A phased array weather radar (PAWR) can complete one volume scan in seconds, thus enabling us to obtain high spatiotemporal resolution echo intensities and wind fields of storms. Using its rapid scanning capability, we investigated the evolution of a convective storm in detail. To describe evolution of convective storms, we used the following definitions. The precipitation cell is defined as a three-dimensionally contiguous region of 40 dBZ or greater. The precipitation core is defined by a threshold of positive deviation greater than 7 dBZ, which is a difference from the average reflectivity during the mature stage of the cell. An updraft core is defined as an updraft region of 1 m s −1 or stronger at a height of 2 km. An isolated convective storm was observed by two PAWRs on 7 August 2015 in the Kinki District, western Japan. The storm was judged as a single cell, according to the above definition. We identified nine precipitation cores and five updraft cores within 49 minutes in the mature stage of the cell. A long-lasting updraft core and its branches moved southwestward or southeastward. Around these updraft cores, the precipitation cores were generated successively. The updraft core with the longest duration lasted 73.5 minutes; in contrast, the lifetimes of precipitation cores were from 4.5 to 14.5 minutes. The precipitation cell was maintained by the successive generations of updraft cores which lifted humid air associated with a low-level southwesterly inflow. The total amounts of water vapor inflow supplied by all the identified updraft cores were proportional to the volumes of the precipitation cell, with a correlation coefficient of 0.75. Thus, the extremely high spatiotemporal resolution of the PAWR observations provides us with new evidence that an isolated convective storm can be formed by multiple precipitation cores and updraft cores.

A stationary line-shaped precipitation system (SLPS), which is one type of mesoscale convective systems (MCSs), is a typical heavy-rain-producing weather system formed during warm seasons in Japan. Although the Kinki district, western Japan, is known as a frequent occurrence region for SLPSs, their formation mechanisms in the region have not been sufficiently clarified yet because of their complex formation processes. This study investigated a SLPS event that occurred on 1 September 2015, using observational data and high-resolution numerical experiments. We also carried out numerical sensitivity experiments with regard to the orography and initial time. The observational data showed that the relative humidity at lower levels was high during the SLPS event. The southwesterly was dominant at middle levels over the Kinki district during the formation of the SLPS. The formation of the SLPS was associated with neither a mesoscale low-pressure system nor a synoptic-scale cold front, demonstrating that these were not necessary conditions for the formation of the SLPS. In the numerical experiments, we found that the SLPS was formed in a low-level convergence zone of the westerly with the warm and moist south-southwesterly from the Kii Channel. New convective cells formed over the north of Awaji Island and are propagated northeastward by the middle-level southwesterly. This cell formation process was repeated and resulted in the formation of the SLPS. The sensitivity experiments for the orography around the occurrence area of the SLPS indicated that the orography was not a significant factor for the formation of the SLPS in this event. The orography can modify the location of the SLPS.

A rapid rise of the lightning activity center in the upper part of a cloud is called a lightning bubble (LB). It remains unclear how LBs occur in thunderstorm clouds. Recently, high-spatiotemporal resolution data obtained by a phased array weather radar enabled observation of temporal changes in the three-dimensional structure of precipitation cores in a precipitation cell. To understand the mechanism by which LBs occur, we examined the relationship between the time-evolution of precipitation cores and the flash initiation points.After a precipitation core developed in an isolated thundercloud, the top height of the core reached its highest altitude and then started to descend. Meanwhile, the echo tops above the core continued to rise, which is termed an upward reflectivity pulse (URP). Over an hour, nine URPs were successively observed in the thundercloud. The average tracking period of the URPs was 3.9 minutes. Flash initiation points appeared near the highest points of the URPs and continued to rise with time. These observational results suggest that URPs cause LBs by enhancing the electric field, via the separation of graupel and ice crystals near the highest points of ascending URPs.