A strange supercell embedded in a linear convective system

In Japan, approximately 10% of convective systems that produce tornadoes are supercell-type storms. Most of them are mini-supercells that form within the outer rainbands of tropical cyclones. However, the case that occurred on September 30, 2018, represents a highly unusual event. Firstly, the convective systems yielding tornadoes located outside the outer rainbands, far from the eye of the Typhoon, ‘Trami’. Furethermore, their arrangement was nearly parallel to the typhoon's radial direction, distinguishing them from typical outer rainband configurations

    The present study aims to clarify the structure and generation mechanism of the parent convective system. We used the data obtained from JMA Muroto operational radar and our own X-band radar network, and the initial values of mesoscale model provided by JMA.

    The radar data showed that the mesocyclones of about 2 km in diameter, persisted for 35 minutes and a tornado vortex was located to the southeast of the mesocyclone. We also observed a vault structure within the mesocyclone. These features confirm that the parent storm was a mini-supercell. The mesocyclone appeared at the southwestern end of a linear rainband, which measured approximately 20 km in length and 4 km in width. Each rainband was organized from merged small cumulonimbi, indicating that the parent storm was an aggregate of small convective cells with a mini-supercell positioned at its southwestern extremity. Their rainbands propagated northwestward, almost perpendicular to their orientations. Such fact shows the presence of a cold pool from the preceding rainband in the forward flank of the parent rainband, and it was found that the mesocyclone was unable to receive moist warm air necessary as energy for the parent storm.

    In this presentation, we will introduce these anomalous features of this unique storm and will pose a question regarding the precise definition of a supercell.