HYDROLOGICAL PROCESSES
Hydrol. Process. 21, 1997– 2008 (2007)
Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6706

Surface flux and atmospheric boundary layer observations
from the LAPS project over the middle stream of the HuaiheRiver basin in China
Hiroki Tanaka,1,2,3 * Tetsuya Hiyama,1 Koh Yamamoto,3 Hatsuki Fujinami,1 Taro Shinoda,1Atsushi Higuchi,4 Satoshi Endo,3 Shoichiro Ikeda,5 Weijing Li6 and Kenji Nakamura1
1 Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya 464-8601, Japan
2 Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
3 Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan
4 Center for Environmental Remote Sensing, Chiba University, Chiba 263-8522, Japan
5 Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
6 National Climate Center, Chinese Meteorological Administration, Beijing 100081, China

Abstract:
Observations of the surface layer and the atmospheric boundary layer (ABL) were collected as part of the Lower Atmosphereand Precipitation Study (LAPS), which investigated the relationship between the surface conditions and the ABL processes.
The LAPS was part of the Core Research for Evolutional Science and Technology (CREST) program, under the auspicesof the Japan Science and Technology Agency (JST). Observations began in August 2003 over a flat surface region in midlatitude China at 32Ð55 ° N, 116Ð78 ° E. Observations before, during, and after the Meiyu season in China provided data forsurface conditions varying from relatively dry to moist. Preliminary analysis of the surface and the ABL observations shows
relationships between the surface fluxes and the ABL structure. ABL depth was enhanced by sensible heat flux. Fluctuationsin the ABL depth corresponded to plume-like wind structures within the ABL. Day-to-day variability in ABL depth wascontrolled mainly by buoyancy flux over the surface during dry periods. It was also affected by vertical motion at the top of
the ABL, especially during wet periods. Copyright  2007 John Wiley & Sons, Ltd.
KEY WORDS

surface heat flux; atmospheric boundary layer; wind profiler radar; Meiyu in China

Received 29 June 2005; Accepted 1 February 2006

INTRODUCTION
Interactions between the surface and the atmosphericboundary layer (ABL) at various temporal and spatial
scales are an important research topic in air pollution
studies, hydrology, meteorology, and climatology. Surface conditions, including soil moisture and vegetation
cover, affect the ABL structure, cloudiness, and precipitation (Wai and Smith, 1998). Simultaneous observations ofsurface and ABL processes under various surface conditions are required to understand the interactions involved.
Wind profiler radar (WPR) recorded ABL measurements
concurrently with flux measurements above the surface
in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) (Sellers
et al., 1992) in 1989. Wai and Smith (1998) showedthat soil moisture affected the ABL development. Furthermore, biophysically induced ABL forcing, associatedwith a gradient in organized vegetation cover, can sustain mesoscale circulations within a ¾16 km2 domain.
Angevine et al. (1998) reported the preliminary resultsof the simultaneous observations of WPR and surface* Correspondence to: Hiroki Tanaka, Graduate School of Environmental
Studies, Nagoya University, Nagoya 464-8601, Japan.
E-mail: hiroki@hyarc.nagoya-u.ac.jp
Copyright  2007 John Wiley & Sons, Ltd.

flux from the Flatland Project in 1996, and described thediurnal and seasonal variations in the ABL depth and thesurface flux for flat terrain in Illinois, USA.
Simultaneous observations of the WPR and the surface
flux measurements have not yet been reported for the
Asian monsoon region, although many individual surface
or ABL observations have been recorded under the
Global Energy and Water Cycle Experiment (GEWEX)
Asian Monsoon Experiment (GAME) and GAME-related
projects (e.g. Hiyama et al., 1999, 2003; Ohta et al.,1999, 2001; Sugita et al., 1999; Asanuma et al., 2000;
Reddy et al., 2001; Tanaka et al., 2001; Toda et al.,
2002; Komatsu et al., 2003; Choi et al., 2004; Gaoet al., 2004; Strunin et al., 2004; Strunin and Hiyama,
2004; Gao, 2005; Shinoda et al., 2005). The simultaneous
observations reported here will fill this gap. The Asianmonsoon over Far East Asia has unique characteristics,
such as the Meiyu frontal activity that persists during
the summer rainy season in this region and plays animportant role in the regional climate as well as theglobal climate. The Lower Atmosphere and Precipitation
Study (LAPS) is a post-GAME project sponsored by theCore Research for Evolutional Science and Technology(CREST) program of the Japan Science and Technology
Agency (JST). This program seeks to clarify precipitation

1998

H. TANAKA ET AL.

onset mechanisms, including the relationships between
the surface, ABL, clouds, and precipitation processes.
Changes in surface conditions alter the surface heat andthe water vapour fluxes that in turn affect the ABL
structures. ABL processes are one of many triggers
that generate clouds and precipitation. The ABL is theinterface between the surface and the free atmosphere,
and it strongly influences the vapour transport, sensible
heat flux over the surface and convective precipitation
systems. Clarifying ABL development mechanisms on
diurnal and seasonal time scales will help elucidate
regional hydrological budgets.
ABL depth, or the height of the top of the daytime
convective mixed layer is an important parameter inthe studies of the ABL. WPR is a kind of Doppler
radar, which detects the atmospheric echo and its Doppler
shifts due to the wind. Due to the Bragg scattering,
the turbulence structure with the scale of half radar
wavelength (/2 ³ 0Ð12 m in this study) enhances toreflect the WPR beam. WPR detects the ABL top, as
the level with maximum radar scattering or WPR echo,
because the entrainment layer has large scalar gradientsand therefore many turbulent structures (Angevine et al.,
1994, 2001; Hashiguchi et al., 1995; White et al., 2002).
The WPR echo intensity, or backscattered power, PR , isgiven by the radar equation as
PR D PrG2 2  2 h10242 ln 2r 2

1

where Pr is the transmitted peak power, G is the antenna
gain,  is the radar wavelength, r is the range to thebackscatter volume,  is the beam width, h is the pulse
length, and  is the radar reflectivity per unit volume,
which is proportional to the refractive index structure
parameter Cn 2 as follows: D 0Ð38C2n 1/3

2

The structure parameter Cn 2 represents the intensity of
turbulence, and strongly depends on gradients of vapour
density.
Tennekes (1973) used conservation of virtual temperature within the ABL to estimate the ABL depth fromthe accumulated surface virtual temperature flux and thevertical motion at the top of the ABL. If vertical motion
at the top of the ABL and horizontal advection are bothnegligible, ABL depth is related to surface heat flux asfollows (cf., Garratt, 1992):
z0 C z2 D z0 2 C

21 C 2εQS3Here z0 is the initial depth of the ABL, z is theincrement of the depth for a reference duration,  is thelapse rate of potential temperature above the ABL, ε isan entrainment parameter, and QS is the surface virtual
temperature flux that accumulated during the reference
duration.
The main target of observations in this study was thesurface-ABL relationship before, during, and after the
Copyright  2007 John Wiley & Sons, Ltd.

Meiyu season in China. One goal was to understandhow the drastic change in surface moisture conditions
during the Meiyu season affects the ABL structure
through surface flux interactions. The monitoring site
is located in a transition area between humid and dry
regions in China. The land surface in this vicinity issuddenly moistened during the Meiyu. Humidity persistsin the lower atmosphere during the Meiyu season evenwhen the Meiyu front temporarily moves south of theregion. Thus, another goal was to describe the vapour
source that provides or maintains such high humidity
throughout the season. The presence of high humidity
allows relatively small triggers to generate clouds andprecipitation. Synoptic-scale water vapour transport andhuman activities, such as agricultural irrigation andcultivation, can influence the ABL moisture conditions
through surface-ABL interactions. Surface and ABL
monitoring in this flat region before, during, and after
the Meiyu will provide important data.
This paper describes the surface and ABL observationsin the LAPS project over the middle stream of the HuaiheRiver basin in China. Then, initial data from tower observations, WPR observations, and vegetation research arepresented. Finally, relationships are presented between
the ABL depth and the surface heat fluxes and otherparameters, such as the vertical motion at the top of
the ABL.

MATERIALS AND METHODS
Site description
The study site was a flat surface region located inShouxian, China, at 32Ð55 ° N and 116Ð78 ° E, 22Ð7 m
A.S.L. The site is located near the Huaihe River in
Anhui Province, China (Figure 1). Instrumentation at
the study site included an ABL wind profiler radar(L-28, Sumitomo Electric Industries, Ltd, Japan), an
acoustic wind profiler SODAR (XFAS, Scintec, A.G.,
Germany), a microwave water vapour profiler (TP/WVP3000, Radiometrics, Co., USA), and a surface-layer
observation system (C-PRJ30 and C-FLJ30, Climatec,
Inc., Japan) at the Shouxian Meteorological Observatory,
Anhui Meteorological Bureau, Chinese Meteorological
Administration. The site is located at the southern edgeof residential Shouxian, 2 km south of the city center.
Surrounding land are mainly agricultural fields, withsome residences and small buildings that are often
surrounded by trees. A paved road passes 200 m westof the site, and there is an irrigation canal 2 km west of
the road. Lake Wabuhu is 7 km southeast of the site, andthe Huaihe River passes 9 km northwest.
Monitoring for the LAPS project started in August
2003 and is ongoing, although interruptions occurredbecause of power supply problems, instruments affectedby strong wind or lightning, and other problems. Intensive observations, including vegetation research, wererecorded during the intensive observation period (IOP)
from 24 May to 16 July in 2004 (IOP-2004). Land use
Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp

1999

100°E

130°E

120°E

110°E

SURFACE AND ABL OBSERVATION IN SHOUXIAN

40°N

30°N

30°N

20°N

20°N

100°E

130°E

120°E

110°E

40°N

Figure 1. Location of the study site, Shouxian, at 32Ð55 ° N, 116Ð78 ° E, and 22Ð7 m A.S.L., near the Huaihe River in Anhui Province, China. The
study site is indicated as the marker ()Table I. Variables observed during the intensive observation period from 24 May to16 July 2004 (IOP-2004) in the Lower Atmosphereand Precipitation Study (LAPS)
Variables

Intervals

Notes

Water temperature
Soil temperature
Surface temperature

1 h during daytime, 08 : 00–17 : 00 LT
1 h during daytime, 08 : 00–17 : 00 LT
1 h during daytime, 08 : 00–17 : 00 LT

Vegetation height

Once per day

Water depth
SPADa

Once per day
Once per day

Leaf area index (LAI)

5 times, 25 MAY, 16 JUNE, 24 JUNE, 30
JUNE, 7 JULY
Twice per day

Cloud amountNo water before 14 JUNE
At 0Ð1 m below-ground
Soil surface before 14 JUNE; Water
surface after 15 JUN
No vegetation between 31 MAY and 15
JUNE
No water before 14 JUNE
No vegetation between 31 MAY and 15
JUNE
No vegetation between 31 MAY and 15
JUNE
Using fish-eye sky photos and weather
condition records

a An index of the chlorophyll density obtained by a chlorophyll sensor (SPAD-502, Konica Minolta, Japan).

surrounding the site changed from wheat field to rice
paddy field during IOP-2004.
Observation items
IOP-2004 included observations of vegetation coversurrounding the study site. Table I summarizes parameters observed manually at the study site. Surfacelayer variables include turbulent fluxes, radiation fluxes,
wind speed, wind direction, air temperature, atmospherichumidity, soil temperature, soil moisture, and precipitation rate. These were all measured with a surface-layer
observation system on a 32Ð2-m tower and two 3Ð5-m
masts. Table II summarizes the observed items and individual instruments within the system. The turbulent fluxes
Copyright  2007 John Wiley & Sons, Ltd.

obtained at the height of 32Ð0 m were used for analyses
in this study, because the dependency on the wind direction was less detected than those obtained at other lower
levels.
ABL parameters, including wind vectors and a turbulence structure parameter, were measured using a1Ð29-GHz WPR. Table III summarizes the WPR operational specifications. The maximum height of the observation was approximately 8000 m. The resolution of
the observed wind velocities was 0Ð1 ms1 (horizontal)
and 0Ð01 ms1 (vertical). The accuracy was less than
1 ms1 in horizontal and less than 0Ð1 ms1 in vertical. The observation range was 0–90 ms1 (horizontal)
and 0–30 ms1 (vertical). The original WPR data have
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H. TANAKA ET AL.

Table II. Observed variables and instruments for the surface layer observation system
Variables
Wind vector and virtual temperature
CO2 and H2 O density
Atmospheric pressureWind velocity
Wind direction
Radiation fluxes (four components)
Air temperature and humidity
Surface temperature
Soil moisture content
Ground heat flux
Ground soil temperature
Precipitation
Water level

Instruments

Height or depth (m)C-SAT-R3-50 (1210R3), Gill
C-CS7500 (LI-7500), Li-Cor
CVS-PTB210, Vaisala
C-PR-010C, MetOne
C-PR-020C, MetOne
C-PR-CNR1, Kip & Zonen
C-PR-45D (HMP-45D), VaisalaC-303F
C-CS616-30, Campbell
C-PRHF01, Huxflux
C-PT100
COT-34T, Ota Keiki
Floating gauge

32Ð0, 12Ð2, 3Ð5
32Ð0, 12Ð2, 3Ð52Ð532Ð2, 21Ð9, 11Ð9, 3Ð0, 1Ð6
31Ð8
31Ð8
30Ð7, 20Ð7, 10Ð8, 2Ð8, 1Ð5
30Ð7, 2Ð9
0Ð1, 0Ð2, 0Ð4
0Ð01, 0Ð01, 0Ð01
0Ð05, 0Ð1, 0Ð2, 0Ð4, 0Ð4
0
0

Table III. Operation specifications for the wind profiler radar
Variables

ValuesRadar operational frequency
Pulse length
Sampling intervalInter-pulse period
Number of height points
Number of coherent integrations
Number of FFT points
Number of incoherent integrations
Number of beam directions
Direction of beam(azimuth, zenith) (deg)1290 MHz666 ns
666 ns80000 ns
80
32
128
18
5
(0, 0), (0, 15), (90,15),(180, 15), (270, 15) xtr , hr   Xt, h DD

D2 <1Ð5

1
D2
D2 <1Ð52

where Xt, h is the interpolated value at time t (s) andheight h (m), xtr , hr  is the original data at time tr (s) andheight hr (m), and D2 is the square of the dimensionlessdistance defined as follow;

2

D D

h  hr
100

2



C

t  tr60

2

5

The time interval of the gridded data is 30 s, and theheight resolution is 50 m.

0
10
20
30
40

100
80
60
40

Volumetric water content

4

0.5

20

Relative humidity (%)

Daily precipitation (mm)a time interval of about 59 s, and a height resolution of99Ð9 m for vertical beams and 96Ð4 m for oblique beams.

For ease data inspection, data in this study were griddedwith two-dimensional interpolation as;

0.4
0.3
0.2
-150

-120

-90

-60

-30

0
30
60
Time (d, 2004/01/01 = 0)

90

120

150

180

Figure 2. Seasonal variation of the daily precipitation (upper column), daily relative humidity (middle column), and volumetric soil water content(bottom column) at 0Ð4 m depth (solid), 0Ð2 m depth (dots), and 0Ð1 m depth (dashes) in non-irrigated ground near the flux tower for approximately
1 year, from 8 August 2003 to 16 July 2004
Copyright  2007 John Wiley & Sons, Ltd.

Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp2001

SURFACE AND ABL OBSERVATION IN SHOUXIAN

0.6

4
3

0.4
0.2

2

0.0

1

0.18

0

Leaf area index

Height or depth (m)0.8Albedo

0.16
0.14
0.12
0.10
0.08
0.06
140 145 150 155 160 165 170 175 180 185 190 195 200
Time (d, 2004/05/20 = 140)

Figure 3. Changes in plant height (dashed line, left axis), leaf area index (rectangle, right axis), irrigated water depth (solid line, left axis), and albedo(solid line with circles) during IOP-2004

Figure 4. Land-use photographs. (a) Mature wheat on 27 May 2004, (b) harvested wheat on 1 June 2004, (c) burned field on 3 June 2004, (d) plowed
field on 9 June 2004, (e) planted rice on 16 June 2004, (f) growing rice on 26 June 2004
Copyright  2007 John Wiley & Sons, Ltd.

Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp

2002

250
200
150
100

0

500
400
300
200
100
0

-0.0
-0.4
-0.8

Albedo0.20-1.2

CO2 flux (mgCO2 m-2 s-1)

Latent heat flux (Wm-2)

50

Sensible heat flux (Wm-2

H. TANAKA ET AL.

0.15
0.10
0.05
-150

-120

-90

-60

-30

0

30

60

90

120

150

180

Time (d, 2004/01/01 = 0)

Figure 5. Seasonal variation of midday virtual sensible heat, latent heat, and CO2 flux averaged between 08 : 00 and 16 : 00 LT, and short-wave
albedo averaged between 11 : 00 and 13 : 00 LT

Estimation of the ABL depth from WPR data
The ABL depth can be estimated theoretically or
empirically, as the height with the maximum echo intensity received by the WPR. Angevine et al. (1994) proposed the so-called ‘median filtering method’ to estimate
the ABL depth as the median value of the maximum echoheights for certain durations (e.g. 30 min). However, such
estimates sometimes fail because of large returns fromclouds above the ABL or from strong near-surface turbulence. Such failures can be avoided by estimating the
ABL depth over much longer time periods (e.g. morethan 2–4 h), or by limiting the height range in which
the ABL top can appear. The latter method allows fluctuations within a short time scale and was chosen forthis study. Assumptions that limited the possible height
range were: the convective mixed layer started to develop
just after sunrise, around 06 : 00 local time (LT); thepeak height appeared between 12 : 00 and 16 : 00 LT;
the height remained within approximately the same range
after 16 : 00 LT. This diurnal evolution of the ABL height
was usually valid for days without significant clouds or
precipitation.
RESULTS AND DISCUSSION
Overview of the climate and surface conditions
Figure 2 shows seasonal changes in the volumetricsoil water content of the non-irrigated ground near the
Copyright  2007 John Wiley & Sons, Ltd.

tower, the relative humidity on the tower, and the daily
precipitation from 8 August 2003 to 16 July 2004. Soilwater content increased following precipitation events.
Atmospheric humidity was high during the summer. Soils
were generally moist (the soil porosity was approximately
0Ð45) during the entire year, except near the surface
during some dry spells in winter and early spring.
Figure 3 shows plant height, leaf area index (LAI),
irrigated water depth, and albedo (˛) during IOP-2004.
A land-use change is illustrated by the photographs in
Figure 4. Mature wheat (˛ ¾ 0Ð14) was the dominant
vegetation between 24 May (time D 144 in Figure 3) and
30 May (time D 150), at which time it was harvested(˛ ¾ 0Ð16). Wheat litter was burned (˛ ¾ 0Ð07) between
1 June and 4 June (time D 152–155). The black ash
was plowed down (˛ ¾ 0Ð12), and rice seedlings wereplanted and flooded (˛ ¾ 0Ð09) on 14 June (time D 165).
The planted seedlings were 0Ð2–0Ð3 m tall. The seedlings
grew to 0Ð7–0Ð8 m (˛ ¾ 0Ð15) on 16 July (time D 197),
the end of IOP-2004.
Water vapour was transported by synoptic-scale southerly winds on 14 June (time D 165), when irrigation
started. After this day, moisture persisted in the lower
atmosphere (Figure 2), even when drier air blow in fromthe north, i.e. when the Meiyu front moved south. The
sudden increase in atmospheric moisture may have been
driven by synoptic-scale moisture transport, and partly
by irrigation. Maintenance of high humidity within the
Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp

2003

SURFACE AND ABL OBSERVATION IN SHOUXIAN(a) Horizontal Wind Velocity (ms-1)
3000

20

Height (m)

2500
2000

15

1500

10

1000

5

500
0

0
(b) Wind Direction (degree)
3000

360

Height (m)

2500
2000

270

1500

180

1000

90

500
0

0
(c) Vertical Wind Velocity (ms-1)
3000

2

Height (m)

2500
2000

1

1500

0

1000

-1

500
-2

0
3000

(d) Height Corrected Echo Power (dB)40

Height (m)

2500
2000

30

1500

20

1000

10

500
0

0
Energy flux (Wm-2)(e) Energy Fluxes
700
600
500
400
300
200
100
0
-100

-4

-2

0

2

4

6

8
10 12 14 16 18 20
Time (h, 2004/05/31 00:00 LT = 0)

22

24

26

28

30

Figure 6. Typical diurnal changes in the ABL structure and the surface flux in on 31 May 2004 when vegetation cover is not present. (a) Horizontal
wind speed, (b) wind direction, (c) vertical wind velocity, (d) echo intensity, and (e) energy fluxes including net radiation (black), virtual sensible
heat flux (red), and latent heat flux (blue)

ABL resulted likely from the surface water during thewet season.
Figure 5 shows the seasonal changes in midday virtual
sensible and latent heat fluxes and CO2 flux averaged
between 08 : 00 and 16 : 00 LT, and the short-wave albedo
averaged between 11 : 00 and 13 : 00 LT. Virtual sensible
heat flux, rather than the sensible heat flux, is shownin Figure 5 and subsequent figures to account moreclearly for buoyancy effects. The two fluxes differ byapproximately 6 Wm2 when the latent heat flux is
100 Wm2 . Virtual sensible heat flux Hv is defined as:
HV D H C 0Ð514CP TV E
Copyright  2007 John Wiley & Sons, Ltd.

6

where CP is the specific heat of air, Tv is the virtual
temperature, and E is the water vapour flux. Changes inland-use clearly affect the parameters. Rice fields were
present during summer, and high rates of CO2 absorption
and evapotranspiration, and small values of sensible heat
flux indicate growing rice during August 2003. Mature
rice is indicated in September 2003 by decreased CO2
absorption and evapotranspiration and increased sensible
heat flux. Low albedo and small fluxes accompany
sparse vegetation from October through January 2004. As
wheat germinated and grew between February and April,
CO2 absorption and evapotranspiration increased. Wheat
Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hypH. TANAKA ET AL.

800
600
400
200
0
-200

Height (m)

800
600
400
200
0
-200

Height (m)

800
600
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0
-200

Height (m)

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600
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0
-200

3000
2500
2000
1500
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500
0
3000
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2000
1500
1000
500
0

Height (m)

800
600
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200
0
-200

3000
2500
2000
1500
1000
500
0

3000
2500
2000
1500
1000
500
0

Height (m)

800
600
400
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0
-200

2004/01/28

3000
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0

Height (m)

800
600
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0
-200

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2500
2000
1500
1000
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0

Height (m)

-2
-2
-2
-2
-2
Energy flux (Wm-2) Energy flux (Wm-2) Energy flux (Wm ) Energy flux (Wm ) Energy flux (Wm ) Energy flux (Wm ) Energy flux (Wm )

2004

3000
2500
2000
1500
1000
500
0

2004/03/06

2004/04/24

2004/05/20

2004/07/03

4

6

8

10

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14
Time (h)

16

18

20

10

20

30

dB
40

18

20

2004/03/06

2004/04/24

2004/05/20

2004/05/31

2004/06/22

0

2004/01/28

2004/05/31

2004/06/22

2004/07/03

4

6

8

10

12

14

16

Time (h)Figure 7. Diurnal changes in energy fluxes over the surface on 7 clear
days: 28 January, 6 March, 24 April, 20 May, 31 May, 22 June, and 3
July. Black line indicates net radiation, red line indicates virtual sensible
heat flux, and blue line indicates latent heat flux

Figure 8. Time-height sections of echo intensity obtained from windprofiler radar observations on several typical days (same as in Figure 7).
Atmospheric boundary layer depth was estimated every 2 min (blackdots)

matured during May, when CO2 absorption and vapour
flux decreased, sensible heat flux was large, and albedo
decreased slightly. Rice seedlings were planted again in
mid-June 2004, and subsequently grew until mid-July as
CO2 absorption, water vapour flux, and albedo increased,
and sensible heat flux decreased.

on 31 May (time D 151 in Figure 2). The figure includes(a) horizontal wind speed, (b) wind direction, (c) verticalwind velocity, (d) echo intensity, and (e) net radiation,
virtual sensible heat flux, and latent heat flux. Echo intensity (Figure 6d) depicts turbulence intensity, or the turbulence structure parameter Cn 2 (refer Equations 1 and
2). A convective mixed layer was present during the day
and strong WPR echoes were detected. The ABL depth
clearly increased from soon after sunrise till the afternoon. There were strong horizontal winds and sudden

Diurnal development of the ABL depth
Figure 6 shows typical diurnal changes in ABL structure and surface fluxes when no vegetation cover existed
Copyright  2007 John Wiley & Sons, Ltd.

Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp2005

SURFACE AND ABL OBSERVATION IN SHOUXIAN

Seasonal changes in the ABL depth

ABL depth (m)ω (Pa s-1)

Figure 7 shows the diurnal changes in energy fluxes
over the surface on seven clear days. Wheat was sprouting around 28 January (time D 27 in Figure 2), growing on 6 March (time D 65) and 24 April (time D 114),
mature on 20 May (time D 140), and harvested on 31
May (time D 151). Subsequently, rice was growing on
22 June (time D 173) and 3 July (time D 184). Virtual
sensible heat flux is simply referred as sensible heat fluxin the following sentences, because the two fluxes differ
little (refer Equation 6). Sensible and latent heat fluxes
had similar diurnal traces that tracked the diurnal changes

in the net radiation before the wheat was harvested. When
the wheat was most actively growing, the latent heat flux
slightly exceeded the sensible heat flux (on 24 April).
The latent heat flux decreased between wheat harvest andfield flooding (31 May). Sensible heat flux dropped andthe latent heat flux increased dramatically after the field
was flooded and rice seedlings were planted (22 Juneand 3 July); the peak latent heat flux shifted later in the
day because the high heat capacity of the irrigation water
allowed higher temperatures to persist till the evening.
Figure 8 shows the echo intensities and the ABL
depths, recorded by the WPR on the same days as
in Figure 7. The ABL depth was derived every 2 minusing the procedures outlined above. The ABL depth
reached 1500 m around noon, before irrigation, except
in winter: the ABL was less than 500 m deep on 28
January. Strong sensible heat flux can enhance the ABLdevelopment in the morning. The ABL merged with theresidual layer, and consequently the ABL depth increased
abruptly at around 10 : 30 LT on 6 March, 11 : 30 LT
on 24 April, and 09 : 30 LT on 31 May. On the other
hand, the residual layer remained unchanged till 13 : 30
LT on 22 June, a day when rice seedlings were growing
after irrigation. Much stronger echoes within the ABLon that day indicate turbulent structures associated withlarge humidity gradients. High humidity resulting fromthe strong water vapour flux over the surface affected
the ABL structure. Daily maximum ABL depth occurred
around 1500 m on 22 June and 3 July, even though thevirtual sensible heat flux was smaller. Surface buoyancy
flux does not explain such ABL development.
Figure 9 shows the daily maxima of the ABL depth andthe virtual sensible heat flux averaged between 08 : 00 and
14 : 00 LT. There is a relationship between the ABL depth
and the midday averaged heat flux before the field was
irrigated, especially in winter. However, the relationshipis less clear after irrigation. Also plotted in the figure is

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changes in wind direction above the top of the ABL in
daytime (cf., Figure 6(a) and (b)). The ABL depth fluctuated as echo intensity within the ABL varied, being
periodically enhanced by plume-like structures in the
upward wind (red in Figure 6(c)), which alternated withdownward wind structures (blue in Figure 6(c)) between
09 : 00 and 17 : 00 LT. The period was 30–60 min on
typical fine days. The plume-like upward wind extended
to the top of the ABL at midday. Plumes did not reachthe top of the ABL in late afternoon.
Much stronger echoes were often detected at night,
especially just after sunset. Heights often exceeded
2500 m. Similar phenomena had been recognized in thepast as contamination from migrating birds. Wilczak
et al. (1995) considered possible reasons behind theappearance of strange echoes. Kobayashi et al. (2005)
concluded that migrating birds can scatter the WPR beam,and proposed a method to eliminate this effect fromobserved wind velocity. However, no migrating birds
were observed when the anomalous echoes were present
in this study, and it seems unlikely that they would
be present every night. The reason for these anomalous
echoes will be the subject of future study. The following
sections focus on ABL development during the day.

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Figure 9. Seasonal changes in the observed daily maximum ABL height (bottom panel, bars), observed virtual sensible heat flux (bottom panel,
rectangle with lines) between 08 : 00 and 16 : 00 LT, and the 925-hPa vertical pressure velocity (ω; upper panel, bars) extracted from the NCEP/NCAR
re-analysis at 00Z (08 : 00 LT) at 32Ð5 ° N, 117Ð5 ° E. Negative ω indicates upward motion
Copyright  2007 John Wiley & Sons, Ltd.

Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp

2006

H. TANAKA ET AL.

the vertical pressure velocity ω at 925 hPa at 00Z (08 : 00
LT) at 32Ð5 ° N and 117Ð5 ° E from the National Center
for Environmental Prediction (NCEP)/National Center
for Atmospheric Research (NCAR) re-analysis (Kalnay
et al., 1996), which represents an atmospheric verticalmotion. The plotted vertical motion is near or slightly
above the top of the ABL, which was usually close toor below 925 hPa at 08 : 00 LT. The vertical motion
is representative of a 2Ð5 ð 2Ð5-degree area between
31Ð25–33Ð75 ° N and 116Ð25–118Ð75 ° E, a region that
includes the study site. Vertical motion can be linked

to the discrepancies between the ABL depth and theheat flux, especially when the heat flux was low after
irrigation.
Figure 10 shows the relationships between the daily
ABL depth and the midday averaged virtual sensible
heat flux for (a) the entire study, and for five periods including (b) winter (time D 0–59), (c) early spring(time D 60–89), (d) the growth period of wheat (time D
90–139), (e) the period with no vegetation after thewheat had been harvested and before irrigation (time D
140–164) and (f) after irrigation (time D 165–197). A

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ABL depth (m)(a) 3000

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Figure 10. Relationships between the daily ABL depth and the midday averaged virtual sensible heat flux. Filled circles indicate neutral motion(0Ð02 < ω < 0Ð02), upward open triangles indicate ascending motion (ω < 0Ð02), and downward filled triangles indicate descending motion(ω > 0Ð02), where ω is the 925-hPa vertical pressure velocity from the NCEP/NCAR re-analysis, as shown in Figure 9
Copyright  2007 John Wiley & Sons, Ltd.

Hydrol. Process. 21, 1997– 2008 (2007)
DOI: 10.1002/hyp2007

SURFACE AND ABL OBSERVATION IN SHOUXIAN

positive trend is evident in all periods except in period(f) after irrigation. The scatter in Figure 10 can be
explained partly by atmospheric vertical motion after irrigation and in other periods except period (e). Downward
motion suppressed the ABL development (downward triangles in Figure 10), whereas upward motion enhanced
it (upward triangles in Figure 10). Other explanations for
scatter may include residual layer effects and horizontal
advection effects, which should be considered in futurestudies.

CONCLUDING REMARKSThe Surface and ABL observations in the LAPS project
began in August 2003, and are ongoing. A preliminary
analysis of the surface and ABL observations revealed
relationships between the surface fluxes and the ABL
structure. Buoyancy flux enhanced ABL development.
Fluctuations in ABL depth were related to plume-likewind structures within the ABL. Daily variations in the
ABL depth were controlled mainly by the buoyancy
flux over the surface during the dry periods. Variations
were strongly affected by atmospheric vertical motion
during the wet periods, such as the Meiyu season. Further
analysis to investigate how the ABL depth is affectedby the combined effects of temperature and humidity
profiles, in addition to surface fluxes, is warranted.
The source of the increased atmospheric humidity during the onset of Meiyu, and the mechanism that maintains humidity during the Meiyu season, may be thesurface vapour flux caused by irrigation. Humid conditions affect the ABL structure and its development. In
addition, synoptic-scale vapour transport may increase
atmospheric humidity during the onset of Meiyu. Relationships between local phenomena and synoptic-scale
humidity must be studied with further observations,satellite remote sensing, and re-analysis data such as
NCEP/NCAR. Projections of water vapour transport atsynoptic scales, i.e. weather forecasts, influence agricultural schedules. Interactions between anthropogenichydrological activities and atmospheric phenomena also
warrant future study.

ACKNOWLEDGEMENTS

This part of the Lower Atmosphere and Precipitation
Study (LAPS) was conducted under the Core Research
for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency(JST) and the Hydrospheric Atmospheric Research Center (HyARC) at Nagoya University. The authors thank
LAPS members for their cooperation and helpful discussions, CREST research manager Dr Shigeo Masuda forkind collaboration, and Ms Tomoko Tanaka for hard workwith logistics. The authors thank Sumitomo Electronic
Industries, Ltd., and Climatec, Inc. for the establishment of the observation systems. We also thank Shouxian Meteorological Observatory, Anhui Meteorological
Copyright  2007 John Wiley & Sons, Ltd.

Bureau, and the China Meteorological Administration for
their collaboration in this study.

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