A regional lightning climatology of the UK and Ireland and sensitivity to alternative detection networks

Abstract

A total lightning (cloud‐ground and cloud‐cloud) climatology of the UK and Ireland is presented combining three different ground‐based lightning location systems over a 12‐year period (2008–2019). The study area is divided into seven geographical regions using k‐means clustering to identify areas with distinctive seasonal distributions of lightning flashes per km2/year (referred to as flash density [FD]). Different regions exhibit contrasting summer thunderstorm seasons (e.g., from April to August in the southeast of England and May to July in southern England coastal regions). Summer FD peaks in July in the English Channel and southeast and midland areas of England range from 0.1 to 0.3 FD whilst the southern England coastal region sees FDs in the range 0.03–0.06 FD. Regions more prone to winter thunderstorms are identified as having northwest facing coastlines (<0.02 FD in Northwest Scotland). Diurnal lightning distributions are also shown to have regional dependence with stronger afternoon peaks over‐land (0.05–0.1 FD in the south of England), whilst in the South coastal and English Channel regions early morning or overnight peaks (0.03–0.09 FD) are more pronounced relative to afternoon FDs (0.015–0.03). This study has demonstrated the benefit of using multiple lightning detection networks to mitigate the effects of inhomogeneities within any one data source. It is also shown that significant additional insight comes from taking a regional approach to analysing temporal distributions of lightning.

Thunderstorm tracking in Northwest Europe for enhanced hazard preparedness

“…Pattern 27 has a low mean flash count of 151 (relative to the other examples in Table 2), a medium mean duration and flashes per minute, but has the second fastest mean speed of 28.9 km/h. This low mean flash count is likely due to the location and season of the greatest frequency of thunderstorm occurrence which for pattern 27 is shown to be in coastal regions, particularly northerly facing coastlines in the summer (Hayward et al, 2022), where thunderstorms are much less likely to be as electrically active as land‐based summer thunderstorms.…”

“…Daytime warming over land areas may also trigger convection during spring and autumn producing thunderstorms which can move out to sea and persist overnight. Patterns 23, 26 and 30 are also identified by Hayward et al (2022) as frequently producing thunderstorms in northwest-facing coastlines in the UK and Ireland year-round where the airmass is cold (originating from north and Northwest Atlantic) relative to the sea and in continental based regions and the English Channel in the autumn and winter. In these regions during winter storms originating from the northnorthwest and west may penetrate a short way inland under these weather patterns.…”

“…The ATDnet waveform propagation style lightning detection is known to be affected by variation in the height of the Ionosphere (Hudson et al, 2016) which may cause some diurnal variation in lightning flash detection rates. Ideally, such inhomogeneities could be mitigated by using multiple lightning detection datasets (Hayward et al, 2022), however, diurnal lightning activity variation is beyond the scope of this study.…”

“…The patterns which produce a west-east direction of movement all show a clear westerly flow, although for patterns 14 and 30 this westerly flow is concentrated in the south of the study area (where it is likely that the highest number of thunderstorms are located). Fourteen is identified as a pattern frequently producing thunderstorms by Hayward et al (2022) in the southeast of the UK during the summer, autumn and winter. This pattern is likely providing air that is relatively cooler than ocean areas in the winter and the autumn and cooler than the continental land areas in the summer as it originates from the north and north-west.…”

“…The European continental scale research shows that thunderstorm frequency and lightning flash density decrease from southeast to northwest (Anderson & Klugmann, 2014; Enno et al, 2020; Taszarek et al, 2021; van Delden, 2001). The spatial distributions of thunderstorms and lightning flash density also vary seasonally, which is noted both on the continental scale (Anderson & Klugmann, 2014) and the regional scale (Hayward et al, 2022; Holley et al, 2014; Holt et al, 2001; Stone et al, 2022). This shows a general pattern of peaks in activity occurring in spring and summer over land areas, moving to coastal areas in the autumn and focused along northern/western facing coastlines in more northern latitudes in the winter.…”