Tornadoes (and waterspouts) are not uncommon in New Zealand, around 20 are reported each year 1), although their size and intensity are not as great as those which famously occur in parts of North America.Their occurrence within range of our high resolution radar, however, is rare and provides an interesting example of fine scale precipitation structure. During initial field trials of the Trailer Radar on 25th June 2005, a small tornado passed near to the Ardmore field station2).
In the morning of 25th June, several small convective cells passed over the radar before a large thunderstorm arrived at approximately 11:30. An arm of intense precipitation extended from the rear of the storm cell. By the time the arm terminus entered the radar’s range it had already developed a tornadic vortex. After 10 minutes, approaching a range of of hills, this vortex spawned several other vortices. Only one of these persisted longer than one minute. The original vortex dissipated just before reaching the ridgeline. The second, longer lived, vortex continued in a slightly deviated path for just over 10 minutes before itself dissipating.