Intensification of single cell storms prior to lightning onset

Abstract

Single cell storms in the United Kingdom can produce lightning, despite apparently only having developed to towering cumulus rather than cumulonimbus. Such marginal thunderstorms still present severe weather hazards but are difficult to identify and predict and therefore provide a warning. Observations from the Met Office radar mosaic and ATDNet (Arrival Time Difference Network) show that these single cell storms demonstrate a characteristic increase in the area of high reflectivity storm core during the 15 min prior to the first lightning. By using the Met Office Unified Model to investigate reflectivity development in modelled storms, a microphysical explanation for the observed reflectivity increase is identified. During a rapid reflectivity increase, the updraft area at the melting layer, the peak updraft velocity and the storm graupel mass increase. The three quantities examined are linked to each other and to the generation of charge within the storm. The production of graupel is promoted by the increase in updraft area and charge separation is enhanced by the faster peak updraft velocity. This explains some of the physical differences between single cell storms that produce lightning and apparently similar storm systems which do not. It also provides a new basis with which to predict lightning hazard for marginal storms.

Exploring relationships between weather patterns and observed lightning activity for Britain and Ireland

“…While this study choses to report even small‐scale activity (e.g., isolated storms such as those in Courtier et al ., 2019) because of their risk to aviation (e.g., Wilkinson et al ., 2013), the most disruptive weather will naturally occur on significant thunder days. Therefore, it would be useful to provide a forecast of the probability of the region of study experiencing a significant thunder day in any given weather pattern (Figure 6).…”

“…Of these, Courtier et al . (2019) discuss thunderstorm events which occur in weather patterns 7 (cyclonic south‐westerly with low close to UK) and 13 (anticyclonic north‐westerly with a high centred south west of Ireland). However, it should be noted they aimed to find simple single‐celled storms rather than larger lightning events, and thus the weather pattern numbers in the two cases studied by them tend to give lower thunder areas.…”

“…Courtier et al . (2019) note that in their case focussing on single‐celled storms, 37 out of 71 storms (52%) which look similar in radar reflectivity and despite showing obvious intensification did not produce any lightning. As mentioned earlier, each ATDnet lightning flash recorded has been quality‐controlled using radar data, giving confidence that weak storms are being detected rather than false signals from ATDnet.…”

“…The 16‐km radius used is slightly larger than the typical size of single‐celled systems in the UK: Courtier et al . (2019) examine cross sections of a single‐celled storm that is roughly 10 km in radius at its maximum extent. Hanley et al .…”

“…In practise, this value can be adjusted to produce larger or smaller thunder areas if desired. The 16-km radius used is slightly larger than the typical size of single-celled systems in the UK: Courtier et al (2019) examine cross sections of a single-celled storm that is roughly 10 km in radius at its maximum extent. Hanley et al (2015) studied six cases and found the maximum storm diameter observed was close to 30 km (15 km radius), with most storms having a smaller cell size.…”