Okinawa 2004 Field Campaign of observing precipitation
Katsuhiro NAKAGAWA1, Yasushi KITAMURA1, Hiroshi HANADO2, Koyuru IWANAMI3,CREST/LAPS observation member, and CREST/GSMaP observation member
1 Okinawa Subtropical Environment Remote-sensing Center, National Institute of Information andCommunications Technology, 4484 Aza-Onna, Onna, Kunigami, Okinawa 904-0411, Japan
2 JAXA/EORC, Tsukuba, Japan
3 National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan
1.

INTRODUCTION

In the 2004 rainy season in Japan, a field campaignof observing precipitation in Okinawa (okn-baiu04) was
conducted as part of the Lower Atmosphere andPrecipitation Study (LAPS/CREST) and the global
satellite mapping of precipitation (GSMaP/CREST)
project [1]. One of the important observations fromokn-baiu04 was simultaneous measurement ofprecipitation with COBRA (CRL Okinawa bistatic
polarimetric radar, C-band) [2], a 400-MHz wind profiler(WPR) [3], and ground-based measurements of therain drop size distribution (DSD).
In this paper, as a step toward improving themicrophysics model and the rain distribution models
used in the rain retrieval algorithms, we analyze thevertical characteristics of rainfall that are related to therainfall type (convective rainfall and stratiform rainfall),as observed by COBRA, the 400-MHz WPR, and a 2-D
video distrometer.
2.

On the other hand, MP-Ka is a cloud radar, with highsensitivity to cloud particles, high space resolution, anda very sharp antenna beam, though its rainfall
attenuation is very large when the rainfall intensity isstrong. Considering these strengths and limitations,
MP-Ka was installed at NICT Ogimi. This radar wasoperated in vertical pointing mode. The 400-MHz WPR
could simultaneously observe the atmosphericturbulence echo and the echo from precipitation. Table
2 shows the specifications of the 400MHz WPR.
Therefore, by analyzing the echo power spectrum of
the received signal, it was possible to obtain rain drop
size distribution measurements in which the effects ofwind speed, the intensity of atmospheric turbulence,and background winds had been removed.

Volume Scan
RHI ScanRHI Scan

FIELD CAMPAIGN

The field campaign of observing precipitation inOkinawa, Japan (okn-baiu04), was conducted during
the 2004 rainy season. Fig. 1 shows an outline of theprecipitation observation network used in okn-baiu04.
At NICT Ogimi, a 400-MHz WPR performed
synchronous observation with the COBRA system. The
ground-based Ka-band radar (MP-Ka) of the National
Institute for Earth Science and Disaster Prevention wasalso operated. Ground-based measurements of therain drop size distribution were obtained by a Joss-type
disdrometer and a 2-D video distrometer every minute.
The rainfall intensity was also observed every minutewith a tipping-bucket rain gauge and an optical rain
gauge. Vertical profiles of the air temperature, humidity,pressure, and horizontal wind were observed by a GPS
radiosonde every three hours. COBRA was operated in3-D observation mode and range-height indicator (RHI)
scan mode every 10 minutes. The 3-D observation
took six minutes, and the RHI scan took four minutes.
Two RHI scans were carried out along lines connecting
to NICT Okinawa and NICT Ogimi. Although COBRA isaremote-sensing
instrument
thatcaninstantaneously and continuously observe anextremely wide area, it does not have enoughsensitivity with respect to solid-phase hydrometeors.
Table 1 shows the specifications of the COBRA system.

MP-Ka
400MHzWPR

NICT Okinawa- Disdrometer
NICT Nago-ORG- Micro Rain Radar- 2D-Video Disdrometer- Meteorological obs.

1.3GHz WPRNICT Ogimi- Disdrometer - ORG- Meteorological Observations- Micro Rain Radar- 2D-Video Disdrometer- GPS radio sondeFig. 1 Outline of the precipitation observation network
used in okn-baiu04.
Table 1 Specifications of the COBRA system
Frequency

5340 MHz (C-band)

Peak Power250 kW×2 (Dual Klys tron ）Puls e width

0.5, 1.0, 2.0 µs

PRF

250-3000 Hz, 1-Hz s tep

Antenna

4.5-m Φ parabola (8-m Φ radom e)

Antenna gain

45.6 dBi (H) / 45.7 dBi (V) (include radom e)

Beam width

0.91 deg (H) / 0.91 deg (V)Sidelobe level< -31.8 dB (AZ), < -28.3 dB (EL)

Cros s pol ratio

39.2 dB (H) / 40.3 dB (V) (integ in a beam )Doppler estim ation

Puls e-pair / FFT

Range bin num> 2000 bin

Antenna s can

PPI : 0.5-10 rpm , 0.1-rpm step
RHI : 0.1-3.6 rpm , 0.1-rpm step

The X-band Doppler radar of Nagoya University
was installed at Katsuren on the main island ofOkinawa (referred to here as the “Katsuren radar”). Fig.
2 shows the locations of the COBRA radar site, theKatsuren radar, and the 400-MHz WPR.

Table 2 Specifications of 400MHz WPRRadar TypeFrequency
Peak Power
Average Power
Pulse Length
Pulse Repetition Frequency
Antenna Type
Antenna Size
Beam Width
Beam Steerability Azimuth
ZenithObservation Items

Pulsed Doppler Rader
443.0 MHz
20 kW
2 kW
1.33，2.0，4.0 µs
6.25，20 kHz
24 × 24 element crossed array
10.4 m × 10.4 m
3.3°
0 - 360°
0 - 15°
Verticul profiles of wind velocity,
rainfall intensity, and virtual temperature

Fig.2 Locations of the ground-based remote-sensing
radar site. The gray line circles represent thedual-Doppler-radar observation area for COBRA andthe Katsuren radar. The thin line circle represents theKatsuren radar’s observation area, while the thick line
circle represents COBRA’s observation area (less than
400 km in diameter).

island to the southeastern part. A bright band existed at
an altitude of 5 km. We confirmed from the GPS
radiosonde data that a melting layer had existed nearthat altitude.
In examining the collected data, we first analyzed
the vertical characteristics of each rainfall type by using
the polarimeric information observed by COBRA. Fig. 6
shows vertical profiles of the radar reflectivity ZHH, thedifferential reflectivity ZDR, and the median volume
diameter D0, for (a) convective rainfall, and (b)stratiform rainfall. The variation in the vertical profile ofZHH was greater for convective rainfall than forstratiform rainfall. In other words, the height
dependency of the amount of convective rainfall wasgreater than that for stratiform rainfall. This result
corresponds to the analysis by Nakagawa et al. using a
vertically pointing VHF Doppler radar in Japan, called
the Middle and Upper (MU) radar [5]. For convective
rainfall, ZDR varied from 0 to 2.0 dB. On the other hand,for stratiform rainfall, ZDR was approximately 1.0 dB.
This suggests that the vertical variation in the rain drop
size distribution was greater for convective rainfall thanfor stratiform rainfall. For stratiform rainfall, D0 was notcomputed at a height of more than 4 km, because thehydrometeors there were solid. The vertical variation inD0 for convective rainfall was very large, varying from 1
to 3.5 mm. In contrast, for stratiform rainfall, the verticalvariation in D0 was small. From the vertical variations ofZDR and D0 for stratiform rainfall, it is clear that the rain
drop size was uniform.

3. OBSERVATION RESULTS
Fig. 3 shows a time series of the vertical profile of
the radar reflectivity observed by COBRA at NICTOgimi on July 1, 2004. The rainfall type wascategorized as convective rainfall or stratiform rainfall
by using the PPI and RHI data observed by COBRA.
The dotted line and thick line boxes show the periodsof convective and stratiform rainfall, respectively.
Figures 4 and 5 show the radar reflectivity distributionsof each rainfall type, as observed by COBRA. Therainfall attenuation was determined by the ZPHI
method [4]. Figures 4(a) and 5(a) show the horizontal
distributions for an elevation angle of 1.4 degrees.
Figures 4(b) and 5(b) show the vertical distributions for
RHI scan mode along a line connecting to NICT Ogimi.
As seen in Fig. 4, a baiu frontal rain band existed
parallel to the Okinawan main islands, and a strong
rain band area existed over the northern part of theseislands. The strong rain band was confirmed from theRHI scan data shown in Fig. 4(b). On the other hand,
as shown in Fig. 5, the baiu frontal rain band moved
from the northwestern part of the Okinawan mainFig. 3 Time series of the vertical profile of the radarreflectivity on July 1, 2004. The dotted line and thickline boxes show the periods of convective andstratiform rainfall, respectively.

（a）PPI scan data of radar reflectivity (EL = 1.4 [deg])

1412Height [km]10864（b）RHI scan data of the radar reflectivity
Fig. 4 Radar reflectivity distribution observed by
COBRA for convective rainfall (2004/06/01 05:26(UTC)). The RHI scan data was taken along a line
connecting to NICT Ogimi.

202040ZHH [dBZ]

60-1012ZDR [dB]

3 -1012D0 [mm]34(b) Stratiform rainfall
Fig. 6 Vertical profiles of the radar reflectivity, thedifferential reflectivity, and the median volume
diameter, for (a) convective rainfall and (b) stratiform
rainfall. The dots indicate the average values, witherror bars for the standard deviations.

（a）PPI scan data of radar reflectivity (EL = 1.4 [deg])

Finally, we analyzed the vertical profile of the DSD
by using the 400-MHz WPR data. The profile wasestimated from Doppler spectra by using Kobayashi’salgorithm [6]. An outline of the process of estimating
the DSD from a Doppler spectrum is given below. The
turbulent spectrum St(v) is assumed to have aGaussian distribution, given by

S t (v ) =⎡ (v − v0 ) 2 ⎤ ,exp⎢−⎥
2σ 2 ⎦
2πσ 2
⎣
1(1)

where σ is the spectral width, v is the Doppler
velocity, and v0 is the mean vertical air velocity. The
spectral density Sobs(v) measured with the WPR isgiven by

S obs (v) = Pt S t (v) + S D (v) ∗ S t (v) + Pn , (2)（b）RHI scan data of radar reflectivity
Fig. 5 Radar reflectivity distribution observed by
COBRA for stratiform rainfall (2004/06/01 13:36(UTC)). The RHI scan data was taken along a line
connecting to NICT Ogimi.

S D (v ) = C ⋅ N ( D ) ⋅ D 6 ⋅1412dD ,dv(3)

where C is a radar constant and D is the rain drop
diameter. This algorithm works well for stratiform
rainfall.

10Height [km]where SD(v) is the precipitation spectrum, Pt is the echo
power of the air turbulence, Pn is the noise power, and* indicates the convolution. Then, the precipitation
spectrum SD(v) is given by864202040
ZHH [dBZ]

60-1012ZDR [dB]

3 -1(a) Convective rainfall

0123D0 [mm]4Fig. 7 shows the Doppler spectrum observed by
the 400-MHz WPR at 12:36 (UTC) on July 1, 2004. It isclear that the Doppler spectrum from the falling rain
drops could be detected very well below the meltinglayer. When only small differences between air motion
and precipitation fall velocity above the melting layer
are observed in the Doppler spectrum in stratiform
rainfall, the spectrum peak of air motion is difficult tofind from the observed Doppler spectrum. However,when the Doppler spectrum for air motion can be foundfrom the observed Doppler spectrum, DSD can beestimated using Kobayashi algorithm. Therefore, wedeveloped new algorithm above the melting layer. Fig.
8 shows the vertical profile of raindrop size distribution
by the proposed algorithm. The shape of DSD above
the melting layer almost does not vary with heights(Fig.8 (a)). The trend vertical variation of DSD does
small change below the melting layer. There is a small
number of raindrops and the maximum diameter ofraindrops ranged from 3 to 5 mm below the melting
layer (Fig.8 (b)). The number of raindrops andmaximum diameter of raindrops are large near theground.

Log10(N(D)) (mm-1 m-3)

Fig. 7 Doppler spectrum observed by the 400-MHz
WPR for stratiform rainfall (2004/6/1 12:36, UTC).

5500m

5000m

20-2-412345612345612345

From the vertical variation in the radar reflectivity
observed by COBRA, we found that the height
dependency of was greater for convective rainfall thanfor stratiform rainfall. This result corresponds to theresults of an earlier analysis using the MU radar. From
our analysis of the vertical variations in the differentialreflectivity and the median volume diameter for
stratiform rainfall, it is clear that the rain drop size wasuniform.
We developed new algorithm above the meltinglayer. The vertical profile of the rain drop size
distribution was analyzed after estimating it from theDoppler spectrum observed by the 400-MHz WPR.
The shape of DSD above the melting layer almost does
not vary with heights. There is a small number ofraindrops and the maximum diameter of raindrops
ranged from 3 to 5 mm below the melting layer. The
number of raindrops and maximum diameter ofraindrops are large near the ground.

6. ACKNOWLEGMENT

46000mproject (okn-baiu04) was simultaneous measurementof precipitation with COBRA, a 400-MHz wind profiler,and ground-based measurements of the rain drop size
distribution (DSD).

6This research was supported by the Core
Research for Evolutional Science and Technology(CREST) project, of the Japan Science andTechnology Agency (JST).

Diameter (mm)(a) Above the melting layer

7. REFERENCES

43500m3000m

2500m

2000m

1500m

1000m

2Log10(N(D)) (mm-1 m-3)

0-2-4420-2-4
123456123456123456Diameter (mm)(b) Below the melting layer
Fig. 8 Vertical profile of the rain drop size distribution,
derived from the Doppler spectrum observed by the400-MHz WPR, at various heights (2004/6/1 12:36,UTC).
5. SUMMARY
A field campaign of observing precipitation inOkinawa, Japan, was conducted during the 2004 rainyseason. One of the important observations of this[1] K. Okamoto, T. Iguchi, H. Hanado, N. Takahashi,and T. Ushio: The Global Satellite Mapping ofPrecipitation (GSMaP) Project: Part I Outline ofthe Project , The 2nd TRMM International science
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