Electrification and lightning are simulated for a small continental multicell storm. The results are consistent with observations and thus provide additional understanding of the charging processes and evolution of this storm. The first six observed lightning flashes were all negative cloud-to-ground (CG) flashes, after which intracloud (IC) flashes also occurred between middle and upper levels of the storm. The model simulation reproduces the basic evolution of lightning from low and middle levels to upper levels. The observed lightning indicated an initial charge structure of at least an inverted dipole (negative charge above positive). The simulations show that noninductive charge separation higher in the storm can enhance the main negative charge sufficiently to produce negative CG flashes before upper-level IC flashes commence. The result is a “bottom-heavy” tripole charge structure with midlevel negative charge and a lower positive charge region that is more significant than the upper posit...

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Simulated Electrification of a Small Thunderstorm with Two-Moment Bulk Microphysics

[1] It is generally thought that the lower positive charge region serves to enhance the electric field at the bottom of the main negative charge region and thereby facilitate the launching of a negatively-charged leader toward ground. On the other hand, the presence of excessive lower positive charge region may prevent the occurrence of negative cloud-to-ground discharges by “blocking” the progression of descending negative leader from reaching ground and thus “converting” the potential cloud-to-ground flash to an intracloud (or cloud-to-air) one. Assuming that the preliminary breakdown pulse train is a manifestation of interaction of a downward-extending negative leader channel with the lower positive charge region, we qualitatively examine the inferred dependence of lightning type on the magnitude of this charge region.

/pdf/some-inferences-on-the-role-of-lower-positive-charge-region-43q3kg3cbz.pdf

Some inferences on the role of lower positive charge region in facilitating different types of lightning

The electrical structure inside thunderstorm clouds and the changes with time of this structure are fundamental parameters for atmospheric electricity studies on Earth. This paper gives a review of the current state of knowledge about thunderstorm charge structure based primarily on in-cloud vertical profiles through different types of deep convective clouds, cirriform anvil clouds, and stratiform precipitation regions associated with mid-latitude mesoscale convective systems. Results on the dynamical evolution of the vertical charge structure during the early and decay stages from series of balloon soundings through small thunderstorms are described, along with recent findings regarding the importance of the charge structure in determining the current contribution of small and large thunderstorms to the Earth’s global electrical circuit.

Charge Structure and Dynamics in Thunderstorms

[1] We examine the various conceptual cloud charge configurations and scenarios leading to production of positive lightning with a view toward an explanation of its observed properties. Data for 52 positive cloud-to-ground flashes containing 63 strokes recorded in Gainesville, Florida, in 2007–2008 are presented and discussed. The U.S. National Lightning Detection Network (NLDN) located 51 (96%) of the positive strokes at distances of 7.8 to 157 km from the field-measuring station. In 42 (81%) of the 52 flashes, the first (or the only) leader-return stroke sequence was not preceded by in-cloud discharge activity. The average number of strokes per flash is 1.2. Single-stroke flashes constitute 81%. In three (38%) of eight two-stroke flashes, the second stroke likely followed the channel of the first stroke, and in five (62%) flashes it likely created a new termination on ground. Eight (15%) of the 52 positive flashes had detectable preliminary breakdown pulse trains in their electric field records. Electric field waveforms of 14 (27%) first strokes (also of one third stroke) are preceded by pronounced pulses, apparently indicative of leader stepping. NLDN-reported peak currents for 48 positive strokes in 40 flashes ranged from 20 to 234 kA with a geometric mean value of 75 kA. The median charge transfers in 1, 2, and 40 ms after the beginning of return stroke electric field change for an assumed height of 12 km were estimated to be 13, 18, and 34 C, respectively. Two bipolar lightning discharges are additionally examined.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JD017545

Positive lightning: An overview, new observations, and inferences

A new prognostic, spatially and temporally dependent variable is introduced to the Weather Research and Forecasting Model (WRF). This variable is called the potential electrical energy (Ep). It was used to predict the dynamic contribution of the grid-scale-resolved microphysical and vertical velocity fields to the production of cloud-to-ground and intracloud lightning in convection-allowing forecasts. The source of Ep is assumed to be the noninductive charge separation process involving collisions of graupel and ice particles in the presence of supercooled liquid water. The Ep dissipates when it exceeds preassigned threshold values and lightning is generated. An analysis of four case studies is presented and analyzed. On the 4-km simulation grid, a single cloud-to-ground lightning event was forecast with about equal values of probability of detection (POD) and false alarm ratio (FAR). However, when lighting was integrated onto 12-km and then 36-km grid overlays, there was a large improvement in the forecast skill, and as many as 10 cloud-to-ground lighting events were well forecast on the 36-km grid. The impact of initial conditions on forecast accuracy is briefly discussed, including an evaluation of the scheme in wintertime, when lightning activity is weaker. The dynamic algorithm forecasts are also contrasted with statistical lightning forecasts and differences are noted. The scheme is being used operationally with the Rapid Refresh (13 km) data; the skill scores in these operational runs were very good in clearly defined convective situations.

Predicting Cloud-to-Ground and Intracloud Lightning in Weather Forecast Models

[1] Electrical characteristics of thunderstorms on the central Tibetan Plateau at an altitude of 4508 m have been studied. The evolution of surface electric (E) field and the E field changes produced by lightning flashes under a representative thunderstorm revealed a tripole charge structure with a larger-than-usual lower positive charge center (LPCC). The storms appear to begin with the lower dipole of a normal tripole structure, rather than with the upper dipole followed by the development of a weaker lower positive charge. The flash rate is quite low and the average value is usually 1 fl/min. The IC flashes were usually polarity-inverted and occurred in the lower dipole. The large LPCC did not cause positive CG flashes to occur during the whole storm lifetime, and only negative CG flashes were observed in the late stage of the storm.

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Wuzhou Wei

Papers

The lower positive charge center and its effect on lightning discharges on the Tibetan Plateau