Wind Speed Fluctuations on Scale of 10km and less than 1km Observed in Typhoon Violet

Typhoon Violet approached the Kanto district , Japan on September 22, 1996, bringing heavy rain and strong winds. The Narita Airport Doppler radar observed the Typhoon, and the data were recorded with spatially and timely high resolution.

In the region of the northwest quadrant of the typhoon, 10km-scale linear echoes arose. These linear echoes had an orientation of NNE to SSW. The reflectivity and Doppler velocity moved in a wave-like pattern, and the phases of both were synchronized. The ridges of the field of reflectivity and the ridges of the field of Doppler velocity were concurrent, as were the troughs. The interval between each linear echoes was about from 10 to 20 km; the speed of the echoes was approximately 30 m/s; the amplitude of the Doppler velocity fluctuation was 5 to 8 m/s. These linear echoes moved continuously from west to east, maintaining this phase synchronization like the propagation of waves. The structure of that synchronization of reached an upper altitude of 5 to 6 km from the lowest level of altitude.

Wind speed fluctuations on a scale of less than 1 km were observed by radar also. Magnifying the figure of the Doppler velocity field in the area of a 5 km circle around the radar site showed that striping was apparent. The stripes run NE to SW with an interval of less than 1 km, and that had a wavelength of less than 1 km with an amplitude of approximately 5 m/s. But the reflectivity has no such fluctuation. They reached an altitude of less than 1 km above ground level.

Although the author is researching the mechanism of these phenomena, 10km scale phenomenon of Typoon Violet was similar to one which Gall et al.(1998) observed in the rainband on a scale of 10 kilometers near the eye wall , and the phenomenon on a scale of less than 1km similar to one which Wurman and Winslow (1998) observed in Hurricane Fran. Both of the phenomena of these studies were the roll convection in the boundary layer.

Gall, R., J.Tuttle, and P. Hildebrand, 1998: Small-Scale Spiral Bands Observed in Hurricanes Andrew, Hugo, and Erin. Mon. Wea. Rev., 126, 1749-1766.

Wurman, J., and J. Winslow, 1998: Intense Sub-Kilometer-Scale Boundary Layer Rolls Observed in Hurricane Fran. Sci. Mag., 280, 555-557.