1. Petr Kašpar, Ondřej Santolík, and Ivana Kolmašová
A physics-based model of the electric field pulses occurring during the lightning initiation
Petr Kašpar, Ondřej Santolík, and Ivana Kolmašová
Institute of Atmospheric Physics, CAS, Prague, Czech Republic
EGU
4. Comparison with measurements
As an example of a PBP pulse train, which consists of several electric-field pulses, we show the short sequence labelled as event D by Kolmašová et al. [2016] measured at 308 km from the lightning position.
We show two additional examples of the modeled PBPs together with data (event G and I) both measured at 332 km from the lightning position.
5.Estimations of the PBP peak currents We simulate the PBP peak currents for 9 events investigated in detail by Kolmašová et al. [2016], and compare them with the PBP peak amplitudes at 100 km and with the return stroke peak currents obtained from the lightning detection network. We show that our model (blue dashed line) gives approximately three times higher currents than the simplified estimations by Kolmašová et al. [2016] (green dashed line) based on return stroke waveforms.
1. Introduction
We investigate properties of the electric field pulses generated by lightning initiation processes. We use a generalization of electrostatic and transmission-line models. We show how the shapes of the electric field waveforms of individual pulses depend on various parameters of the model. We show how the preliminary breakdown peak currents are related to the preliminary breakdown electric field amplitudes and return stroke peak currents. We compare our modeling results with observations.
2. Model formulation
We use the model of preliminary breakdown pulses initially developed by da Silva and Pasko [2015], coupled to SOR (successive over-relaxation) calculation of the ambient electrical potential. We numerically solve the following set of equations:
The potential Uamb is obtained by solving Poisson equation for a given charge distribution. After these equations are solved for the discharge current, electric field is computed by the formula given e.g. by Uman et al. [1975].
Propagation effects are included in the same way as in Kolmašová et al. [2016].
α × 20dB gives the amplitude attenuation in dB per 100 km.
We simulate a single PBP (the largest pulse in event J as labelled by Kolmašová et al. [2016]) at the distances da=306 km db=416 km, dc=563 km. The green dashed line gives the same PBP peak current from simplified estimates by Kolmašová et al. [2016], I=64 kA, and the blue line gives the modeled estimation I=189 kA. We model exceptionally large PBP pulse, which correspond to very strong charge centres.
Eamb
Uamb
6.Conclusions
We show that the PBP polarity overshoot depends mainly on the characteristic time of the new step development, and also, to a lesser extend, on the initial leader conductivity. If we change the main negative charge centre magnitude the PBP amplitudes change but the waveshape remains almost the same.
We have found that the modeled PBP peak currents are about three times larger than the values from simplified estimates by Kolmašová et al. [2016]. Modeled PBP peak currents can be comparable or even larger than the measured return stroke peak currents, while the peak amplitudes are always lower for PBPs.
We have suitably chosen free parameters of the model to compare simulated PBPs with measurements. Although the PBP waveshapes do not match exactly due to propagation effects, the modeled PBPs agree well with observations.
3. The shape of the preliminary breakdown pulses
We show how the shape of the PBPs depends on the magnitude of the main negative charge layer Q-, channel conductivity G, and the characteristic time of a new step development τEach particular configuration also gives us the peak current Ii.
7. References
da Silva, C. L., and V. P. Pasko (2015), Physical mechanism of initial breakdown pulses and narrow bipolar events in lightning discharges, Journal of Geophysical Research: Atmospheres, 120(10).
Uman, M. A., D. K. McLain, and E. P. Krider (1975), The electromagnetic radiation from a finite antenna, Am. J. Phys, 43(1).
Kolmašová, I., O. Santolík, T. Farges, S. A. Cummer, R. Lán, and L. Uhlíř (2016), Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud-to-ground lightning discharges, Geophysical Research Letters, 43(3).
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