Analysis of an anomalously severe thunderstorm system over Northern Ireland

In Northern Ireland, as elsewhere in Europe, attempts to reduce the societal impacts of severe convective storms are constrained by underestimations in frequency and intensity of extreme events within the present-day climate regime. Such underestimations also present difficulties in estimating probabilities of extreme storm frequency and intensity in relation to future climate change scenarios. Detailed analyses of past extreme events enhance the robustness of hydrometeorological and climatological models, and improve human perception of the true nature of present climate. In Northern Ireland, extensive thunderstorm development is infrequent due to relatively low relief and limited severe convectional activity in summer. On 25–26 July 1985 extensive thunderstorm activity occurred accompanied by hailstones 3 cm in diameter, some of the largest on record to fall over the Province. The event predates operation of the Northern Ireland component of the UK rainfall radar network. Nevertheless, utilizing quality-controlled autographic rain gauge records and radiosonde data, the synoptic situation was examined and the mesoscale precipitation signatures determined throughout the duration of thunderstorm activity. Within the cyclonic circulation covering Northern Ireland for much of the day, potential instability existed at three levels and on release the buoyant upward motion resulted in clouds of more than 11 km in depth. While upland areas recorded 30 to 50 mm, the greatest precipitation totals of up to 85 mm were received in lowland areas around Lough Neagh. Several lowland sites recorded rainfall totals and intensities with frequencies of about 1 in 100 years. Significant thunderstorm activity occurred over a period of 11 h and resulted in serious flooding. Observed mesoscale precipitation patterns displayed limited evidence of orographic enhancement within the moist southeast to south airflow. At best, orography assisted in terms of initial uplift, but thereafter precipitation distribution was reliant upon thunderstorm cell structure and movement with input from local site and exposure influences. A factor possibly reducing the potential for marked orographic enhancement was the absence of strong winds at low levels. Mesoscale precipitation patterns suggested enhancement locally around the west and north shores of Lough Neagh. Energy exchanges between the Lough surface and the overlying atmosphere may have had repercussions upon water vapour content and buoyancy within the cyclonic south to southeasterly airflow, leading to this precipitation enhancement. Occurrence in the Mournes of hailstones 3 cm in diameter was indicative of the strength and endurance of the updraught component within individually propagating cells. The remarkable amounts and high intensities of rainfall, accompanied by large hailstone development, emphasized the importance of the synoptic event within the climatological history of Northern Ireland. With an absence of notable orographic enhancement of precipitation, the event also highlighted the occurrence of deviations from the mean precipitation patterns normally expected within cyclonic circulations.