Simulations of lightning optical waveforms as seen through clouds by satellites

We present three-dimensional simulations of photon transport through clouds, specifically designed to address the characteristics and detection of optical lightning waveforms collected by satellites. The model uses a Monte Carlo approach, in which discrete photons are advanced by a standard time step through a distribution of scattering water droplets, whose size and number density distributions are variable. The model is different from previous work, in that it considers both finite and infinite cloud geometries and simulates sources of emission with arbitrary spatiotemporal properties. The model outputs are designed to be directly comparable to data obtained by the FORTE satellite photodiode detector, which records optical waveforms for lightning events with 15 mus resolution. The model treats the light propagation through clouds having a variety of shapes, sizes, and optical depths and constructs the delayed/dispersed/attenuated light curve as seen from arbitrary locations outside of the cloud. We compare the simplest case results to previous models and to data from the FORTE satellite and consider certain special cases such as the signal received from impulses occurring below the cloud. We find that the shape of the cloud and the position of the event within the cloud, rather than the motion or extent of the event itself, are the greatest determinants of the resultant distribution of photons in the sky. We also find that the position of the event within the cloud can be as large a determinant in the apparent attenuation of the signal as the cloud optical depth. We find that the class of FORTE optical waveforms with durations greater than or similar to1 ms cannot be accounted for by photon scattering alone, but rather, the intrinsic source duration must itself be quite long, which is not the case for return strokes.