Formation Mechanism of a Waterspout Associated with a Mini-Supercell near Okinawa, Japan, in March 2024: Insights from Observations and Numerical Simulations

A waterspout was observed on 5 March 2024 over the ocean near Okinawa, Japan, south of the center of an extratropical cyclone. Doppler radar observations revealed a mesocyclone above the waterspout, suggesting that the mesocyclone was responsible for the waterspout formation. A convective system associated with the mesocyclone was likely a mini-supercell because static instability was moderate whereas low-level vertical wind shear was large around Okinawa, and because the echo top of the convective system was not high. Numerical simulations with realistic boundary conditions were performed to investigate the development process of the mesocyclone and waterspout. In the 250-m horizontal resolution simulation, a mini-supercell featuring a low-level mesocyclone and a strong near-surface vortex below it was reasonably reproduced. A vorticity budget analysis indicated that tilting and vertical advection of vorticity contributed to the development of the mesocyclone. The strong near-surface vortex developed along a gust front beneath the mini-supercell. In the 50-m-resolution simulation, the near-surface vortex was even more intense enough to be called a tornado-like vortex (TLV). Strong updraft beneath the low-level mesocyclone promoted the rapid amplification of the TLV through vortex stretching.