Observations of the Intensity and Structure Changes within a Winter Tornadic Storm during Landfall over the Japan Sea Area

抄録

Winter tornadic storms over the Japan Sea area have been observed during the Shonai Area Railroad Weather Project between 2007 and present. The MRI portable X-band Doppler radar has been installed on the roof of Shonai Airport building in Sakata City during this project. On 25 January 2008, a winter tornado passed very close to the radar near the coast. In this paper, the detailed vertical structures of the high-resolution reflectivity and velocity fields of this tornado during landfall are described. Before landfall, the reflectivity field revealed that the center of the tornado had a weak-echo "eye" partially surrounded by an eyewall. After landfall, the eyewall was transformed to spiral structures spiraling outward from the eye. The diameter of the maximum tangential wind contracted from 582 m to 291 m at the height near 150 m. The radar data indicate that the tornado was tilted downstream and the diameter increased with height. The results in the present study will be reflected the information of radar signatures from a winter tornado, which can contribute to the refinement of future wind gust algorithms concerning operational use.