Mesoscale Convective Vortex that Causes Tornado-Like Vortices over the Sea: A Potential Risk to Maritime Traffic

Abstract

AbstractStrong gusty winds in a weak maritime extratropical cyclone (EC) over the Tsushima Strait in the southwestern Sea of Japan capsized several fishing boats on 1 September 2015. A C-band Doppler radar recorded a spiral-shaped reflectivity pattern associated with a convective system and a Doppler velocity pattern of a vortex with a diameter of 30 km [meso-β-scale vortex (MBV)] near the location of the wreck. A high-resolution numerical simulation with horizontal grid interval of 50 m successfully reproduced the spiral-shaped precipitation pattern associated with the MBV and tornado-like strong vortices that had a maximum wind speed exceeding 50 m s−1 and repeatedly developed in the MBV. The simulated MBV had a strong cyclonic circulation comparable to a mesocyclone in a supercell storm. Unlike mesocyclones associated with a supercell storm, however, its vorticity was largest near the surface and decreased monotonically with increasing height. The strong vorticity of the MBV near the surface originated from a horizontal shear line in the EC. The tornado-like vortices developed in a region of strong horizontal shear in the western part of the MBV, suggesting that they were caused by a shear instability.