Surface flux and atmospheric boundary layer observations
over the middle of the Huaihe River basin in China
Hiroki Tanaka*,1, 2, Tetsuya Hiyama1, Koh Yamamoto1, Hatsuki Fujinami1, Taro Shinoda1,
Atsushi Higuchi3, Satoshi Endo1, Shoichiro Ikeda4, Weijing Li5, and Kenji Nakamura1
1

Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya 464-8601, Japan
Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation,
Kawaguchi 332-0012, Japan
3
Center for Environmental Remote Sensing, Chiba University, Chiba 263-8522, Japan
4
Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
5
National Climate Center, Chinese Meteorological Administration, Beijing 100081, China*
E-mail: hiroki@hyarc.nagoya-u.ac.jp (Tel: +81-52-789-3439; Fax: +81-52-789-3436)
2

1. Introduction
Interactions between the surface and the atmospheric boundary layer (ABL) at various temporal and spatial
scales are an important research topic in hydrology, meteorology, and climatology. Surface conditions, including
soil moisture and vegetation cover, affect ABL structure, cloudiness, and precipitation (Wai and Smith, 1998).
Simultaneous observations of surface and ABL processes under various surface conditions are required tounderstand the interactions involved. Wind profiler radar (WPR) recorded ABL measurements concurrently withflux measurements above the surface in the First International Satellite Land Surface Climatology Project(ISLSCP) Field Experiment (FIFE) (cf., Sellers et al., 1992) in 1989. Wai and Smith (1998) showed that soil
moisture affected ABL development. Furthermore, biophysically induced ABL forcing associated with a gradientin organized vegetation cover can sustain mesoscale circulations within a ~16 km2 domain. Angevine et al.
(1998) reported the preliminary results of simultaneous observations of WPR and surface flux from the Flatland
Project in 1996, and described diurnal and seasonal variations in ABL depth and surface flux for flat terrain in
Illinois, USA.
The simultaneous observations reported here will fill the observation gap for the Asian monsoon region.
The Asian monsoon 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 an important role in the regional climate. Changes insurface conditions alter surface heat and water vapor fluxes that in turn affect ABL structures. ABL processes are
one of many triggers that generate clouds and precipitation. The ABL also has important hydrological aspects.
The ABL is the interface between the surface and the free atmosphere, and it strongly influences vapor transport,sensible heat flux over the surface, and convective precipitation systems. Clarifying ABL development
mechanisms on diurnal and seasonal time scales will help to elucidate regional hydrological budgets
ABL depth, or the height of the top of the daytime convective mixed layer, is an important parameter instudies of the ABL. ABL-WPR detects the ABL top as the level with maximum radar scattering, or WPR echo,
because the entrainment layer has large scalar gradients and therefore many turbulent structures (cf., Angevine etal., 1994, 2001; White et al., 2002). The WPR echo, or backscattered power is defined by the radar reflectivity η
per unit volume, which is proportional to the refractive index structure parameter Cn2 and the radar wavelength
as follows:η = 0.38C n2 λ −1 / 3.
(1)where λ is the radar wavelength. The structure parameter Cn2 represents the intensity of turbulence, and strongly
depends on gradients of vapor density.
Tennekes (1973) used conservation of virtual temperature within the ABL to estimate ABL depth from theaccumulated surface virtual temperature flux and the vertical motion at the top of the ABL. If vertical motion at
the top of the ABL and horizontal advection are both negligible, ABL depth is related to surface heat flux asfollows (cf., Garratt, 1992):(z 0 + ∆z )2 = z 0 2 + 2 (a + 2ε )QSΓ(2)Here z0 is the initial depth of the ABL, ∆z is the increment of the depth for a reference duration, Γ is the lapse
rate of potential temperature above the ABL, ε is an 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 the surface-ABL relationship before, during, and after the
rainy Meiyu season in China. One goal was to understand how 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 is suddenly
moistened during the Meiyu. Humidity persists in the lower atmosphere during the Meiyu season even when theMeiyu front temporarily moves south of the region. Thus, another goal was to describe the vapor source thatprovides or maintains such high humidity throughout the season. The presence of high humidity allows relatively
small triggers to generate clouds and precipitation. Synoptic-scale water vapor transport and human activities,
such as agricultural irrigation and cultivation, can influence ABL moisture conditions through surface-ABLinteractions. Surface and ABL monitoring in this flat region before, during, and after the Meiyu will provide
important data.
2. Methods
2.1 Site description and observation items
The mid-latitude study site was a flat surface region located 22.7 m A.S.L. in Shouxian, China, at 32.55 N
and 116.78 E. The site is located near the Huaihe River in Anhui Province, China. Instrumentation at the studysite included an ABL wind profiler radar (L-28, Sumitomo Electric Industries, Ltd, Japan), 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 lands are mainly agricultural fields, withsome residences and small buildings that are often surrounded by trees. A paved road passes 200 m west of thesite, and there is an irrigation canal 2 km west of the road. Lake Wabuhu is 7 km southeast of the site, and the
Huaihe River passes 9 km northwest.
Monitoring for the LAPS project started in August 2003 and is ongoing, although interruptions haveoccurred because of power-supply problems, instruments destroyed by strong wind or lightning, and otherproblems. Intensive observations, including vegetation research, were recorded during the intensive observation
period from 24 May through 16 July in 2004 (IOP-2004). Land use surrounding the site changed from wheat
field to rice paddy field during IOP-2004.
IOP-2004 included observations of vegetation cover surrounding the study site. Surface-layer variables
include turbulent fluxes, radiation fluxes, wind speed, wind direction, air temperature, atmospheric humidity, soil
temperature, soil moisture, and precipitation rate. These were all measured with a surface-layer observation
system on a 30-m tower and two 3-m masts. ABL parameters, including wind vectors and a turbulence structure
parameter, were measured using a 1.29-GHz WPR. The original WPR data have a time interval of about 59 s,
and a height resolution of 99.9 m for vertical beams and 96.4 m for oblique beams. Data in this study were
gridded with two-dimensional interpolation to ease data inspection. The time interval of the gridded data is 30
s, and the height resolution is 50 m.
2.2 Estimation of the ABL depth from WPR data
The ABL depth can be estimated theoretically or empirically as the height with the maximum echo received
by WPR. However, such estimates sometimes fail because of large returns from clouds above the ABL or fromstrong near-surface turbulence. Such failures can be avoided by estimating ABL depth over much longer time
periods (i.e., more than 2-4 h), or by limiting the height range in which the ABL top can appear. The lattermethod allows fluctuations within a short time-scale and was chosen for this study. Assumptions that limited
the possible height range were that the convective mixed layer started to develop just after sunrise, around 06:00
local time (LT), that the peak height appeared between 12:00 and 16:00 LT, and that the height remained withinapproximately the same range after 16:00 LT. This diurnal evolution of ABL height was usually valid for days
without significant clouds or precipitation.
3. Results and Discussion
3.1 Overview of the climate and surface conditions
Seasonal changes in the volumetric soil watercontent of the non-irrigated ground near the flux
tower, the relative humidity on the tower, and daily
precipitation was observed. Soil water content
increased following precipitation events. Atmospherichumidity was high during the summer. Soils weregenerally moist during the entire year, except near thesurface during some dry spells in winter and early
spring. The ground was much wetter during summer,reflecting heavier summer rainfalls. A land-use shift
was observed during IOP-2004. Mature wheat(Albedo α ~ 0.14) was the dominant vegetation
between 24 May and 30 May, at which time it was
harvested (α ~ 0.16). Wheat litter was burned (α ~0.07) between 1 and 4 June. The black ash was
Figure 1. Seasonal changes in midday virtual sensible
plowed under (α ~ 0.12), and rice seedlings were
heat, latent heat, and CO2 flux averaged between 08:00
planted and flooded (α ~ 0.09) on 14 June. The
and 16:00 LT, and short-wave albedo averaged between
11:00 and 13:00 LT.
planted seedlings were 0.2-0.3 m tall. The seedlings

grew to 0.7-0.8 m (α ~ 0.15) on 16 July, the end of IOP-2004.
Water vapor was transported by southerly synoptic-scale winds on 14 June, when irrigation started. After
this day, moisture persisted in the lower atmosphere, even when drier air moved in from the north, i.e., when the
Meiyu front pushed south. The sudden increase in atmospheric moisture may have been driven by synoptic-scalemoisture transport, and partly by irrigation. Maintenance of high humidity within the ABL likely resulted fromsurface water during the wet season.
Figure 1 shows 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 sensible heat flux, is shown in Figure 1 and subsequent figures to account more clearly forbuoyancy effects. The two fluxes differ by approximately 6 Wm-2 when the latent heat flux is 100 Wm-2. Virtual
sensible heat flux Hv is defined as:

HV = H + 0.514C P TV E

(3)
where CP is the specific heat of air, Tv is the virtual temperature, and E is the water vapor flux. Changes in land
use clearly affect the parameters. Rice fields were present during summer, and high rates of CO2 absorption andevapotranspiration, and small values of sensible heat flux, indicate growing rice during August 2003. Mature rice
is indicated in September 2004 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. Wheatmatured during May, when CO2 absorption and vapor 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 vapor flux, and albedo increased, and sensible heat flux decreased.

Figure 2. 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 indicates net
radiation, red indicates virtual sensible heat flux, andblue indicates latent heat flux.

Figure 3. Time-height sections of echo intensity
obtained from wind profiler radar observations on
the same days as in Figure 2. Atmosphericboundary layer depth was estimated every 2 min(black dots).

3.2 Diurnal development of the ABL depth
A convective mixed layer was present during the day and strong WPR echoes were detected. The ABL
depth clearly increased from soon after sunrise through the afternoon. There were strong horizontal winds andsudden changes in wind direction above the top of the ABL in daytime. ABL depth fluctuated as echo intensity
within the ABL varied, being periodically enhanced by plume-like structures in the upward, which alternatedwith downward wind structures. The period was 30-60 min on typical fine days. The plume-like upward windextended to the top of the ABL at midday. Plumes did not reach the 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 have been recognized in the past as contamination from migrating birds. Wilczak et al.
(1995) considered possible reasons behind the appearance of strange echoes. Kobayashi et al. (2005) concluded
that migrating birds can scatter the WPR beam, and proposed a method to eliminate this effect from observed
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 thesubject of future study. The following sections focus on ABL development during the day.
3.3 Seasonal changes in the ABL depth
Figure 2 shows diurnal changes in energy fluxes over the surface on 7 clear days. Wheat was sprouting on
about 28 January, growing on 6 March and 24 April, mature on 20 May, and had been harvested by 31 May.
Subsequently, rice was growing on 22 June and 3 July. Virtual sensible and latent heat fluxes had similar diurnal
traces that tracked diurnal changes in net radiation before the wheat was harvested. When the wheat was mostactively growing, latent heat flux slightly exceeded sensible heat flux (cf., on 24 April). Latent heat flux
decreased between wheat harvest and field flooding (cf., 31 May). Sensible heat flux dropped and the latent heat
flux increased dramatically after the field was flooded and rice seedlings were planted (cf., 22 June and 3 July);
the peak latent heat flux shifted to later in the day because the high heat capacity of the irrigation water allowed
higher temperatures to persist into the evening.
Figure 3 shows the echo intensities and ABL depths recorded by the WPR on the same days as in Figure 3.
The ABL depth was derived every 2 min using the procedures outlined above. 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 ABL development in the morning. The ABL merged with the residual layer, andconsequently 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 distinct until 13:30 LT on 22 June, a day when riceseedlings were growing after irrigation. Much stronger echoes within the ABL on that day indicate turbulent
structures associated with large humidity gradients. High humidity resulting from strong vapor water flux overthe surface affected the ABL structure. Daily maximum ABL depth occurred around 1500 m on 22 June and 3
July, even though the virtual sensible heat flux was smaller. Surface buoyancy flux does not explain such ABLdevelopment.
Figure 4 shows daily maxima of ABL depth and the virtual sensible heat flux averaged between 08:00 and
14:00 LT. There is a relationship between ABL depth and midday averaged heat flux before the field was
irrigated, especially in winter. However, the relationship is less clear after irrigation. Also plotted in the figure isthe vertical pressure velocity ω at 925 hPa at 00Z (08:00 LT) at 32.5 N and 117.5 E from the National Center for
Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al.,1996), which represents an atmospheric vertical motion. The plotted vertical motion is near or slightly above the
top of the ABL, which was usually close to or below 925 hPa at 08:00 LT. The vertical motion is representativeof a 2.5 x 2.5-degree area between 31.25-33.75 N
and 116.25-118.75 E, a region that includes thestudy site. Vertical motion can be linked todiscrepancies between ABL depth and heat flux,
especially when the heat flux was low after
irrigation.
Figure 5 shows the relationships between
daily ABL depth and midday averaged virtual
sensible heat flux for (a) the entire study, and forfive periods including (b) winter (time = 0-59), (c)early spring (time = 60-89), (d) the growth periodof wheat (time = 90-139), (e) the period with no
Figure 4. Seasonal changes in the daily maximum observed
vegetation after the wheat had been harvested andABL height (bottom column, bars), observed virtual
before irrigation (time = 140-164) and (f) after
sensible heat flux (bottom column, rectangle with lines)
irrigation (time = 165-197). A positive trend is
between 08:00 and 16:00 LT, and the 925-hPa verticalevident in all periods except in period (f) afterpressure velocity (ω; upper column, bars) extracted from
irrigation. The scatter in Figure 6 can be explained
the NCEP/NCAR reanalysis at 00Z (08:00 LT) at 32.5 N,
partly by atmospheric vertical motion after
117.5 E. Negative ω indicates upward motion.

irrigation and in other periods except period (e).
Downward motion suppressed ABL development(cf., downward triangle in Figure 6), whereas
upward motion enhanced it (cf., upward triangle in
Figure 6). Other explanations for scatter may
include residual layer effects and horizontal
advection effects, which should be considered infuture studies.
4. Concluding remarks
A preliminary analysis of surface and ABL
observations revealed relationships between
surface fluxes and the ABL structure. Buoyancy
flux enhanced ABL development. Fluctuations in
ABL depth were related to plume-like windstructures within the ABL. Daily variations in ABL
depth were controlled mainly by the buoyancy flux
over the surface during dry periods. Variations
were strongly affected by atmospheric verticalmotion during wet periods such as the Meiyu
season. Further analysis to investigate how ABL
depth is affected by the combined effects oftemperature and humidity profiles in addition tosurface fluxes is warranted.
The source of the increased atmospherichumidity during the onset of Meiyu, and themechanism that maintains humidity during the
Meiyu season, may be surface vapor flux caused byirrigation. Humid conditions affect ABL structure
Figure 5. Relationships between daily ABL depth and midday
and development. In addition, synoptic-scale vapor
averaged virtual sensible heat flux. Filled circles indicate
transport may increase atmospheric humidity
neutral motion (-0.02 < ω < 0.02), upward empty triangles
during the onset of Meiyu. Relationships between
indicate ascending motion (ω < -0.02), and downward filledlocal phenomena and synoptic-scale humidity must
triangles indicate descending motion (ω > 0.02), where ω is
be studied with further observations, satellite
the 925-hPa vertical pressure velocity from theremote sensing, and reanalysis data such as
NCEP/NCAR re-analysis shown in Figure 4.
NCEP/NCAR. Projections of water vapor transport
at synoptic scales, i.e., weather forecasts, influence agricultural schedules. Interactions between anthropogenichydrological activities and atmospheric phenomena also warrant future study.
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