SOLA, 2008, Vol. 4, 033 036, doi:10.2151/sola.2008 009

33

Diurnal Variation of Water Vapor Mixing
between the Atmospheric Boundary Layer and Free Atmosphere
over Changwu, the Loess Plateau in China
Atsuhiro Takahashi1, Tetsuya Hiyama2, Masanori Nishikawa3, Hatsuki Fujinami2,
Atsushi Higuchi4, Wei Li5, Wenzhao Liu6, and Yoshihiro Fukushima1
1
Research Institute for Humanity and Nature, Kyoto, Japan
2
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan
3
Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
4
Center for Environmental Remote Sensing, Chiba University, Chiba, Japan
5
Department of Civil & Environmental Engineering, Pratt School of Engineering, Duke University,
Durham, U.S.A.
6
Institute of Soil and Water Conservation, Chinese Academy of Sciences, Shaanxi, China

Abstract
We investigated the diurnal variation of water vapor
mixing between the atmospheric boundary layer (ABL)and the free atmosphere over the Loess Plateau in
China. Water vapor and wind velocity in the troposphere were observed using a ground-based microwave
radiometer and a wind profiler radar in 2005 and 2006.
On sunny days in early summer, a strong vertical wind
was generated in the afternoon followed by ABL development. Strong convection was enhanced when active
cumulus convection developed in the afternoon. In suchcases, water vapor decreased in the lower atmospherefrom the early morning until late afternoon, while water
vapor increased in the upper atmosphere. This finding
suggests that water vapor was exchanged diurnally
between the ABL and the free atmosphere. The strong
convection in the ABL, which was developed bysensible heat from the land surface, played critical roles
with link to cumulus convection in such vertical mixingof water vapor. Influences of other processes such as alocal circulation and advection of cloud systems werealso discussed.

1. Introduction
Water plays an important role in the Earth’s climate
by transporting energy over long distances, forming
clouds and rainfall, and thereby influencing biologicaland ecological activities worldwide. Water is also
crucial for human lives and activities. It is especiallycritical in arid and semi-arid regions where water resources are limited.
In East Asia, vast areas of such severe climate
regimes are distributed across northern China. Locatedin the middle part of the Yellow River basin, the Loess
Plateau significantly affects the water cycle in the
Yellow River basin (Zhang and Liu 2005).
The Loess Plateau experiences dry conditions inmost periods of the year. Over the dry surface exposed
to intense solar radiation, the atmospheric boundary
layer (ABL) can develop to high altitudes (Gamo 1996).
A strong vertical wind, which can reach the free atmosphere, develops in the ABL over the plateau in earlysummer (Nishikawa et al. 2005). Over the Tibetan
Plateau, dry convection in the ABL and cumulus convection have been shown to play significant roles in
Corresponding author: Atsuhiro Takahashi: Present affiliation:
Graduate School of Environmental Studies, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan. E-mail: atchuing@yahoo.co.jp. ©2008, the Meteorological Society of Japan.

Fig. 1. Topographical maps of the observation site, which ismarked by an asterisk in each map. The upper right figure
shows the MODIS-derived normalized difference vegetationindex (NDVI) values for July 2002 (resolution: 250 m). The
lower left figure was made using data from the global digital
elevation model GTOPO30 (resolution: 1 km), and the lower
right figure was made using data obtained by the Space
Shuttle (resolution: 90 m).

heating the upper troposphere in spring (Taniguchi and
Koike 2007). Thus, the developments of the ABL andclouds are critical factors in atmospheric dynamics andwater cycles.
To clarify the role of cloud activity, it is necessary toinvestigate the temporal variation of the atmospheric
thermodynamic state. Recent progress in collecting
in situ observation variables has made it possible tomeasure precipitable water vapor (PWV) at a high time
resolution (e.g., Takagi et al. 2000). Passive microwave
radiometry, which was adopted in this study, is an efficient method to retrieve continuous profiles of atmospheric humidity and temperature (Lohnert et al. 2007).
In this study, data were also obtained using a windprofiler radar to assess the kinematic structures of the
ABL and the free atmosphere.
The aims of this study were 1) to clarify the diurnal
variation of the atmospheric water vapor over the Loess
Plateau in early summer in 2005 and 2006, and 2) to investigate interactive influences of the ABL development
and cumulus convection on the vertical transport ofwater vapor.

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Takahashi et al., Diurnal Variation of Water Vapor

2. Observations and data set
In May 2004, we established systems for measuring
the ABL over a wheat field of the Changwu AgroEcological Experimental Station [35.24°N, 107.68°E,
1224 m above sea level (a.s.l.)] in the southern part of the
Loess Plateau in China (Fig. 1). Complex terrain with aflat tableland and steep gullies spreads over the plateau.
Broad areas of the tableland are occupied by crop fieldsof wheat, maize, and apples.
The ABL observation systems were installed in May
2004 (Hiyama et al. 2005). Vertical profiles of atmospheric water vapor were measured from the ground to aheight of 10 km using the ground-based microwave radiometry (MR) equipment (TP/WVP-3000, Radiometrics
Corp., Boulder, CO, USA) with a time step of 1 min.
Vertical profiles of wind velocity were also observed
using a 1290-MHz wind profiler radar (WPR; L-28,
Sumitomo Electric Industries, Inc., Osaka, Japan) up to aheight of about 8 km. Turbulence measurements werealso conducted at this site to estimate sensible heat (SH)and latent heat (LH) fluxes on the ground based on theeddy correlation method (EC). An ultrasonic anemometer/thermometer (1210R3, Gill Instruments, Ltd.,
Lymington, UK) and an open-path type H2 O/CO2 gas
analyzer (LI-7500, Li-Cor, Inc., Lincoln, NE, USA) were
installed on the system at a height of 32 m.
Additionally, 30-min average air temperature (Ta ) at aheight of 2 m was measured using a temperature/relative humidity probe (HMP-45A, Vaisala, Helsinki,
Finland). Precipitation was measured using a tippingbucket rain gauge. Visual observations of the amountand types of clouds that appeared in the daytime wereconducted every 1 h from 8:00 Beijing Standard Time
(BST) to 18:00 BST during the intensive observation
periods (IOPs) from 11 May to 13 July 2005 and from 15
May to 10 July 2006.
The neural network of the MR was trained with useof radiosonde data at YanAn (36.60°N, 109.50°E, 959 ma.s.l.), located about 224 km from our observation site.
To check the validity of our MR data, further, thevertical profiles of humidity and air temperature
measured by the MR were compared to radiosonde datameasured at Pingliang (35.55°N, 106.67°E, 1348 m a.s.l.),located about 98 km from our observation site. We useddata measured at 7:00 BST in 32 cases of fine days. The
data from the three measurement sets agreed well witheach other. Root mean square errors were 1.47 g m 3 (r2= 0.78) for humidity and 2.27 K (r2 = 0.83) for air temperature in the atmosphere lower than a height of 2 km
from the ground, and 0.32 g m 3 (r2 = 0.71) for humidityand 2.74 K (r2 = 0.97) for air temperature in the atmosphere higher than 2 km. Atmospheric conditions could
be taken as being closely homogeneous in the area including the three sites on fine days. Thus, we judgedthat the MR data were suitable for use in examining theseasonal and diurnal changes in atmospheric water
vapor and air temperature over the observation site.

height of 2 km, the integrated WVC below 10 km fluctuated greatly, especially in the warm season. The
maximum peak of WVC appeared around July andAugust in 2006. During the period when WVC was high,precipitation occurred frequently and the soil-watercontent increased.
The middle panel of Fig. 2 shows the seasonal variation of the daily-averaged fluxes of SH and LH based on
the EC method at a height of 32 m. Moderate maximum
peaks of SH occurred in spring. When surface soil wasmoistened by precipitation in the rainy seasons beginning around July, LH reached its maximum peaks, andSH decreased gradually.
As depicted in the lower panel of Fig. 3, which shows
the amounts and types of clouds during the IOPs over
the observation site in daytime, the frequency of appearance of active cumulus clouds increased with increases of Ta and WVC (upper panel of Fig. 3) from lateJune in each year. In this study, we defined active
cumulus as cumulus congestus or cumulus castellanus
clouds that developed massively to a high altitude, inFig. 2. Seasonal variations in (upper panel) daily-averaged
values of water vapor content (WVC) in the troposphere, 30min average air temperature (Ta ) at 2 m, (middle panel) flux
densities of sensible heat (SH) and latent heat (LH), and (lower
panel) surface soil water contents. Seasonal variation in daily
precipitation is shown as black bars in the lower panel.

3. Results
The upper panel of Fig. 2 shows the seasonal variation in daily averaged values of atmospheric water
vapor content (WVC) integrated vertically from thesurface to a height of 2 km (light blue line and circles)
and to 10 km (black line and circles) in unit space (1m2) and Ta (green line) at a height of 2 m. Here, we use
WVC to represent the amount of water vapor in acertain depth of the atmospheric layer in order to distinguish this amount from PWV. Data were missing for theperiod from 13 July 2005 to 8 April 2006 due toproblems with the electric power to the measurement
equipment. The WVC varied drastically in the course of
the seasonal march. Compared with WVC below a

Fig. 3. Seasonal variation in (upper panel) daily-averaged
values of water vapor content (WVC) in the troposphere, 30min average air temperature (Ta) at 2 m, and (lower panel) theamounts and types of clouds. The meanings of each line are thesame as those for the upper panel of Fig. 2. In the lower panels,
red bars indicate the presence of active cumulus, blue bars
indicate the presence of forced but not active cumulus, and thewhite bars indicate other types of clouds.

SOLA, 2008, Vol. 4, 033 036, doi:10.2151/sola.2008 009
contrast to fair weather cumulus.
In the presence of active cumulus clouds, characteristic diurnal variation in WVCs was observed in thelower and upper atmospheric layers, as shown in theupper panel of Fig. 4 illustrating the time series of
WVCs during days of the year (DOY) 168 172 in 2005.
Ther grey bar was depicted for showing occurrenceof precipitation, since accuracy of measurement of
humidity profile decreased during events of precipitation. The WVC decreased in the lower layer in daytime
and increased in the upper layer clearly on the DOY 168,
169, and 170. The minimums in the lower layer andmaximum in the upper layer appeared in the evenings.
Such diurnal variations of WVCs had been observed
during the periods from spring to early summer in 2005
and 2006. We could observe 15 and 19 cases in 2005 and2006, respectively. Especially, it occurred frequently inJune in both of years. The lower panel of Fig. 4 shows
the variation of cloud amount and types. Active
cumulus appeared in the afternoon of each day. Notethat the observations of cloud amount and type by
visual survey of the sky from the ground were madeonly during the daytime. To compare this information
with the cloud amount for a larger area, cloud amount
was also estimated using equivalent black body temperature (Tbb) data derived from the Geostationary
Operational Environmental Satellite 9 (GOES-9); thesecloud amounts are depicted by open circles with lines in
the lower panel of Fig. 4. The larger area covered 5°×5°, the center of which was our observation site, and theresolution was 0.05°×0.05°. Areas in which Tbb was
below 270 K were treated as cloud areas. The estimations using GOES-9 data indicated that the cloudamount tended to increase in the afternoon, which wasapparent on DOY 168, 170, and 171, but was relatively
weak on DOY 169 and 172. These differences might
relate to development of active cumulus in the afternoon on each day. Thus, these cloud developments also
likely related to the development of the ABL.
Figure 5 shows the time height sections of thevertical wind velocity measured by the WPR on DOY

35

168, 169, and 170. The wind velocity showed strong
vertical mixing. Updrafts sometimes reached a height ofabout 5 km, in the afternoons of DOY 169 and 170. The
vertical motion in the lower layer was due to development of the ABL. However, the strong vertical motion of
the upper layer cannot be explained only by the ABLdevelopment. Active cumulus clouds, which appeared in
the afternoon as shown in the lower panel of Fig. 4, may
have caused the strong vertical motion, which might
have been linked to the vertical motion in the ABL.
Active cumulus can be considered to act as a pump thattransports water vapor from the lower to the upper atmosphere. This feature can be seen in Fig. 6 (a) illustrating the diurnal variation of the vertical profiles ofabsolute humidity on DOY 170 in 2005, when thevertical winds were strong in the afternoon. Four
profiles at different times are shown. Upward transportof air, which may be caused by convective clouds, canalso affect the atmospheric humidity. Water vapor canheat the upper layer by releasing its latent heat to thesurrounding air as a consequence of condensation.
Thus, the potential temperature increased in the free atmosphere above around 4 km under the condition of
cumulus convection (figure not shown). Such atmospheric heating in the upper troposphere was also
observed in spring at the Tibetan Plateau (Taniguchi
and Koike 2007). In contrast to this case, the verticalFig. 5. Time-height sections of the vertical winds observed on
168, 169, and 170 DOY in 2005. The colors indicate the verticalwind velocity. Red colors indicate upward wind and blue color
indicates downward wind.

Fig. 4. Diurnal variation in atmospheric water vapor content(WVC) measured on days of the year (DOY) 168 172 in 2005
(upper panel). The dashed broken line shows WVC from theground to a height of 10 km, the solid line shows WVC integrated from the ground to a height of 2 km, and the dotted line
shows WVC integrated from 2 to 10 km in height. Grey bar
shows event of precipitation. The lower panel shows theamounts and types of clouds. The colors of the bars have thesame meaning as in Fig. 3. Open circles with lines are cloudamounts estimated using black body temperature (Tbb)
measured by the GOES-9 satellite.

Fig. 6. Vertical profiles of 30-min-averaged absolute humidity
on day of the year (DOY) 170 (a) and 158 (b) in 2005. Dotted
line: 7:30 8:00 BST, dashed dotted line: 11:30 12:00 BST, andsolid line: 15:30 16:00 BST.

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Takahashi et al., Diurnal Variation of Water Vapor

winds were weak and a cumulus cloud was not observed on DOY 158 in 2005. As shown in Fig. 6 (b), thediurnal variations of humidity were relatively small inboth of the upper and lower atmosphere in such a case.

4. Discussion
In addition to the contribution from vertical mixing,
the horizontal advection of water vapor may influence
atmospheric profiles. DOY 168 was cloudy and active
cumulus clouds were observed in the afternoon (lower
panel of Fig. 4). In this condition, WVC also showeddiurnal variation (upper panel of Fig. 4), althoughstrong vertical mixing was not captured by the WPR(upper panel of Fig. 5). In this case, the contribution ofhorizontal advection of clouds might have been
dominant in the variation of WVC, although our observations were limited to vertical distribution, assuming
homogeneity in the horizontal direction.
Several previous studies have reported decreases in
PWV in the daytime over inland, mountainous, andcoastal areas and have concluded that local circulations
are important to this process (Takagi et al. 2000; Bastin
et al. 2007). Under such conditions, convective activity
might be closely related to a local circulation. In thisstudy, WVC in the lower layer began to decrease inearly morning on DOY 168, 169, and 170, as shown in
Fig. 4. A weak upward wind occurred in the lower layerfrom the ground to about 1 km in the morning each
day in Fig. 5. Such air motion may also have contributed to decreasing the WVC in the lower layer. Thus, alocal circulation may have formed over the tablelandand influenced the diurnal variation in WVC (e.g.,Z
angle and Chico 2006), which also might influence
diurnal change of potential temperature above a heightof about 4 km.
On the other hand, diurnal variation of atmosphericwater vapor due to cloud development is also likely to
be influenced by weak synoptic subsidence reaching aheight of about 3 km, which helps active cumulus todevelop easily, and also by the convective activity thatis transported eastward from the Tibetan Plateau (Asaiet al. 1998). The active cumulus clouds on cloudy days
(e.g., DOY 168 and 171) and during the nighttime on
DOY 170 in Fig. 4 were likely caused by active cloud
convection transported from the eastern Tibetan
Plateau, which was shown in the Tbb data from GOES-9(figures not shown). In this context, the period withweak activity of cumulus clouds until mid-June (lower
panel of Fig. 3) may have been related to a resting phaseof cloud activity over the eastern Tibetan Plateau; sucha phase was reported by Fujinami and Yasunari (2001)
and Yasunari and Miwa (2006). These findings suggestthat the convective activity over the Loess Plateau is
influenced by that of the eastern Tibetan Plateau.
However, in the daytime on DOY 169 and 170, active
cumulus was considered to have developed locally, since
the GOES-9 Tbb data showed no horizontal advection ofactive cloud convection from the outside region (figures
not shown). In such a situation, development of the ABL
over the plateau was related to cloud development, andcumulus cloud convection had a critical role in thevertical transport of water vapor.

5. Summary
This study revealed characteristic diurnal variationsof WVC in the atmosphere over the southern part of the
Loess Plateau in summer. Water vapor decreased fromearly morning until late afternoon in the atmosphere
within the ABL and increased in the free atmospherewhen active cumulus clouds developed. A strong
vertical wind was generated in the afternoon followedby the development of the ABL. Such strong convective

activity was especially apparent when cumulus clouds
developed in the afternoon. The enhanced vertical wind
induced by the active cumulus clouds may play a
critical role in diurnal variation in atmospheric water
vapor, although a local circulation may also be generated and contribute to the diurnal variation in WVC.
Future research should investigate whether a local circulation is formed over the complex terrain of the Loess
Plateau. Further studies will also be needed to clarify
the diurnal variations in water vapor in the troposphere
over the Loess Plateau, detailing how such variation is
generated and how it influences the water vapor transport for this region.

Acknowledgments
We thank the two anonymous reviewers and theeditor for their helpful comments to improve this study
significantly. This study was conducted with financial
support from the “Recent Rapid Change of Water Circulation in the Yellow River and Its Effects on the Environment” research project of the Research Institute for
Humanity and Nature, Japan.

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Manuscript received 11 January 2008, accepted 27 March 2008
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