Structures and Evolutions of Meso-β-scale Vortices that Spawned Tornado-like Vortices.

Sudden gusty winds caused by meso-β-scale vortex (MBV) of 30-50 km diameter occurred at the southwestern part of the Sea of Japan between 0300 and 0400 JST (Japan Standard Time; UTC+9 hour) on 1 September 2015. The MBV formed in the northeastern part of the center of extratropical cyclones. A C-band Doppler radar of Japan Meteorological Agency (JMA) detected spiral-shaped reflectivity patterns associated with the MBV. Couplet of positive and negative Doppler velocities exceeding 50 m s^-1was observed near the center of the spiral-shaped reflectivity pattern.The observed convective system is not associated with a quasi-linear convective system nor pre-existing mesoscale convective complexes. Thus, to our knowledge, this kind of MBV and the associated strong gusty winds due to embedded tornado-like vortices over the oceans have not been reported previously, yet they can be a serious threat to marine traffic.

A triply-nested numerical simulation using JMA non-hydrostatic model was performed to clarify the fine structure and evolution of the MBVsthat caused the damaging gusty winds. The simulations with the finest horizontal resolution of 50m and 100 vertical levels successfully reproduced the MBVs with spiral-shaped precipitation systems and associated tornado-like vortices (TLVs) within the MBV. The simulated MBV had the maximum vertical vorticity near the surface. A vorticity budget analysis and a circulation analysis show that the near-surface vorticity of the MBV is strengthened by stretching of vertical vorticity associated with horizontal shear between northeasterly and southeasterly winds in the west of the MBV center. TLVs with maximum vorticity exceeding 1 s^-1grew and decayed repeatedly near the surface in the region of the horizontal shear. It is suggested that TLVs were generated and strengthened by the shear instability. The simulated maximum wind speed near the surface were about 50 m s^-1, which is comparable to Japanese Enhanced Fujita scale of 1~2.

Similar MBVs have been detected with radar observations over the oceans around Japan on 21 August 2011, 22 September 2016, and 16 October 2016. The MBV in 2011 also caused a gusty wind, resulting in a shipwreck. Similar triply-nested simulations succeeded in reproducing fine structures of MBVs and associated strong TLVs. The locations of TLVs with respect to the MBV center, however, varies from case to case: a TLV in August 2011 formed in the southeast of the MBV, TLVs in September 2016 formed in the west of MBV, and TLVs in October 2016 formed in west and east of the MBV. Thus, TLVs in MBVs seem to occur in various regions with respect to the MBV center.