High-speed volumetric observation of downburst using X-band phased-array radar

Downburst is a strong downdraft formed in a severe thunderstorm, causing low-level wind shear (LLWS) which significantly affects the safety of aircrafts under landing and taking off. Japan Meteorological Agency operates the Doppler Radar for Airport Weather (DRAW) to detect the occurrence of LLWS and instantaneously report the location of the area in danger to air traffic controllers and airlines [Ishihara and Hata, 1995]. To make further safety, we need to develop a way to forecast the occurrence of LLWSs 5-10 minutes prior to them, by observing the descending precipitation cores which cool down the surrounding air and result in the downburst initiation. However, weather radars using mechanically-rotating parabolic antenna, including DRAW, require 5-10 minutes to scan three-dimensional volume and is not capable of observing the rapidly descending precipitation cores associated with downburst. This study reports the first high-speed volumetric observation of a downburst in Japan using an X-band phased array radar which is operated at Osaka University. The phased array radar (PAR, hereafter) used in the present study has a 60-km observation range, a range resolution of 100 m and an azimuth resolution of 1.2 degree [Yoshikawa et al., 2013]. PAR scans electronically in elevation and mechanically in azimuth, which provides a volume scan update rate of 30 seconds. In this study, we picked up a downburst event for data analysis, taking the following procedures: (step 1) we listed up all the downburst events (denoted as “microburst”) detected by DRAW at Osaka airport which is located at ~9 km distance from PAR, and then (step 2) we examined the PAR data to find clear divergent wind patterns of LLWS caused by the downburst. We consequently selected a downburst event occurred on the night of September 10, 2014. Between 23:10 and 23:40 (JST), PAR observed the three-dimensional structure of a convection cell which had a vertical scale of 5-6 km and a horizontal scale of 2-10 km, moving toward east-northeast direction. Figure 1 shows successive images of the convection cell observed by PAR. At 23:10, a precipitation core with a radar reflectivity of 44 dBZ appeared at 4-6 km ASL (white color in Figure1). The precipitation core then rapidly grew up both in spatial scale and maximum intensity and gradually started falling down toward the ground. Around 23:21, the descending core arrived at 0.5 km ASL and formed a divergent wind (red color). At the same time, DRAW detected the first LLWS denoted as “microburst”, which had a wind pattern of an outflow with a radial wind speed difference of ~22.3 m/s between a pair of outgoing winds. The PAR-observed divergence and the DRAW-detected LLWS lasted for about 13 minutes and eventually disappeared at 23:34 along with the dissipation of the causative descending precipitation core. While past studies pointed out that a downburst especially in a wet condition has descending precipitation cores at the initial stage, it was not realistic to detect them using conventional weather radar. The present study succeeded in detecting the overall processes of a downburst outbreak: the formation of a precipitation core at higher altitude, their descend to the ground, and the generation of divergent wind at low level, or LLWS. The results obtained here suggest that PAR is an innovative tool to observe the three-dimensional structure of a downburst-producing storm at a sufficiently high time resolution and to develop a new technology for a short-term prediction of the occurrence of LLWS. [Acknowledgment] The DRAW-detected LLWS data were provided by Kansai Aviation Weather Service Center and Osaka Aviation Weather Station. This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research(C) 25350514. [References] Yoshikawa, E., T. Ushio, Z-I. Kawasaki, S. Yoshida, T. Morimoto, F. Mizutani, and M. Wada, MMSE Beam Forming on Fast-Scanning Phased Array Weather Radar, IEEE Trans. Geosci. Remote Sens., Vol. 51, Issue 5, pp. 3077-3088, 2013. Ishihara, M., and K. Hata, 1995: Operational Doppler weather radar for airport in Japan. Preprints, 27th Conf. on Radar Meteorology, Vail, CO, Amer. Meteor. Soc., 723–724.