A TORNADO WITH ANTICYCLONIC ROTATION AT CLOONCAN, CO. MAYO

Abstract

On 31 July 2004 a tornado that was unusual in several ways occurred in the west of Ireland. It had an anticyclonic, rather than the more normal cyclonic, rotation, and it occurred under relatively unpromising synoptic conditions, where there was a slack surface-pressure gradient close to the centre of a ridge of high pressure over north-west Europe. However, regional low-level wind shear and strong convergence are shown to have created a favourable environment over Co. Mayo, aided by a limited amount of convective available potential energy (CAPE) that accumulated beneath an inversion at 816hPa and by a strong vertical moisture profile. The role of the contrasting thermal properties between Lough Carra and the adjacent land surface in producing an anticyclonically rotating tornado from a horizontal vortex that developed within the sheared environment is considered. All of these features are found to be strongly localised. Comparisons are made with the non-mesocyclonic tornado environments documented in US research, and a number of contrasts are noted. The tornado track was only partially traceable due to the terrain, and it reached its widest point of 18m where it entered bogland after 0.75km.