This work evaluates how clouds evolve to thunderstorms in terms of microphysical characteristics to produce the first intracloud (IC) and cloud‐to‐ground (CG) lightning flashes. Observations of 46 compact isolated thunderstorms during the 2011/2012 spring‐summer in Southeast Brazil with an X‐band polarimetric radar and two‐ and three‐dimensional Lightning Location Systems demonstrated key parameters in a cloud's vertical structure that produce the initial electrification and lightning activity. The majority (98%) of the first CG flashes were preceded (by approximately 6 min) by intracloud (IC) lightning. The most important aspect of the observations going into this paper, which came originally from the visual examination of a large number of thunderstorms, is that an initial positive differential reflectivity (ZDR) (associated with supercooled raindrops) evolved to reduced ZDR (and even negative values) in the cloud layer between 0° and to −15°C before and during the time of the initial lightning, suggesting evolution from supercooled raindrops to frozen particles promoting the formation of conical graupel. An enhanced negative specific differential phase (KDP) (down to −0.5° km−1) in the glaciated layer (above −40°C) was predominantly observed at the time of the first CG flash, indicating that ice crystals, such as plates and columns, were being vertically aligned by a strong electric field. These results demonstrate that the observations of ZDR evolution in the mixed layer and negative KDP in the upper levels of convective cores may provide useful information on thunderstorm vigor and lightning nowcasting.

This work analyzes some of the physical and microphysical properties of Mesoscale Convective Systems (MCS) and cloud-to-ground lightning. Satellite data from the GOES-10 infrared and NOAA-18 and TRMM microwave channels and lightning information from the Brazilian Lightning Detection Network (BrasilDAT) were utilized for the period from 2007 to 2009. Based on an automatic MCS detection method, 720 MCS life cycles were identified during the period and region of study, with a lightning detection efficiency of over 90%. During the diurnal cycle, the electrical activity maximum occurred close to the time of maximum convective cloud fraction and 3 hours after the maximum normalized area expansion rate. The diurnal cycles of both properties are modulated by diurnal heating and thus could be used to monitor the diurnal variability of lightning occurrence. The area growth during the initial phase of the MCS exerted a strong influence on their size-duration, and potential for electrical activity during their life cycle. The average lightning life cycle exhibited a maximum close to MCS maturation, while the maximum of the average lightning density occurred close to the initial MCS life cycle stages. The growth rate of electrical activity during the early stages can indicate the strength of convection and the possible duration of systems with lightning occurrence. The strong condensation processes and mass flux during the growth phase of the systems can provide favorable conditions for cloud electrification and lightning occurrence. A comparison of high microwave frequencies with lightning data showed a strong relationship of the vertically integrated ice content and particle size with lightning occurrence. The polarization difference in the 85 GHZ channel showed that electrical activity increases linearly with polarization reduction, associated with a high value of Pearson's correlation coefficient. This suggests that regions with more intense electrical activity are predominantly located in areas with a high concentration of larger ice particles that are preferentially oriented vertically, due to the existence of intense updrafts and the electric field. These results demonstrate the potential use of thermodynamic, dynamic and microphysical characteristics for analyzing storm severity and as additional information for monitoring of electrical activity over large regions that lack ground-based lightning sensors and for nowcasting.

This paper analyzes the cloud microphysics in different layers of storms as a function of three‐dimensional total lightning density. A mobile X‐band polarimetric radar and very high frequency (VHF) sources from Lightning Mapping Array (LMA) observations during the 2011/2012 Brazil spring‐summer were used to determine the microphysical signatures of radar vertical profiles and lightning density. This study quantified the behavior of 5.3 million vertical profiles of the horizontal reflectivity (ZH), differential reflectivity (ZDR), specific differential phase (KDP), and correlation coefficient (ρHV). The principal changes in the polarimetric variables occurred only for VHF source rate density greater than 14 VHF sources per km2 in 4 min. These storms showed an enhanced positive KDP in the mixed 1 layer (from 0 to −15°C) probably associated with supercooled liquid water signatures, whereas regions with negative ZDR and KDP and moderate ZH in the mixed 2 layer (from −15 to −40°C) were possibly associated with the presence of conical graupel. The glaciated (above −40°C) and upper part of the mixed 2 layers showed a significant trend to negative KDP with an increase in lightning density, in agreement with vertical alignment of ice particle by the cloud electric field. A conceptual model that presents the microphysical signatures in storms with and without lightning activity was constructed. The observations documented in this study provide an understanding of how the combinations of polarimetric variables could help to identify storms with different lightning density and vice versa.

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