Three-dimensional Fine Structure of Localized Heavy Rainfalls Measured by Phased Array Weather Radar

In recent years, severe weather disasters caused by localized heavy rainfalls or tornadoes have emerged as a social issue in Japan. Since it is difficult to predict the rapidly developed severe storms, the real-time monitoring of the severe phenomena should be helpful to reduce the damage of disasters. Although weather radar is a powerful instrument to capture these phenomena, it is inadequate to measure three dimensional phenomena developed rapidly in a local area. As the general performance of traditional weather radar, a volume scan by a parabolic antenna requires substantial time of more than 5 minutes. Furthermore, the volume scan of a limited number of elevation angles cannot provide filled data in a three-dimensional space. We have completed X-band one-dimensional phased array weather radar (PAWR) with an electronical scan in the elevation direction and a mechanical rotation in the azimuth direction. To increase the number of samples in a limited time, we adopt a combination of fan-beam transmitting using 24 antenna elements with T/R module and DBF (Digital Beam Forming) receiving using 128 antenna elements to form a number of pencil beams simultaneously. As leading observation modes, seamless (an elevation angle number above 110) 3D observation is possible in 10 seconds at detailed observation mode of 20 km radius observational range, and in 30 seconds at standard observation mode of a 60 km radius. In May 2012, we installed PAWR at Osaka University Suita Campus on the 13th floor rooftop of a building. After the installation, we have continued test observation except short maintenance and improvement work periods. In this study, we introduce two localized heavy rainfall cases from the test observation results. The first case on July 22 is that convective echoes were organized into a line-shape in a mountainous area. In this case, a new precipitation tower appeared in the rear of an existing cell or between the existing cells. Through some precipitation towers are developing and falling in turn, the precipitation area is widening to 40 km in a horizontal direction, and the echo top altitude is 12 km at the highest shown in Fig. 1. A raingauge under the precipitation towers recorded rainfall amounts of over 70 mm in 2 hours. In the second case on July 26, the evolution of isolated cumulonimbus clouds were observed. As shown in Fig. 2, the total echo size is only 5 km in horizontal and 8 km in vertical. A first echo appeared at an altitude of 4 km was falling to the ground for a few minutes. However, the next first echo which appeared at the rear of the old echo at the same altitude rapidly grew and the enlarged precipitation was falling to the ground like a waterfall. Consequently, it brought heavy rainfall to a limited small area. 3D Visualization data of radar reflectivity in every 30 seconds observed by PAWR revealed the process of growth of heavy precipitation. Studying this type of observation case is expected to provide insight on what type of echoes rapidly develop and cause heavy rainfalls. Moreover, if we can process this kind of 3D data in real-time, it is thought that this will lead to the forecast information on local heavy rains in shorter periods of time.