Characteristics and Evolution of Clear Air Echoes Observed by
Okinawa Polarimetric Doppler Radar (COBRA)
Shinsuke Satoh1, Fumie Akimoto Furuzawa2, Kenji Nakamura2,
Katsuhiro Nakagawa1, Shinya Sekizawa1, and LAPS team2
1 National Institute of Information and Communications Technology (NICT),
Okinawa Subtropical Environment Remote-Sensing Center
4484 Aza-Onna, Onna, Kunigami, Okinawa 904-0411, JAPANe-mail: satoh@nict.go.jp
2 Hydrospheric Atmospheric Research Center (HyARC), Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan

1. Introduction
Clear air echoes (CAEs) observed by weather radar have been recognized since the 1940s (Atlas,1990). The scattering objects include not only birds and insects but also atmospheric turbulence. TheCAE formed by turbulence is observed particularly over land warmed by solar radiation. Although aweather radar usually measures precipitation particles through the Rayleigh scattering, it also measuresvariations of atmospheric refractivity caused by turbulence through the Bragg scattering as a wind profiler.
A C-band (5 GHz) weather radar is at a disadvantage to measure turbulence due to the shorter wave
length (Ralph, 1995), while some results about the CAE observation have been reported (Kusunoki,2002; Takahashi, et al., 2004, 2005).
Convection in atmosphere is called by some names, which are plume, thermal, cumulus andcumulonimbus, according to their scales. Plume is a small-scale thermal with a root which is a fixedheat source. Although plume and thermal are usually invisible, cumulus and cumulonimbus are visible
by clouds. Cumulus convection is generated in the environment of convective instability. When an air
mass with water vapor is lifted up to a condensation level, cloud droplets are generated by condensation.
There are some reasons of the first opportunity of lifting up the moisture air mass. Wind convergence atsurface, which is caused by the discontinuity of temperature, roughness, topography, and so on, is one of
the reasons. In addition, the fluctuation of the surface temperature caused by solar radiation should be
another reason. In most cases, the first convection is occurred in the atmospheric boundary layer (ABL).
In the ABL with alive turbulence, many thermal convections generated by the solar heating. The thermal
may be one of the causes of an outbreak of cumulus convection over the warmed land surface.
However, the relation between the thermal and the cumulus convection has not been cleared.
In this study, the C-band Okinawa Bistatic polarimetric Radar (COBRA) developed by NICT
(Nakagawa, et al., 2003) is used to investigate the characteristics of CAE and its evolution into cumulus
convection.
The advanced C-band radar, which measures polarimetric parameters and Doppler
velocity, is expected to reveal the convective evolution from thermal to cumulus.

2. Observation and Data
From June 29 to July 10, 2005, a campaign experiment was conducted by NICT Okinawa and HyARC
Nagoya university members as one of Lower Atmosphere and Precipitation Study (LAPS) projects. In
addition to the COBRA observation, GPS radiosonde sounding every two or three hours, optical video
observation, and 400 MHz wind profiler (WPR) observation were carried out. The contents in detail ofthe IOP experiment is reported by Furuzawa (2005). The COBRA observation sequence every 10
minutes includes three PPI scans at 1.4, 7.0, and 8.0 degrees in elevation angle, and continuous fourteen
RHI scans every 30 seconds at 41.2 degrees in azimuth angle. The azimuth angle is a direction fromthe COBRA to the WPR site. The COBRA observation range is 20 km, the range resolution is 75 m with
0.5 µs pulse width, and the elevation angle resolution is about 0.4 degrees (the antenna beam width is0.9 degrees). The polarization observation is conducted by alternate H and V transmitting pulse bypulse and simultaneous H and V receiving to measure differential reflectivity (ZDR), linear depolarization
ratio (LDR), and so on. Doppler velocity and phase information, such as differential phase shift (φDP),cross correlation coefficient (ρHV), are also recorded.

3. Extraction of CAE

HEIGHT (km)HEIGHT (km)The COBRA data near the land surface during the clear air observation is contaminated by groundclutter echoes. Although the side-lobe level of the COBRA antenna is less than –28 dB in the elevation
direction, the RHI scan drags the intense ground echoes over 60 dBZ into the upper clear regions.
Figure 1 shows an example of the observation data. Although most of the echoes appeared inhorizontal reflectivity (ZHH) are ground clutter, the valid Doppler velocity region less than about –2.5 m/slooks like concrete CAEs. Since the ground clutter echoes do not move, it is possible to extract theclutter echoes by subtracting around zero Doppler. In this study, the condition of both –0.5 < V < +0.5
m/s and NCP < 0.2 is used to extract the ground clutter echoes. Where, V is Doppler velocity, and NCPis Normalized Coherent Power. The NCP condition is used to subtract the noise-level data, because theNCP is effective to distinguish the valid velocity data from noise-contaminated data. According to thesimple procedure, the CAE distribution is revealed as shown in Fig. 1 (b). The inversed U-shape echo
indicates a vertical slice of the hanging bell-shaped echo by thermal (Hardy and Ottersten, 1969; Konrad
and Robinson, 1970). Note the reflectivity values in the ranges less than 2.5 km are weakened, which
may be caused by a hard-ware problem.
dBZ(a)(b)

Extracted

DISTANCE FROM RADAR (km)dBZm/s(c)(d)DISTANCE FROM RADAR (km)Fig.1 Vertical sections (RHI) of (a) observed reflectivity (ZHH), (b) extracted CAEs, (c) observed
Doppler velocity (d) observed NCP.

4. Characteristics of CAE
Figure 2 shows three kinds of CAEs represented by ZHH, ZDR, and ρHV. Although a typical CAE,
which has the maximum altitude less than 2 km, has been shown in Fig. 1 (b), here, other significant
CAEs are shown in Fig. 2. The developed CAE shown in Fig. 2 (a) achieves the altitude of more than 2km, and the horizontal scale is between 1 km and 1.5 km. The ZDR value around zero in Fig. 2 (b)means the difference between horizontal and vertical size of the targets is small. The higher ρHVaround 0.9 in Fig. 2 (c) appeared at the upper regions of the CAEs. Although the ρHV represents theshape and the rotation of the targets, it is difficult to explain the distribution of ρHV in the CAEs because
the turbulence structure in the inertial region seems to be isotropic. The larger CAE shown in Fig.2 (d)achieves the altitude of more than 2.5 km, and the horizontal scale is between 2 km and 3 km. Since
the turbulence scale increases with altitude, the observation result is consistent with the theory. Two orthree tower echoes like plume expand to 3 km in altitude. While the ZDR in Fig. 2 (e) values are aroundzero, the ρHV values in Fig. 2 (f) within the plume-like echoes are almost one, which is the same value forsmall precipitation particles. Figure 2 (g), (h), (i) show the quite different distributions of ZHH, ZDR, andρHV at the horizontal distance between 4 km and 9 km comparing with the above mentioned CAEs. Theshape and characteristics of the echo are different from the turbulence echoes, though the echo is also
observed in clear air. The strong reflectivity over 18 dBZ, the higher ZDR, and the lower ρHV may
indicate the biological echoes, such as birds and insects. This group echo moves to downwind direction
at the wind speed. The similar echoes were observed two or three times during the IOP 2005.

HEIGHT (km)HEIGHT (km)HEIGHT (km)0441Z, 07JUL2005

0341Z, 08JUL2005

0605Z, 07JUL2005

(a)

(d)

(g)

(b)

(e)

(h)

(c)

(f)

(i)

DISTANCE FROM RADAR (km )DISTANCE FROM RADAR (km )DISTANCE FROM RADAR (km )Fig. 2 Vertical sections (RHI) of (a) reflectivity (ZHH), (b) differential reflectivity (ZDR), (c) crosspolarization correlation coefficient (ρHV) observed at 0441Z, 07 July 2005. (d)(e)(f) are the sameas (a)(b)(c) except at 0341Z, 08 July, (g)(h)(I) are the same as (a)(b)(c) except at 0605Z, 07 July.

5. Cumulus evolved from CAE
Figure 3 shows the time change of a significant plume-like tower echo, which develops rapidly up toan altitude of 3.5 km at 0513Z. The reflectivity over 10 dBZ appeared around a distance of 8 km at0513Z, 0514Z, and 0515Z is larger than the usual turbulent echoes. The horizontal size of the towerecho is about 500 m. Since the mean wind direction is a little different from the RHI slice direction, thehorizontal cross section is shown in Fig. 4. Figure 4 (a) reveals a doughnut-shaped echo around x=3 km
and y=4.5 km, which indicates a horizontal slice of the hanging bell-shaped CAE. The relatively lower
ρHV in the doughnut-shaped echo in Fig. 4 (c) seems to represent the horizontal slice of the lower
altitude of the bell-shaped CAE. Although the inversed U-shaped echo is not clear in the vertical crosssection in 45 seconds in Fig. 4 (d), the horizontal slice line along the PPI at the elevation angle of 8.0
degrees indicated by a solid line in Fig. 4 (d) locates below the developing CAE at the distance between 5km and 8 km. Figure 4 (d) also shows the higher ρHV region appears in the tower echo. The center ofthe doughnut-shaped echo at 0511Z in Fig. 4 (a) will moves onto the distance of about 7 km at 0513Z inFig. 3 (d) according to the mean wind speed and direction (referred by an arrow in Fig. 4 (a)) observed by
the 400 MHz WPR. This result indicates that the rapid development of the tower echo shown in Fig. 3(c) and (d) appears above the doughnut-shaped CAE.

HEIGHT (km)4HEIGHT (km)(d)(b)

(e)

(c)

(f)

2104dBZ

3210432102468101214DISTANCE FROM RADAR (km )161820 02468101214DISTANCE FROM RADAR (km )1618Fig. 3 Time change of reflectivity (ZHH) in the vertical section (RHI) at 41.2 degrees in azimuth at(a) 0505Z, (b) 0508Z, (c) 0511Z, (d) 0513Z, (e) 0514Z, and (f) 0515Z, 07 July 2005.

(c)
AZ=41.
2AZ=41.
2(a)DISTANCE FROM RADAR (km)0HEIGHT (km)HEIGHT (km)(a)3(d)(b).
EL= 8DISTANCE FROM RADAR (km)0.
EL= 8DISTANCE FROM RADAR (km)Fig. 4 Reflectivity (ZHH) on the (a) PPI at 8.0 degrees in elevation and (b) RHI at
41.2 degrees in azimuth at 051059Z, and 051143Z, respectively. (c) and (d)are the same as (a) and (b) except cross correlation coefficient (ρHV).

020

6. Summary
The rapidly developing tower echo is considered as a cumulus convection. The reasons are stronger
reflectivity over 10 dBZ and higher ρHV around one comparing with the turbulence CAE. Since theC-band weather radar cannot detect small cloud droplets, in which the reflectivity may be less than –10dBZ, the cloud droplets in the cumulus convection seem to grow up into larger precipitation droplets
detected by the COBRA. The horizontal scale of the tower echo is about 500 m, which is smaller thanthe bell-shaped echo. Since the COBRA detects only the core part with precipitation of the cumulus
convection, however, the actual size of the cumulus cloud must be larger. An important point is that thetower echo develops above the bell-shaped thermal echo. This observation result indicates that a partof thermals in the ABL penetrates into the free atmosphere above the ABL, and causes the cumulus
convection.
Using the C-band multi-parameter radar which can detect both turbulent echoes and precipitation
echoes, the relationship between thermal in the ABL and cumulus convection is revealed. However, toinvestigate the detailed evolution process from thermal to precipitation through cumulus cloud, it isnecessary to use a cloud detecting remote-sensor such as a high-frequency cloud radar.

References
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