Mesoscale Vortex that Caused Marine Accidents due to a Sudden Gusty Wind in the Southwestern Part of the Sea of Japan on 1 September 2015

A sudden gusty wind accompanied by a mesoscale vortex of 30km diameter occurred in the Tsushima Strait at the southwest part of the Sea of Japan between 0300 and 0400 JST (Japan Standard Time) on 1 September 2015. It upset 6 fishery boats, causing 4 fatalities and 2 missing people. Some of the survived fishermen reported that they were hit by a waterspout. The mesoscale vortex was located near the warm front about 300 km northeast of the center of an extratropical cyclone. The structure and evolution of this mesoscale vortex are examined using observational data set and a numerical simulation.

A C-band Doppler radar of Japan Meteorological Agency (JMA), which is located at Fukuoka, about 130km south east of the area of the marine accidents, detected a spiral-shaped reflectivity pattern associated with a convective system to the east of Tsushima island at around 0300 JST. The convective system propagated northward with time. At 0320 JST, a couplet of positive and negative Doppler velocities exceeding 50 m s^-1 started to be observed near the center of the spiral-shaped reflectivity pattern, which later transformed into a circular reflectivity pattern with an eye-like weak echo region at the center by 0400 JST.

A numerical simulation using JMA Non-hydrostatic Model (JMANHM) with horizontal resolution of 2km and 50 vertical levels successfully reproduced the mesoscale vortex. The simulated vortex had a diameter of about 10-50 km, and was formed in the northeast quadrant of the extratropical cyclone at around 0300 JST. It had a warm core structure at low levels. The vortex developed between 0330 and 0400 JST and the associated wind exceeded 20 m s^-1 near the surface. An additional numerical simulation with a finer horizontal resolution have been performed to clarify more detailed structure and evolution of the vortex and to reproduce small-scale features that caused the damaging gusty wind. The results of the additional experiment will be presented at the conference.