No abstract

[1] A two-dimensional axisymmetric model of charge relaxation in the conducting atmosphere is used in conjunction with a probabilistic lightning model to demonstrate how realistic cloud electrodynamics lead to the development of blue and gigantic jets. The model accounts for time-dependent conduction currents and screening charges formed under the influence of the thundercloud charge sources. Particular attention is given to numerical modeling of the screening charges near the cloud boundaries. The modeling results demonstrate the important role of the screening charges in local enhancement of the electric field and/or reduction of net charge in the upper levels of the thundercloud. The charge relaxation model presented in this work confirms the previous results obtained with a simpler model by Krehbiel et al. (2008), specifically that the accumulation of screening charges near the thundercloud top produces a charge configuration leading to the initiation of blue jets, while the effective mixing of these charges with the upper thundercloud charge may lead to the formation of gigantic jets.

[1] The direct comparison of lightning mapping observations by the New Mexico Tech Lightning Mapping Array (LMA) with realistic models of thundercloud electrical structures and lightning discharges represents a useful tool for studies of electrification mechanisms in thunderstorms, initiation and propagation mechanisms of different types of lightning discharges as well as for understanding of electrical and energetic effects of tropospheric thunderstorms on the upper regions of the Earth's atmosphere. This paper presents the formulation of a new three-dimensional probabilistic model for investigating the structure and development of bidirectional positive and negative lightning leaders. The results closely resemble structures observed by the LMA during intracloud discharges. The model represents a synthesis of the original dielectric breakdown model based on fractal approach proposed by Niemeyer et al. (1984) and the equipotential lightning channel hypothesis advanced by Kasemir (1960) and places special emphasis on obtaining self-consistent solutions preserving complete charge neutrality of the discharge trees at any stage of the simulation. A representative simulation run is compared to a typical intracloud discharge measured by LMA in a New Mexico thunderstorm on 31 July 1999. Following the conclusions from Coleman et al. (2003), the comparison of the model and observed discharges reveals that an adequate choice of the electrical structure of the model thundercloud permits the development of a model intracloud discharge reproducing principal features of the observed event including the initial vertical extension of the discharge between the main negative and upper positive charge regions of the thundercloud, and the subsequent horizontal propagations in these regions. Also consistent with observations (e.g., Coleman et al., 2003), negative and positive leaders mainly develop in the upper positive and main negative charge regions, respectively. For the particular model case presented in this paper, the total charge transfer, the vertical dipole moment and the average linear charge density associated with the development of bidirectional structure of leader channels are estimated to be 37.5 C, 122 CÁkm, and 0.5 mC/m, respectively, in good agreement with related data reported in the refereed literature. The model results also demonstrate that the bulk charge carried by the integral action of positive and negative leaders leads to a significant (up to 80%) reduction of the electric field values inside the thundercloud, significantly below the lightning initiation threshold.

[1] Blue and gigantic jets are transient luminous events in the middle atmosphere that form when conventional lightning leaders escape upward from the thundercloud. The conditions in the Earth's atmosphere (i.e., air density, reduced electric field, etc.) leading to conversion of hot leader channels driven by thermal ionization near cloud tops to nonthermal streamer forms observed at higher altitudes are not understood at present. This paper presents a formulation of a streamer-to-spark transition model that allows studies of gas dynamics and chemical kinetics involved in heating of air in streamer channels for a given air density N under assumption of constant applied electric field E. The model accounts for the dynamic expansion of the heated air in the streamer channel and resultant effects of E/N variations on plasma kinetics, the vibrational excitation of nitrogen molecules N 2 (v), effects of gains in electron energy in collisions with N 2 (v), and associative ionization processes involving N 2 (A 3 S u + ) and N 2 (a′ 1 S u − ) species. The results are in excellent agreement with available experimental data at ground and near-ground air pressures and demonstrate that for the air densities corresponding to 0-70 km altitudes the kinetic effects lead to a significant acceleration of the heating, with effective heating times scaling closer to 1/N than to 1/N 2 predicted on the basis of similarity laws for Joule heating. This acceleration is attributed to a strong reduction in electron losses due to three-body attachment and electron-ion recombination processes with reduction of air pressure.Citation: Riousset, J. A., V. P. Pasko, and A. Bourdon (2010), Air-density-dependent model for analysis of air heating associated with streamers, leaders, and transient luminous events,

Gigantic jets are atmospheric electrical discharges that propagate from the top of thunderclouds to the lower ionosphere. They begin as lightning leaders inside the thundercloud, and the thundercloud charge structure primarily determines if the leader is able to escape upward and form a gigantic jet. No observationally verified studies have been reported on the thundercloud charge structures of the parent storms of gigantic jets. Here we present meteorological observations and lightning simulation results to identify a probable thundercloud charge structure of those storms. The charge structure features a narrow upper charge region that forms near the end of an intense convective pulse. The convective pulse produces strong storm top divergence and turbulence, as indicated by large values of storm top radial velocity differentials and spectrum width. The simulations show the charge structure produces leader trees closely matching observations. This charge structure may occur at brief intervals during a thunderstorm’s evolution due to the brief nature of convective pulses, which may explain the rarity of gigantic jets compared to other forms of atmospheric electrical discharges.