Analysis of the structure and mechanism of the tornado at the periphery of Typhoon Mangkhut

In this study, an enhanced Fujita 2 scale tornado that struck Sanshui District of Foshan City on the morning of September 17, 2018, was simulated at a resolution of 49 m using reanalysis data and the WRFV3.6.1 model. The structure of the tornadic supercell and low-layer tornado-like vortex (TLV) were analyzed using the mode data. Furthermore, shearing wind helicity was introduced to diagnose the dynamic reasons for the formation of TLV. The results revealed that the tornado was produced by a supercell in the spiral rainband of Typhoon Mangkhut. The supercell exhibited typical characteristics, such as a hook echo, an inflow notch, a bounded weak echo region, and an overhanging echo. The overshooting top, anvil, and wall cloud can be observed in the vertical cross-section of the hydrometeor mixing ratio. The vertical circulation in the supercell was formed by the updraft (UD), front flank downdraft, and rear flank downdraft (RFD). Moreover, TLV occurred between the low-level UD and RFD. The life history of TLV could be divided into the initial stage, the developing stage, the mature stage, and the weakening stage. During the mature stage of TLV, the inside sank and the outside rose. The diagnosis of the vertical vorticity equation indicated that the torsion term was more critical to the strengthening and upward spread of TLV during the developing stage, while the stretching term dominated the changes in TLV during its mature stage.