Wind Gusts Associated with Small-Scale Vortices in the Core of Typhoon GONI (2015)

The very strong typhoon GONI (2015) passed over the Yaeyama Islands in Japan during the mature stage on 23 August 2015 and caused severe wind damage. The minimum central pressure of the typhoon was estimated at 935 hPa and the record-breaking maximum wind of 71.0 m/s was observed on Ishigaki Island. Unprecedented finescale surface observations in the core of the typhoon by seven weather stations, which are densely deployed in Yaeyama Islands, successfully captured several small-scale disturbances inside the eye and within the eyewall. Strong winds with pressure drop and wind direction change were periodically observed on the passage of the kinks of the polygonal eyewall in the radar reflectivity pattern. Spectral analysis of wind data revealed that about 1000-s period is dominant. Extreme wind gusts accompanied by sudden pressure drop with about 4 hPa were also observed in the eyewall in a time scale of less than a minute. It is found that wind gusts and extreme strong winds are caused by the superposition of wind field associated with small-scale disturbances within an eyewall and larger-scale basic field of the typhoon circulation. Moreover, observed wind field showed that a number of vortices exist inside the typhoon eye, however their pressure perturbations are smaller than those in the eyewall.