Ensemble Experiments of Meso-Beta-Scale Vortex that Caused a Sudden Gusty Wind over the Sea

On 1 September 2015, a sudden gusty wind caused 6 shipwrecks in the Tsushima Straight at the southwestern part of the Sea of Japan, resulting in 5 fatalities and a missing person. Near the location of the shipwrecks, a meso-beta-scale vortex (MBV) that formed in the northeast quadrant of an extratropical cyclone was observed by a C-band Doppler radar observation of JMA. Our previous study performed a triply nested high-resolution numerical simulation with horizontal resolution of 50 m and has suggested that tornado-like vortices that formed within MBV caused the shipwrecks. In the present study, ensemble forecasts for 100 members using ensemble Kalman filter analyses are performed to understand favorable atmospheric condition for the development of the MBV.

A composite analysis is performed to clarify the differences in structure and environment of MBVs between strongest 10 members (STRONG10) and weakest 10 members (WEAK10). At the early developing stage (0230 JST), the near-surface cyclonic horizontal shear between southerly and southeasterly winds in the south region of the MBV center for STRONG10 is larger than that for WEAK10. At the later developing stage (0430JST), the near-surface cyclonic horizontal shear between northeasterly and easterly winds in the north region of the MBV center for STRONG10 is stronger than that for WEAK10. These differences of near-surface horizontal shear are closely related to the structure of the extratropical cyclone. For STRONG10, cyclonic horizontal shear of winds in the northeast quadrant of the extratropical cyclone is larger compared to WEAK10, while the strength of extratropical cyclones is comparable to WEAK10. An ensemble-based sensitivity analysis also shows that strength of the MBV is sensitive to cyclonic circulation associated with horizontal shear of winds around the MBV.

A vorticity budget diagnosis reveals that the stretching of vertical vorticity for STRONG10 is larger than that for WEAK10, while tilting of horizontal vorticity is smaller for STRONG10. It is suggested that the strength of cyclonic horizontal shear associated with the extratropical cyclone is important for the development of the MBV.