Data-constrained simulations of the return stroke plasma properties

Abstract:

Historically, quantitive estimates of the return stroke channel properties, including its conductivity, temperature, radius, electric field, energy deposition rate, etc. have always been difficult to make due to the lack of direct measurements and physics-based computational models. Astonishingly, with limited exceptions, not much work has been done comparing spectroscopic temperature measurements with computational models since the seminal works of Plooster [Phys. Fluids, 1971] and Paxton et al. [Phys. Fluids, 1986], which leveraged the data collected by Orville [J. Atmos. Sci., 1968]. Recent spectroscopic measurements with microsecond temporal resolution encompassing the soft ultraviolet, visible, and near infrared portions of the optical spectrum [Walker and Christian, JGR, 2017, 2019] have created the opportunity for advancing physics-based models of the return stroke and refining quantitative estimates of the channel properties. Here, we use a computationally-efficient return stroke model, where the gas dynamic equations are parameterized as a series of ordinary differential equations [da Silva et al., JGR, 2019]. The simulations are driven by the directly-measured current to ground and the initial conditions are determined from a parametric search that identifies the best match between calculations and key measured variables, including temperature and electron density. This approach allows for the evaluation of unknown channel properties with high level of confidence. We present calculations of key properties of the return stroke channel, which are instrumental in determining the energetic and chemical impacts of lightning, in the atmosphere and in man-made structures.