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.

In the last three decades mm wavelength radars within the Ka-bands and W-bands have proved to be paramount tools for cloud and precipitation process studies (Kneifel & Moisseev, 2020;Ori et al., 2020) and long-term cloud monitoring (Kollias et al., 2020;Stein et al., 2015;Tridon, Battaglia & Watters, 2017). This success has been driven by their larger sensitivity to small droplets and ice crystals (Lhermitte, 1990), their superior spatial resolution, and their reduced susceptibility to Bragg scattering compared with centimeter-wavelength radars in the S, C, and X bands (Kollias et al., 2007), whereas their portability and compactness have made them suitable instruments to be deployed on ships (e.g., Protat et al., 2016), aircraft (e.g., Walker McLinden et al., 2021, and spacecraft. The latter application is demonstrated by the W-band CloudSat and the Ka-band GPM radars (Battaglia et al., 2020, and references therein). Synergistic measurements which combine multifrequency centimeter and millimeter wavelength radars (e.g.,