3D Monte-Carlo simulations of lightning optical waveforms and images observable by on-board operational instruments

Atmospheric scattering and absorption of the flash radiations by the thundercloud are more or less important depending on the microphysics and density of the cloud, the spatio-temporal characteristics of the flash, or the wavelength of observation. Previous studies modeled these aspects but do not allow to fully interpret optical instrument measurements due to inaccurate representation of thunderstorm clouds and lightning in their simulation. The "Three-dimensional polarized Monte-Carlo atmospheric radiative transfer model" (3DMCPOL) used in this study allows now to characterize the lightning light propagation through heterogeneous clouds in three dimensions. To show the capabilities of 3DMCPOL code used with a lightning source, the propagation of lightning optical signals through a supercellular storm has been simulated during its mature phase, and its observations by the photometers and cameras on-board the ASIM mission at lambda = 777 nm. The simulation results from the non-hydrostatic mesoscale model Meso-NH provide the microphysical properties of the cloud. The conversion of the cloud microphysical properties into optical properties reveals that the extinction of radiation by graupel, which are located in the convection core of the cloud, is significantly higher compared to other hydrometeors. Consequently, most of the radiation observed from space comes from the discharges in the upper regions of the cloud, particularly the anvil composed of ice crystals. The 3DMCPOL code becomes thus a reference to study the propagation of optical signals through clouds, as the micro/macrophysical properties of the cloud and the characteristics of the flash source can be represented very precisely.