Vertical Circulation in the Convective Boundary Layerover the Huaihe River Basin in China Observed by Wind Profiler Radar∗Satoshi ENDO1 , Taro SHINODA1 , Hiroki TANAKA2 , Tetsuya HIYAMA1 ,Atsushi HIGUCHI3 , Hatsuki FUJINAMI2 , Koh YAMAMOTO1 , Shoichiro IKEDA4 ,Hiroshi UYEDA1 , and Kenji NAKAMURA1( 1:HyARC,Nagoya Univ. 2:JST/CREST 3:CEReS,Chiba Univ. 4:Okayama Univ.)

1. Introduction
Atmospheric Boundary Layer (ABL) has an important role as an interface between free atmosphere and land surface.
Land surface provideheat,momentum,water vapor,and other scalars to atmosphere through ABL process. ABL also shows avariety of behavior aﬀected by the land surface andsynoptic condition. In particular, surface forcings are
dominant in ABL by deﬁnition (Stull 1988).
Research of upper ABL has progressed with development of wind proﬁler radar (WPR) (Ecklund etal., 1988; Angevine et al., 1998; Yi et al. 2000). But
less observation using WPR had carried out over humid continental area. Lower Atmosphere and Precipitaion Study (LAPS) / Core Research for Evolutional Science and Technology (CREST) have implemented continuous observation at Shouxian,Anhuiprovince,China. This site is located in the large
plain along the Huaihe River (Fig. 1). Farm landaround the site is nearly uniform and ﬂat, and two
crops,wheat and paddy,are cultivated a year. This
site is suitable for boundary layer experiment to study
the eﬀect of land surface.
In order to understand the convective boundary
layer (CBL) under the condition, we focus on verticalcirculations which characterize the CBL. Turbulence driven by buoyancy is organized into local
updrafts called thermals or plumes. The thermal
updrafts and interthermal downdrafts forms circulation (Stull 1988). These circulations transportheat,momentum,water vapor,and other scalars. As aresult,a quasi-uniform CBL is formed, and the various
physical values are distributed to each level.

2. Data and Analysis Method
The monitoring observation usisng WPR,Doppler
sodar,microwave radiometer,and ﬂux tower has been
continued since August 2003. In this study,we used
WPR and surface data obtained during IOP (fromMay to July,2004). During IOP, we have manual observational data and high quality of monitoring observational data. Table 1 shows operating characteristicsof the WPR. The frequency is 1290 MHz for observation of lower atmosphere. The height and temporal resolutions are 100 m and about 59 s,respectively.
Surface ﬂuxes were calculated from the data observed
by ﬂux sensor and radiative sensor set at a height of30 m of the tower.
We used two methods to estimate the CBL top fromthe Signal to Noise Ratio (SNR) of the WPR. Proﬁlesof the SNR averaged during 30 minutes are used inboth methods. The ﬁrst is the height where the SNR
is maximum (Angevine et al. 1994). The SNR depends on refractive index structure parameter,C2n . InCBL,the C2n is greatly aﬀected by gradient of watervapor. Since there is much diﬀerence of water vapor
between free atmosphere and CBL, the SNR showsits maximum at a CBL top. The second is the height
where the gradient of the SNR is minimum. The SNR
sometimes shows a large value in a humid CBL. TheSNR decreases steeply with height over the CBL top.
Table 1.
WPR.

Fig. 1. Topography around observation
point,Shouxian (◆).

Corresponding author:

Satoshi ENDO, Hydrospheric At-

mospheric Research Center, Nagoya University,Nagoya,4648601,Japan.
Tel: +81-52-789-3494

Fax: +81-52-789-3436

E-mail: endo@rain.hyarc.nagoya-u.ac.jp

Operating characteristics of theFrequency
AntennaBeam width
Peak power
Pulse width
IPP
Gate spacing
Temporal resolution

1290 MHz
Phased array5◦ or less
2 kW666 ns
80000 ns
100 m
59 s

Both methods are not perfect but we use these as
rough indications.

20040531 04-21 Range Collected SNR3000height [m]3. Results
3.1 Surface variables during IOP
During IOP,vegetation changed from mature wheatﬁeld to bare ﬁeld,and to paddy ﬁeld in turn.
There is obvious diﬀerence between the former period which vegitation is mature wheat or bare ﬁeld
and the latter period which vegitation is paddy. Although net radiation in the latter period is larger thanthat in the former period,sensible heat is smaller. In
the latter period,much energy was partitioned to latent heat ﬂux as a result of much water existance atsurface paddy ﬁeld.

3010002010surface fluxes [Wm-2]

0045678910111213141516171819204567891011121314151617181920800
600
400
200
0-200

local time [hour]Fig. 2. Time-height cross section of SNR(upper picture),and time series of surﬀace
ﬂuxes on 31 May. An open square (□) indicates the height where the SNR shows its maximum at a time. An open circle (○) indicatesthe height where the gradient of SNR showsits minimum at a time. Broken,solid,and dotted lines represent net radiation,sensible heat
ﬂux, and latent heat ﬂux,respectively.

20040531 13-15, w, WPR at Shouxian

height [m]

3000

2000

1000

0
13.0

13.5

14.0
local time [hour]

14.5

15.0

Fig. 3. Time-height cross section of verticalmotion on 31 May. The contours are drawn
every 2 m s−1 . Shaded area indicates down
draft.

20040622 04-21 Range Collected SNR
3000

height [m]3.3 Vertical circulation in CBLIn order to ensure a presence of a periodic circuration, and determine its frequency, we calculate
power spectra from vertical velocity during 2 h ateach height. (Figs. 5 and 6) A larger spectral density
represents larger energy.
On 31 May,the case over bare ﬁeld, periodic circulation existed in the CBL (Fig. 5). The height andfrequency of the circulation broadened with the evolution of the CBL. On the spectra calculated fromvertical motion between 09 and 11 LST, a peak appeared only at 0.0004Hz (40 min. cycle) near a heightof 1000 m. On the spectra beteen 13 and 15 LST,
peaks spread from 0.0005 to 0.0018 Hz (from 10 to 30min. cycle), and ranged from a height of 400 to 1700
m. The height of the peaks was limited below the40[dB]20004030100020100

surface fluxes [Wm-2]

3.2 Surface fluxes and CBL development
We select two clear days,31 May and 22 June, as arepresentative of each period.
On 31 May,a deep CBL developed early andrapidly. The diurnal change of surface ﬂuxes and CBLstructure on 31 May is shown in Fig. 2. The wheat
had already been harvested.The land surface was bareﬁeld. Sensible heat ﬂux kept between 200 and 400 W
m−2 from 09 to 16 LST. The CBL provided enoughsensible heat started to increase its depth from earlymorning. About 14 LST,the height of CBL top attained its maximum. The height was about 2300 m.
Updraft and downdraft repeated periodically in theCBL (Fig. 3).
On 22 June,a shallow CBL developed late andslowly (Fig. 4). Much water existed at paddy ﬁeld.
Therefore,sensible heat ﬂux was much smaller thanlatent heat ﬂux. Sensible heat ﬂux was about 100
W m−2 . In contrast,latent heat ﬂux was about 500
W m−2 . Before the real CBL developed, the largest
SNR was marked at a height of about 1000 m duringmorning. Marked large SNR over the real CBL top
was also seen in afternoon. The height of the real CBL
top increased the height gradually after late morning.
A maximum height of the CBL top was about 1400 m.
A repitition of the vertical motion was not noticeable(not shown).

[dB]

2000

04

56

7

8

910

1112

13

14

15

16

17

18

1920

4

5678

91011

12

131415

16

1718

19

20

800
600
400
200
0-200

local time [hour]Fig. 4. Same as Fig. 2 except for on 22 June.

Power Spectral Density with Height
3000

Power Spectral Density with Height
3000(a) 09-11LST

2000

2000

1500

1500

1000

m2s-1
1000

500
0.002

3000

0.004
0.006
frequency [Hz]

0.008

500

800

0.000

0.002

400

2000
1500

0

height [m]

0.004
0.006
frequency [Hz]

3000
200

1000

0.008

(b) 13-15LST

2500

400

200

2000
0

1500
1000

500
0.000

m2s-1
600

600

(b) 13-15LST

2500

1000

height [m]0.000(a) 09-11LST

2500
height [m]height [m]

2500

500
0.002

0.004
0.006
frequency [Hz]

0.008

0.000

Fig. 5. Power spectra of vertical motion ateach height during 2 hour. (a) is 09 - 11 LST,(b) is 13 - 15 LST on 31 May.

4. Discussion
The vertical circulation in the CBL concurrently
developed with the development of the CBL. The
mixing by this circulation made a quasi-uniform CBL
proﬁle. Therefore,the maximum SNR was seen at theCBL top. A driving force of this circulation was sensible heat ﬂux from land surface. Larger sensible heatﬂux made higher CBL as shown in the comparison of
the two cases. Heated air ascend,and form thermals.
The horizontal scales of the thermals were calculated
from the horizontal wind speed and the duration of
the updrafts (not shown). The scales were between
720 and 3600 m at early afternoon on both days.
Over bare ﬁeld,the thermal updrafts were strong. Themaximum vertical motion in the thermals was about
3 m s−1 . Over paddy ﬁeld,the thermal updrafts wereweak. The maximum vertical motion in the thermals
was about 1 m s−1 . Sensible heat ﬂux aﬀected inten-0.004
0.006
frequency [Hz]0.008

Fig. 6. Same as Fig. 5 except for on 22 June.
sity of the thermal updrafts.
A structure of the circulation was seen in vertical motion proﬁles averaged during 2 hours (Fig. 7).
The mean vertical motion was positive from surface
to the middle of the CBL. A percentage of updrafts
was about 50 %; a sum of the duration time of theupdrafts equaled that of the downdrafts. Both theupdrafts and the downdrafts showed its maximum inthis range, but the updrafts were stronger than thedowndrafts. The positive mean vertical motion was20040531 13 - 153000

(a)

(b)

2500

3000

2500

Zi
height [m]height of the CBL top estimated from SNR. Horizontal scales calculated using horizontal wind averaged
during 2 hours at each height were from 3 to 12 km(not shown).
On 22 June, the case over paddy ﬁeld, weak andwide peaks were detected only at the low altitude below the height of the CBL top (Fig. 6). The CBL was
too shallow to adopt the analysis between 09 and 11
LST. On the spectra calculated from vertical motion
between 13 and 15 LST, weak peaks spread discretelyfrom 0.0004 to 0.0034 Hz (from 5 to 60 min. cycle)and ranged from a height of 300 to 800 m. The calculated horizontal scales of the circulations were from
1.3 to 12 km (not shown). The large SNR over theCBL top corresponded with the large variance froma height of 1500 to 2300 m.

0.002

2000

2000

1500

1500

1000

1000

500

500

00
-1.5

-1.0

-0.5

0.0

0.5

mean w [m s -1]

1.0

1.5

025 50 75 100

percentage of updrafts [%]Fig. 7. Averaged vertical motion proﬁles(a) and a percentage of the sum of the duration time of the updrafts (b) between 13 and
15 LST on 31 May. Solid line in (a) indicates an average of all vertical motion during 2hours. Broken and dotted lines indicate averages of updrafts and downdrafts, respectively.
Zi indicates the range which the CBL top wasestimated from the maximum SNR during 2hours.

caused by the intensity of the thermal updrafts. The
mean vertical motion was negative from the middleof the CBL to little above the CBL top. Both the updrafts and the downdrafts weaken near the CBL top.
The percentage of the updrafts markedly decreasedwith height. The negative mean vertical motion wascaused by the long duration time of the interthermal
downdrafts. Considering the law of conservation ofmass, the time-averaged vertical motion should equal
zero. These values would mean local convergence anddivergence which have a larger scale than the circulation.
In order to understand the observed cases better,it needs to examine three dimensional structure. For
the purpose,we are simulating the CBL using highresolution numerical model now.

5. Conclusion
The CBL development over the humid continental area were observed by WPR. Over bareﬁeld,the CBL provided large sensible heat ﬂux developed rapidly,early,and deep. Over paddy ﬁeld,theCBL provided small sensible heat ﬂux developed
slowly,late,and shallow. By the analysis using power
spectra of vertical motion with height, the characteristics of the vertical circulation and its time evolution
were investigated about both case. The peaks of thecirculation ranged only below the height of the CBL
top, and broadened the height and frequency withthe evolution of the CBL. The mean vertical motion
was positive from surface to the middle of the CBL
because of the intensity of the updrafts. The mean
vertical motion was negative from the middle of theCBL to little above the CBL top because of the long
duration time of the interthermal downdrafts.

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