Observed Near-Surface Wind Structure in the Inner Core of Typhoon Goni (2015)

The very strong Typhoon Goni passed over the Yaeyama Islands in southwestern Japan during the rapid intensification stage on August 23, 2015. Surface data collected by the dense network of weather stations as well as Doppler radar observations over the islands revealed a finescale structure in the inner core of the typhoon near the surface. Goni had a clear eye surrounded by a square-shaped eyewall with intense convection. The surface observations revealed that several vortices with a diameter of ~7–10 km accompanied by a pressure deficit were present inside the eye. From the Doppler velocity field, mesovortices approximately 10 km in diameter were found at the apexes of the square-shaped eyewall. These mesovortices and the inner rainbands emanating outward from the apexes of the polygonal eyewall generally exhibited features typical of vortex Rossby waves. The mesovortices were accompanied by a pressure deficit at the surface and enhanced surface winds. The data also indicated the first observational evidence of near-surface mixing between the eye and eyewall through the mesovortices, that is, the transport of high equivalent potential temperature in the eye toward the eyewall. The radar data revealed that many radar-reflectivity filaments that had a pleated shape with lengths of a few kilometers extended perpendicularly from the inner edge of the eyewall at low levels. The filaments associated with wind perturbations at low levels caused significant wind gusts accompanied by sudden pressure drops and shifts in wind direction at the surface.