Okinawa 2004 and 2005 IOP Experiments
Fumie Akimoto Furuzawa1 , Jin Takahashi1 , Kenji Nakamura1 , Katsuhiro Nakagawa2 , Shinsuke
Satoh2 , Shinya Sekizawa2 and LAPS team.
1: Hydrospheric Atmospheric Research Center, Nagoya University
Furocho, Chikusaku, Nagoya 464-8601, Japan
2: National Institute of Information and Communications Technology (NICT) Okinawa SubtropicalEnvironment Remote-Sensing Center
4484, Aza-Onna, Onna, Kunigami, Okinawa 904-0411 Japan
Tel: +81-52-789-5467, Fax: +81-52-789-5467
akimoto@hyarc.nagoya-u.ac.jp
Abstract
Okinawa 2004 and 2005 Intensive Observation Period (IOP) experiments of Lower Atmosphereand Precipitation Study (LAPS) project were performed over land mainly during summer fairweather 14 days with high pressure conditions, continuously during daytime and nighttime. It isthought that the solar heating of land surface causes thermal plumes to rise and expand until athermodynamic equilibrium is reached at the top of the atmospheric boundary layer (ABL) andthe moisture transferred by the thermal plumes forms convective clouds. Our scientific objectivesof these experiments are understandings of structures of the ABL and the formation and evolution.
Because the convective air motions in the ABL generate intense turbulent mixing, we can explore
them by analyzing clear-air echoes (CAE) formed by the turbulence through Bragg scattering, andobserved by C-band polarimetric radar and 400 MHz wind profiler. GPS radiosonde soundings,
automatic weather stations, and optical VIDEO observation were also done. Following results wereobtained; 1) a height of CAE corresponds to that of large gradient in air refractive index. 2) even
if a strong synoptic wind blows, CAE is able to be observed. 3) one type of shapes of CAE is like atemple bell. It is filled with echo under developing phase, and it is not filled under developed phase.
Moreover, the CAE is rising and expanding under developing phase as moving with synoptic wind.
Another type is a stratiform type. It appears mainly at nighttime. 4) the higher the CAE heightis, the larger the turbulent scale. However, the structure of the atmospheric boundary layer has
not yet been clarified enough.

1

Introduction

To investigate the Atmospheric Boundary Layer (ABL), we performed field experiments in Okinawa, subtropicalregion of Japan from August 30 to September 4 on 2004 and from June 29 to July 10 on 2005 as one of Lower
Atmosphere and Precipitation Study (LAPS) projects. There is a 400 MHz-band Wind Profiler Radar (WPR) at
Ogimi site of National Institute of Information and Communications Technology (NICT) Okinawa SubtropicalEnvironment Remote-Sensing Center and a CRL Okinawa Bistatic polarimetric Radar (COBRA) at Nago siteof the center. Their site positions are plotted in Fig. 1. The view of the 400 MHz WPR is shown in Fig. 2,
and the design parameters and performance of the 400 MHz WPR are summarized in Tab. 1. Those of the
COBRA are shown in Fig. 3 and Tab. 2. Details of this WPR and COBRA are written in Adachi et al. (2001)
and Nakagawa et al. (2003), respectively. Automatic weather station (AWS) and other various atmosphericsensors are also established in these sites. They are always operated. But, the COBRA observation mode usedduring these IOP experiments was changed to observe the ABL between Nago site and Ogimi site, in detail,both spatially and both temporally. This mode includes Plan Position Indicator (PPI) observations with theelevation angle of 7 and 8 degrees (1.4 degree are added during 2005 IOP) and 14 Range Height Indicator (RHI)
observations with the azimuth angle of 41.2 degree from the north to the east during 10 minutes, as shown in
Tab. 3. We also performed GPS radiosonde soundings observation at the Ogimi site, every two or three hours
during both daytime and nighttime. GPS radiosonde soundings of Vaisala RS80-15G were used. Moreover,
we did video observation at two points (at 2004) or three points (at 2005) around Ogimi site towards the ∼1
km aloft of the Shioya Bay, where COBRA ground clutter is very small and which is the best point to see theclear-air echoes (CAE) of the COBRA. Information of video observation is summarized in Tab.4. Satoh et al.
will present the results of COBRA data observed during the 2004 IOP experiment in this workshop.

2

CAE theory

Generally, rain radars, like the COBRA, radiate strong microwave and observe weak microwave reflected byrain drops through Rayleigh scattering or Mie scattering. At fair-weather days, they can observe variations ofFigure 2: View of the 400

Figure 3: View of the

MHz WPR.

COBRA.

Figure 1: Map of observation sites.
refractive index due to atmospheric turbulence at the top of ABL through Bragg scattering. At the scale ofatmospheric turbulence, is a half of radar wavelength, the back scattering (CAEs) is the strongest (Marshall1972). The atmospheric refractive index, n, is approximated by a following equation (Bean and Dutton 1968);
n2 ∼ 1 + M v a T .

(1)Here, Mv is the molecular number density and aT is related to molecular polarization. Because the n is nearly
equal unity, a refractivity N (N unit) defined by a following equation is practically used;N ≡ (n − 1) × 106 =

4810e
77.6
(p +
).
T
T

(2)

Here, T is temperature (K), p is atmospheric pressure (hPa) and e is water vapor pressure (hPa). This equation
indicates that a e is the most effective. Rain radars can not always observe the back scattering signals. They
can be observed only when the difference of N at each air layer is large. Moreover, because this back scattering
is very weak, the radar sensitivity must be very high. On the other hand, wind profiler can observe weakvariations of ABL. It is worthful to compare back scattering signals between the COBRA and 400 MHz WPR.

3

Data and Analysis

Two-dimensional data sets of reflective intensity; Z, Doppler velocity; v [m s−1 ] and several polarization parameters (differential reflectivity; ZDR (= ZHH /ZV V ), differential propagation phase; φDP [deg] (two ways)(= φHH − φV V ), specific differential propagation phase; KDP [deg km−1 ] (= φDP /two-way-distance), linear
depolarization ratio; LDR (= ZHV /ZHH ), cross correlation coefficient between ZHV and ZHH ; ρHV ) during
two IOP experiments were obtained from the COBRA data. The ZDR and so on depend on the difference
between vertical and horizontal size of targets. For spherical case, ZDR is nearly 0 dB, LDR is small (−34 ∼−27) and ρHV is nearly unity. On the contrary, for non-spherical case, |ZDR | is large, LDR is large, and ρHV is

Table 1: Design parameters and performance of the 400 MHz WPR
frequency
wavelength
peak power
pulse widthantenna styleantenna size
beam widthbeam direction
pulse repetition frequency (PRF)
maximum detectable distanceposition

443 MHz67.7 cm
20 kW
1.33, 2.0, 4.0 µs
Two sets of 24-elements linear array (orthogonal arrangement)
10.4 m × 10.4 m
3.3◦
0◦ (zenith angle) / 10.5◦ (zenith angle), 0, 90, 180, 270◦ (azimuth)
6.25, 20 kHz
24 km @ PRF = 6.25 kHz, 7.5 km @ PRF = 20 kHz
Ogimi: 26.6781◦N, 128.1589◦E (sea-level 225 m)

Table 2: Design parameters and performance of the COBRA
frequency
wavelength
antenna diameter
redome diameterTransit Polarization pattern
peak power
antenna gain
beam widthside-lobe level
pulse width
PRF
Doppler estimation
cross polarization discrimination (XDP )antenna rotation velocity
position
spatial resolution (line of sight)
PRF [kHz]
integration number
noise level
maximum observable wind velocity [m s−1 ]maximum detectable distance [km]maximum recorded distance [km]Table 3: Sequenceof the COBRA
time
PPI scanRHI scan10 min.
7, 8◦ @2004
7, 8, 1.4◦ @2005
41.2◦ × 14

5340 MHz (C-band)
5.6 cm
4.5 m φ (parabola)
8mφ
H,V,45◦ (45◦ @ 2004, HV @ 2005)
250 kW (Dual Klystron)> 42 dB (included redome)
0.91◦< −25 dB (one way)
0.5 µs (used), 1.0 µs, 2.0 µs
250 ∼ 3000 Hz
Pulse-pair / FFT> 36 dB (integrated within the beam)
0.5 ∼ 10 rpm (2 rpm used) @ horizontal (0.1 rpm step)
0.1 ∼ 3.6 rpm (0.5 rpm used) @ vertical (0.1 rpm step)
Nago: 26.5872◦N, 128.0644◦E (sea-level 343 m)
31 m @ 2004PPI, 75 m @ (2004RHI, 2005PPI, 2005RHI)
0.75 @ 2004PPI, 1. @ 2004RHI, 1.111 @ 2005PPI, 1.786 @ 2005RHI
64 @ 2004RHI, 128 @ 2005RHI−104.4 dBm @ pulse width of 0.5 µs± 10.5 @ 2004PPI, 14 @ 2004RHI, 7.8 @ 2005PPI, 12.5 @ 2005RHI
200 @ 2004PPI, 150 @ 2004RHI, 135 @ 2005PPI, 84 @2005RHI
20 @ (2004PPI, 2004RHI, 2005RHI), 40 @ 2005PPI

Table 4: VIDEO information
IOP
2004
2005

position
26.675◦N, 128.155◦E
26.664◦N, 128.122◦E
26.676◦N, 128.148◦E
26.664◦N, 128.122◦E
26.647◦N, 128.133◦E

azimuth∼221◦∼130◦∼222 ◦∼138◦∼342 ◦

elevation∼7◦ (from Ogimi to COBRA)∼15◦ (perpendicular)∼5 ◦ (from Ogimi to COBRA)∼15◦ (perpendicular, north-side)∼10 ◦ (almost perpendicular, south-side)smaller than unity. The differences between CAE and rain are that Z of CAE is very small and ρ HV of CAE islower than 0.9. The difference between CAE and ground clutter is that v of CAE is nearly equal to the synoptic
wind though that of ground clutter is nearly zero. To mask the ground clutter component dominant pixels, thecriteria that v is not zero, 0.4 < ρHV < 0.9 were used.
Profile data sets of reflective intensity and spectrum of wind velocity were from the 400 MHz WPR. Thoseof temperature, mixing ratio, wind direction and speed, and refractive index, etc were from GPS radiosonde
soundings. Surface temperature and surface humidity, etc were from the AWSs at two observation sites.

4

Results

4.1

Radio Sonde

We show the mixing ratio and wind information observed during 2004 and 2005 IOP experiments in Figs. 4
and 5, respectively. The ABL height was determined because potential temperature and humidity are nearly
constant with height at the lower altitude than the ABL, due to the intense turbulent mixing generated by theconvective air motions. We have fair weather except for Sep. 4 and afternoon of Sep. 2. Especially, Okinawa
had placed at the south side of Baiu front and had been covered by anticyclone during 2005 IOP. From Aug.
30 to Sep. 2 on 2004, the wind speed was very low, but other days have strong wind. Especially, from Jul. 3
to 10 on 2005, wind speed exceeded 10 m s−1 . From noon at Sep. 2 to Sep. 3 on 2004, the northeast wind due
to typhoon was dominant. Moreover, south – southwest wind was dominant during 2005 IOP.
From Jun. 30 to Jul. 3, atmosphere at the middle – lower level had gradually gotten to be drier. Especially,
from Jul. 1 to 3 (every 10 – 6 JST), one tiny mass and another small mass of drier air were descending fromthe height of 2.5 km and penetrating down to 1 km. Haze was seen from Jul. 3 to 5, maybe due to humid air at
lower level. Atmosphere at upper level was very dry during this period. On Jul. 7, it rained near Ogimi site atnoon. Mixing ratio also increased to upper layer. After that, mixing ratio at the surface exceeded 20 g Kg −1 .
until next day, high mixing ratio was kept. On Jul. 9, thought a mixing ratio of upper layer was getting to bedry, visibility at the surface was not good due to fog or haze.

Figure 4: Time-height cross section of mixing ratio, with wind barb representing wind speed and wind direction,observed by radio sonde soundings during 2004 IOP (Aug. 31 – Sep. 3). Unit is g Kg −1 . Gray zone shows that thealtitude is lower than Ogimi site.

Figure 5: Same as Fig. 4 but for 2005 IOP (Jun. 30 – Jul. 5 (top) and Jul. 5 – Jul. 10 (bottom)).

4.2

Video, COBRA and WPR

At 2004 IOP experiment, cloud systems were observed by two videos. On Aug. 31, CAE had been continuously
seen in 2-dimensional COBRA RHI data since 10 JST, both when a cloud street of horizontal roll convection
was seen and when there was no clouds. The COBRA Z and v observed on 1536 JST are shown in Fig. 6.
CAEs with undulating boundary layer top and filled with high Z signals are shown. Figure 7 is a time-heightcross section of N obtained from sonde data with CAE height obtained from COBRA RHI data. This figure
shows that the large gradient of N is coincident with CAE height. Moreover, we show a time-height crosssection of 400 MHz WPR peak reflective intensity of upward beam with CAE height in Fig. 8. At 10–14 JST,COBRA CAE height is at the bottom of the regions with reflective intensity of >15 dB, but it is at the top ofthem at 14–20 JST. Downward doppler velocity was −2 m s−1 at 14 JST, which means that this is not due toa scattering by rain drops but a Bragg scattering by atmospheric turbulence. On Sep. 1, there was a cumulus.
CAE had been intermittently seen since Aug. 30. At 12 JST, the CAE height reached to the maximum of 2.4km, corresponding to the increased mixing ratio and passage of a large cumulus. At 5 JST on Sep. 2, there wasno clouds, but shallow CAE was seen. On Sep. 4, CAE was seen until rain started at evening due to typhoon.
At 2005 IOP experiment, three videos were used. On Jul. 2, CAE appeared at the height of 0.5 km at
08 JST and it developed to 0.9 km at 10 JST, and the structure became a inverse-U character outline like atemple bell. After that, the height became lower. On Jul. 3, atmosphere was not clear due to haze, but thesimilar CAE diurnal variation was observed. At 06 JST on Jul. 5, CAE appeared at the height of 0.5 km, afterthat it developed to the height of 1.7 km and the structure looked like a temple bell. At 15 JST, a layer withhight mixing ratio was lifted highest, and CAE height was also simultaneously highest. At 18 JST, stratiform
CAE was appeared at the height of 0.5 km. On Jul. 6, the haze disappeared and a cumulus was frequently
seen. The similar CAE diurnal variation was observed and the maximum height reached 2.0 km. On Jul. 7,
it rained near Ogimi site in a short time at noon and black clouds was observed. COBRA PPI observation
also showed some local rain systems. The CAE diurnal variation was not so different. On Jul. 8, it rained at
0:30 JST and black clouds was observed but we had too weak rain to detect with optical raingauge. On Jul.
9, atmosphere was not clear due to fog, but CAE was observed. It was generally thought that an appearanceof CAEs depends on surface boundary conditions and it prefers a fair-weather day with no wind and wet land
surface. But, concludingly, the CAEs were seen everyday during 2005 IOP, although wind speed was very high.

4.3Structure of CAE

COBRA CAE revealed two types of structure; stratiform type and temple-bell-like shape. When CAE has atemple-bell-like shape, the right-hand top of CAE moves to right and the left-hand top of CAE moves to left,that is, the top of CAE expands, as shown in Fig. 6. It shows that vertical motions of rising thermal plumes
cause irregularities of CAE top. When CAE has a stratiform shape, CAE was observed at morning or evening
at sometimes higher altitude and sometimes same altitude, compared with the height of a temple-bell-like CAE.
This structure of nocturnal boundary layer is thought to be a result that the remnants of the daytime convective
boundary layer form a residual layer because the radiative cooling and surface friction stabilize the lowest partof the ABL. Moreover, the CAE has two cases; one is filled and another is opened. Figure 6 is a filled case. At
first, a filled CAE appeared. Next, the CAE developed and the height increased, and then, an opened CAE
appeared. So, Fig. 6 is in a underdeveloping phase. Correlation between CAE height and turbulence scale
shown in Fig. 9 indicates that the higher the height, the larger the horizontal size of CAE. Moreover, echoheight was low and turbulence scale was also small before 13 JST (underdeveloping phase). Echo height andturbulence scale became large twice at 14 and 17 JST. But the height was lower than Mar. 2004, drier season.

5

Summary(1) C-band rain radar with high sensitivity could observe variability of the CAE fine 2-dimensional structure.
(2) CAE was seen not only at the condition of weak wind but also strong wind. (3)CAE has two types ofstructure, stratiform shape and temple-bell-like shape. These echoes were sometimes filled and sometimes not
filled. (4) the larger the horizontal size of CAE, the higher the height of CAE. (5) CAE height was coincident
with large gradient of refractive index indicating a top of the ABL. This confirms that CAE is made throughBragg scattering by atmospheric turbulent.
Acknowledgments.: This work was supported by the Japan Science and Technology Corporation (JST). These experiments
were performed in cooperation with many graduate students of Nagoya University and Okayama University.

Figure 6: COBRA RHI image of Z [dBZ] and v [m s−1 ]observed at 1536 JST on Aug. 31, 2004. X-axis is the distance from COBRA, and y-axis is the altitude (sea-level).
The COBRA is placed at x = 0, Ogimi site is at x = 14 km,
and video-focused point is at x = 10 km. Lowest altitudewith no signals is lower than COBRA antenna altitude. Red
color (v < 0) of bottom panel means that it goes away from
COBRA, and blue color (v > 0) means that it comes closer
to COBRA. In a blue circle, three CAEs are seen, and a righthand component of a CAE goes away form COBRA and aleft hand component comes closer to COBRA. It means that
top of each CAE expands. Synoptic wind is very weak.

Figure 7: Time-height cross section of N at N unit observed by radio sonde on Aug. 31, 2004. Diamonds show
the CAE height observed by the COBRA and error bars
show the maximum and minimum heights of CAE.

Figure 9: Correlation between CAE
height and turbulence scale (horizontal size of CAE) observed
by
COBRA on Aug. 31,
2004. Color means
observed local time(JST).

Figure 8: Time-height cross section of reflective intensityof 400 MHz WPR estimated from Fourier spectrum power(arbitrary unit) on Aug. 31, 2004. Diamonds are same as
Fi.g. 7.

References(1) Adachi, T., Y. Masuda, and S. Fujii, 2001 The Third International Symposium on Asian Monsoon System (ISAM3),
309-313.
(2) Nakagawa, K., H. Hanado, S. Satoh, N. Takahashi, T.
Iguchi, and K. Fukutani, 2003, 31st AMS Conference on Radar
Meteorology, Vol. II, 863-866.
(3) Marshall, J.S., A.M. Peterson, and A.A. Barnes Jr., 1972,
J. Appl. Opt., 11, 108-112.
(4) Bean, B.R. and E.J. Dutton, 1968, Dover Publications, Inc.,New York, pp. 435.