Development of Phased-Array Weather Radar System for 3D Observation of Cumulonimbus Clouds

In Japan especially in the urban area, the disasters caused by extreme weather are recently increasing and lives of citizen are at risk. Severe weather phenomena such as localized heavy rainfalls, gust and tornadoes are mainly caused by rapid growth of cumulonimbus clouds which grows more than 10 km attitude. Generally, lifecycle of a cumulonimbus cloud is short as 10 to 30 minutes. However, conventional weather radar system with parabolic antenna requires approximate 5 to 10 minutes for full volume scanning to observe three dimensional structure of a cumulonimbus cloud, which has inadequacy capability in temporal and spatial resolution for observing the behavior of cumulonimbus clouds. In order to achieve precise three dimensional observations of cumulonimbus clouds for predicting severe weather, weather radar is expected to observe the full volume of meteorological phenomena within 1 minute. Toshiba's newly developed X-band Phased-Array Weather Radar (PAWR), which shown in Figure 1 has capability of observing cumulonimbus cloud within 1 minute. Figure 2 describes Conceptual image of Phased Array Weather Radar Operation.

Toshiba's newly developed X-band PAWR, which already installed in Osaka University (Japan) under a grant of NICT, has a 128-slotted array antenna and applied Digital Beam Forming (DBF) technique to simultaneously generate the multiple vertical beams for covering the elevation angle from 0 to 90 degrees. PAWR antenna is embraced with a mechanical drive for azimuth angle and an electronic scanning for elevation angle. For elevation angle, the transmitted beam is formed as a fan beam, and the received beams are formed as multi-beam using DBF technology. The main feature of PAWR is to perform densely 3-D volume scanning within 10 to 30 sec without making gaps between each beam. Table1 describes major specification of PAWR. This system has two observation modes. One is "Rapid Mode" which able to update the full volume scanning in 10 seconds with 20km radius range. Another is "Wind Range Mode" which observes 60 km radius range in 30 seconds. Compare with conventional parabolic antenna weather radar, PAWR in "Wind Range Mode" has 100 elevation angles which are 10 times more than conventional radar, and scanning time had reduced to 30 seconds which was 5 minutes in conventional radar. In total, performance is 100 times improved by PAWR. Comparison of PAWR and conventional radar are shown in Figure 3.

Figure 4 shows the active phased array antenna used in PAWR which consisted of 128 slot antennas and it aperture length is 2m by 2m. This antenna has receiving and transmitting units, DBF units, and those units are located in rear side of antenna. Figure 5 shows Transmitting and receiving unit. Actual measured antenna pattern is shown in Figure 6 and Figure 7. Azimuth beam width is about 1 degree and side lobe is less than -23 dB. Elevation beam width is about 4 degrees for transmitting by using 24 elements (24 slots), and about 1 to 1.2 degrees for receiving by DBF technique.

Figure 8 shows the three dimensional image of actual observation result observed in July 26, 2012. One cumulonimbus echo was about vertically 3km and horizontally 8 km. Observation data showed small indication of heavy rainfall (we call "egg of heavy rainfall") in attitude of 4 to 6 km has exponentially grown and then became localized heavy rainfall within few minutes. By using PAWR, any changes in localized meteorological phenomena can be observed every 30 seconds or less. We can expect Phased Array Weather Radar system to be used for preventing disasters by forecasting localized heavy rainfalls, gust and tornadoes.