shown in Fig. 8a. At the two

change and rotates clockwise. The seasonal change of the northern stations and the two stations located in the lower

surface wind direction also shows signature of the seasonal reach of the Yangtze river, the dominant wind is westerly

variation of large-scale circulation over this region. wind at all the time of a day. On the top of the two southern

123

R. Yu et al.: Diurnal variation of surface wind over china 1093

Fig. 5 Spring (a), summer (b),

autumn (c) and winter (d)

surface wind at 0500 LST (blue

vectors) and 1500 LST (red

vectors). The shading denotes

terrain heights (in m). Three key

regions (the southeastern coastal

region, the western mountain

region, and the northern plain

region) are approximately

marked by black lines in (a)

mountains, southerly wind is found at all the eight local stations show southward anomalies in the afternoon (1500

times. At the mountain station in the middle reach of the LST) and the vectors turn to pointing southwest at 1800

Yangtze river, southwesterly wind is found between mid- LST. In the evening (2100 LST), anomalous southeasterly

night and early morning and southeasterly wind dominates wind appears at mountain stations. Similar clockwise

the rest of a day. Although there are different diurnal rotation can be found in the 850-hPa wind eld in reana-

aspects among the seven stations, a common feature is that lysis data (ERA40 and JRA25) (Figure omitted). This

the wind direction rotates clockwise in the night time. After similarity gives us con dence that mountain stations wind

subtracting daily mean surface wind, Fig. 8b shows the data can be used to analyze the diurnal variation of the low-

anomalous winds at 3-h intervals. The anomalous wind level jet.

vectors exhibit clockwise rotation diurnally. Generally, in Figure 9 shows the seasonal variation of the anomalous

the late night (0300 LST), northern (southern) mountain wind at 0300 LST, when the wind speed at the top of

stations show southeastward (northeastward) anomalous mountains is strong. In autumn, except the two stations in

wind. In the early morning (0600 LST), most stations South China exhibit anomalous easterly wind, the other

exhibit northwesterly anomalous wind. The southern ve stations show anomalous westerly wind. During winter,

123

1094 R. Yu et al.: Diurnal variation of surface wind over china

Fig. 6 Same as Fig. 5 except

for a selected western mountain

region

anomalous wind turns into northwesterly and it becomes from 452 stations during 1991 2007. The results reveal

westerly in spring. In summer, all the seven stations show some interesting spatio-temporal features. The major con-

strong southwesterly anomalies and the late-night wind clusions are summarized below.

speed reaches the maximum in a year. From winter to

1. The surface wind speed averaged over the central

summer, the wind anomalies rotate anticlockwise, which

eastern China exhibits a notable diurnal cycle, with the

presents the signi cant in uences from the large-scale

highest (lowest) wind speed occurring in the afternoon

tropospheric circulation.

(early dawn).

2. The diurnal amplitude of the surface wind speed varies

seasonally. The wind speed amplitude over the south-

6 Summary and discussion

western (northern) China peaks in March (April). The

surface wind over the southern China reaches the

The diurnal surface wind variation over the central eastern

maximum diurnal amplitude in July.

China is investigated using hourly surface observations

123

R. Yu et al.: Diurnal variation of surface wind over china 1095

Fig. 7 Scatter plot of zonal

wind (X-axis) versus meridional

wind (Y-axis) (m/s) over the

northern plain region of China

(114 122 E, 32 40 N).

Surface wind at 0500 LST

(1500 LST) is presented by blue

circles (red crosses). a d are

for spring, summer, autumn and

winter, respectively

3. The diurnal variation of the surface wind direction is The results of this study can help us understand the

signi cantly modulated by the topography. Generally, previously revealed unique diurnal features of precipitation

at stations around coastal (mountain) regions, the winds over eastern China (Yu et al. 2007a). The afternoon pre-

blow toward inland areas (mountains) in the afternoon vailing sea breeze, against the mountains in the southern

and blow toward sea (valleys) in the early dawn. China, results in stronger low-level convergence and more

4. The surface wind over the northern plain region and water vapor which contribute to the late-afternoon rainfall

the southeastern coastal region rotates seasonally. maximum. The upslope wind in the afternoon and down-

However, the surface wind over the western mountain slope wind in late-night can explain the afternoon rainfall

region exhibits little seasonal change in direction, maximum over mountains and the night to early morning

indicating that the local thermal contrast between rainfall maximum in the valley areas. An interesting point

mountain and valley plays the most important role in of this study is that the wind data of the mountain stations

determining the wind direction. are helpful supplements to analyze the diurnal cycle of

5. Stations at the top of mountains show distinct diurnal low-level winds. The low-level jet (LLJ) could play an

features. The wind speed at those stations reaches the active role in modulating the diurnal cycle of summer time

maximum in pre-dawn and gets the minimum in the rainfall by supplying warm moist air (Higgins et al. 1997),

afternoon. The wind direction exhibits clockwise but the observational data of low-level wind are rather

rotation, similar to the diurnal variation of the limited and the 6 h reanalysis data are too coarse to resolve

troposphere low-level wind. the diurnal variation. The aforementioned diurnal speed

6. The wind at mountain stations show signi cant variation and the clockwise veering of the anomalous wind

seasonal change. The maximum southwesterly wind direction offer evidence that the surface winds observed at

tends to occur in summer and the wind anomalies at mountain stations can be used to monitor the low-level

0300 LST rotate anticlockwise from winter to summer. tropospheric circulation. As shown in Figs. 1c and 8, the

123

1096 R. Yu et al.: Diurnal variation of surface wind over china

Fig. 9 Seasonal variation of surface wind anomalies at 0300 LST at

mountain stations. Wind anomalies at different seasons are shown by

different colors. The locations of the Yellow River and the Yangtze

River are drawn as black lines

The results obtained in this study reveal the complexity

in the diurnal variation of surface ow over central eastern

China, and provide an observational basis to validate and

improve model physics, such as land surface uxes and

boundary layer parameterizations. Considering the diurnal

variation of both surface circulation and precipitation is a

useful and necessary step toward a better understanding of

the climate system and would help to comprehensively

evaluate numerical models. Hopefully the analyses pre-

sented herein, as well as previous studies on diurnal cycle

of precipitation, have elucidated issues that will help to

improve the model performance, and ultimately to get

Fig. 8 Diurnal oscillation of annual mean surface wind (a) and its

better forecasts and simulations of weather and climate

anomalies (b) at mountain stations at 3-h intervals. Winds at various

times of the day are shown by color vectors. The locations of the over East Asia.

Yellow River and the Yangtze River are drawn as black lines

Acknowledgments This work was jointly supported by the National

low-level wind, as represented by mountain data, peaks in Natural Science Foundation of China under grant No. 40625014 and

pre-dawn (0300 LST) with northeastward anomalous over 40705025 and the Major State Basic Research Development Program

the south of Yangtze River, indicating a nocturnal of China (973 Program) under grant No. 2004CB418304.

enhancement of southwesterly LLJ. The accelerating LLJ

can transport warm and moist air at night and induce the

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