The Effects of Clouds on the Light Produced by Lightning

Abstract

A Monte Carlo method has been used to simulate the transport of visible (0.45 ?m) and near-infrared (0.87 ?m) photons which are produced by transient light sources, such as lightning, within cubic, spherical and cylindrical clouds. Computations of the total absorption, the fractions of photons which escape various cloud surfaces, the flux density of photons at the cloud surface, and the angular distributions of the photons are given for various source locations. The results show that the total absorption with point and extended sources is negligible in the visible and less than 20% in the near-infrared with total optical depths up to 400. The fractions of photons which escape various cloud surfaces are high when the sources are close to the surface; and if a source is at the optical center of the cloud, these fractions are the same as if there were no cloud present at all, except for the absorption loss. Most cloud discharges and the in-cloud portion of cloud-to-ground flashes occur at or above the ?10 to ?20°C temperature level, an altitude which is close to the optical center of most thunderstorms. The characteristic dimension of the flux density of photons which escape a cloud surface is typically 60?70% of the cloud dimension. The angular distributions of escaping photons show that about half are emitted within 45° of an axis normal to the cloud surface, and that the azimuth angle distribution is isotropic about this axis. The fractions of photons per unit solid angle which could be detected by a satellite optical sensor are given as a function of angle for both point and finite sources within the cloud. The delay and time-broadening of a light impulse by multiple scattering is discussed and can easily be several tens of microseconds or more.