Intensity and Structure Changes of Low-Level Tornadic Vortices during Landfall over the Japan Sea Area

In order to develop a strong gust detection system for railroad, the Shonai area railroad weather project has investigated fine-scale properties of the wind gust using two X-band Doppler radars and the network of 26 surface weather stations since 2007. One of our remarkable findings is that most of the strong wind gusts in the winter season are associated with vortices (misocyclones). Based on the finding, we had made a prototype of the gust detection system that depends on vortex detection and tracking by using a single Doppler radar with a single elevation angle, and have been testing the prototype. The prototype is designed to detect vortices automatically and monitor them in real-time by the algorithm that relies on the continuous change in vortex strength, traveling direction and velocity.

From our observations so far, it is found that vortices often develop over the Japan Sea, intrude into land, and sometimes cause strong wind gusts. In such cases, the property of vortices over the sea is important for the gust detection system. However, if the property of vortices changes rapidly during landfall, the present detection and tracking system that relies on continuous change in vortex property may not work well. Thus, it is necessary for the system design to clarify the change in the properties of vortices during landfall.

In this presentation, we will discuss the intensity and structure changes of tornadic vortices during landfall. Detail analysis of the radar data with high temporal (at about 50-s interval of each volume-scan) and high resolution (analyzed vortices are within 3.5 km from the radar) at the low-level atmosphere (from 100 to 400 m) during landfall of vortices revealed some features as follows: (1) maximum tangential velocity decreases rapidly after landfall, (2) the decrease of tangential velocity can be seen to propagate upward from the lowermost level after landfall, (3) vortices tilt rearward with height before landfall.