On 17 September 2006, three tornadoes occurred along the east coast of Kyusyu Island in western Japan during the passage of an outer rainband in the right-front quadrant of Typhoon Shanshan. To clarify the structure of the tornado-producing storms and the mechanism of tornadogenesis, quadruply nested numerical simulations were performed using a nonhydrostatic model with an innermost horizontal grid spacing of 50 m. Several simulated convective storms in the outermost rainband exhibited characteristics of a minisupercell. One storm had a strong rotating updraft of more than 30 m s 21 and a large vertical vorticity exceeding 0.06 s 21 . This storm spawned a tornado when the low-level mesocyclone intensified. The tornado was generated on the rear-flank gust front near the mesocyclone center when a secondary rear-flank downdraft (RFD) surge advanced cyclonically around the low-level mesocyclone and overtook the rear-flank gust front at its left-front edge. Backward trajectories and vorticity budget analysis along the trajectories indicate that the secondary RFD surge played a key role in tornadogenesis by barotropically transporting the large streamwise vorticity associated with the environmental low-level veering shear toward the surface. When the secondary RFD outflow surge boundary reached the rear-flank gust front, the horizontal convergence was enhanced, contributing to the rapid amplification of the vertically tilted streamwise vorticity. The diagnostics of the vertical momentum equation and several sensitivity experiments demonstrated that precipitation loading in the area of a hook-shaped precipitation pattern was crucial to the behavior of the RFD and the subsequent tornadogenesis.

Life histories of low-level misocyclones, one of which corresponded to a tornado vortex within a winter storm in the Japan Sea coastal region on 1 December 2007, were observed from close range by X-band Doppler radar of the East Japan Railway Company. Continuous plan position indicator (PPI) observations at 30-s intervals at the low-elevation angle revealed at least four cyclonic misocyclones within the head of the comma-shaped echo of the vortical disturbance under winter monsoon conditions. The meso-b-scale vortical disturbance developed within the weak frontal zone at the leading edge of cold-air outbreaks.High-resolution observation of misocyclones revealed the detailed structures of these misocyclones and their temporal evolution. As the parent storm evolved, a low-level convergence line was observed at the edge of the easternmost misocyclone. This convergence line was considered to be important for the initiation and development of the misocyclones and the tornado through vortex stretching. The strongest misocyclone gradually intensified as its diameter contracted until landfall, and then began to dissipate soon after landfall. The temporal evolution of the misocyclones through landfall is discussed.Surface wind and pressure variations suggested a cyclonic vortex passage, which was consistent with the passage of the radar-derived misocyclone. The observed pressure drop was also consistent with that computed from the cyclostrophic equation for the modified Rankine vortex. The observed behavior of two adjacent misocyclones was primarily consistent with the rotational flow associated with the other misocyclone. The generation and development processes of the tornado and misocyclones are discussed.

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A procedure for estimating the precipitable water vapor (PWV) distribution around ground-based stations of the global navigation satellite system (GNSS) on a scale of several kilometers is presented. This procedure utilizes the difference between the zenith total delay above a GNSS station and the zenith mapped slant path delay (SPD). This difference can be used to estimate the PWV gradient in each SPD direction by assuming an exponential distribution for the horizontal water vapor gradient.The procedure was tested using an estimation of the PWV variation associated with the parent storm of an F3 Fujita scale tornado that occurred in Ibaraki prefecture on 6 May, 2012. Differential reflectivity observed by a dual-polarimetric radar indicated the existence of a developed parent cloud approximately 1h before the tornado occurred. A high-resolution numerical weather model simulation suggested the existence of a strong PWV gradient around the parent cloud, made evident by the co-existence of a strong updraft and downdraft within an approximately 5-km radius. The PWV gradient, calculated using the GNSS observation network with an average spacing of approximately 17 km, could not detect such a small-scale, strong PWV gradient. The PWV gradient estimated using the proposed procedure revealed a strong PWV gradient and its enhancement. In this case, a higher-order inhomogeneity component of each SPD played a critical role.