Towards a hybrid model to simulate lightning and associated energetic events in various atmospheres

Thunderstorm processes represent a challenge for numerical models, as they involve numerous processes of various scales, and explosive events producing exponentially increasing numbers of particles in a very short span of time. A phenomenon called a Relativistic Runaway Electron Avalanche can occur under the right conditions, and lead to the production of a Terrestrial Gamma-Ray Flash (TGF), spanning over tens of microseconds, and during which up to 10¹⁷ electrons and photons are produced for the most intense ones, the weaker ones still producing 10¹² to 10¹⁵ energetic photons. While Monte Carlo models are often used to simulate such processes, runtime typically scales with particle number, which leads to poor performance without a way to limit the number of particles computed. On the other hand, a fluid model may be adapted to deal with large particle densities, but it will struggle to deal with the extreme energies and electric fields, and will lose track of the physics of individual particles, which becomes relevant when submitted to such extreme parameters.

In order to accurately and efficiently simulate all these processes, we are developing a fully parallelized 3-D hybrid model. The code is optimised for massively parallel usage on Graphics Processing Units (GPUs), and uses the AMReX library, a software framework for massively parallel, block-structured codes, allowing us to run in parallel with implemented adaptive mesh refinement (AMR), which further improves the accuracy of the model.

With this model, we are aiming at obtaining a better understanding of lightning processes and TGFs, not only in the current Earth atmosphere, but also in the atmosphere of other celestial bodies, and in mixtures likely to have existed in the environment of Primordial Earth.