Radiative Transfer of Lightning Light by Thundercloud and Validation of TARANIS Satellite Observations

Abstract:

Thunderstorms occur all over the world, and produce flashes (optical and radio waves). From space, only the light scattered by the cloud is visible. Understanding the radiative transfer of light produced by the lightning discharges in the clouds is therefore fundamental. Observations made by low orbit satellites for twenty years gave the first global map of electrical activity of thunderstorms. Many on-board instruments can now detect lightning. For the first time, the current generation of geostationary meteorological satellites is equipped with lightning imagers. These satellites strongly contribute to the real-time alert of severe weather associated with thunderstorms. Simultaneously, the ASIM mission on board the International Space Station, and the next low orbit satellite TARANIS, can measure lightning at different wavelengths, from near-UV to near-IR (imagery and photometry). They provide complementary measurements to those of the geostationary satellites.

The present study aims to better quantify the radiative transfer of the light emitted by lightning discharges through the cloud. We characterize optical lightning waveforms and images detected by satellites with three-dimensional simulation of photons transport through clouds. A forward three dimensional radiative code based on a Monte-Carlo approach is used. The light emitted by the lightning source is simulated as a large number of photons with different temporal and spatial distribution. Different wavelengths are taken into account to simulate the cloud scattering differences observed from near-UV to near-IR. Simulation results are compared to previous results in the case of simple homogeneous water clouds. Furthermore, the sensitivity study of lightning scattering through clouds has been extended to more realistic cloud properties, such as the water/ice content, and the size of water droplets and ice crystals. Eventually, first validations with satellite observations will be presented in order to find the trends observed in the simulations.