Simulations of lightning optical waveforms as seen through clouds by satellites

Abstract

Abstract. 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 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 >• 1 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. IntroductionThe FORTE satellite, built by Los Alamos and Sandia Na- spatial resolution optical data set, the optical data analysis is somewhat hindered by the lack of appropriate models of photon transport through cloud. Before we can fully exploit the FORTE optical database, we must better quantify the scattering and absorption of light by clouds. In particular, we must understand how to recover properties of the cloud and discharge from observables such as the measured pulse width and delay. We also seek to understand the detectability of optical transients as a function of event type and viewing angle. This paper presents Monte Carlo simulations of photon transport in clouds designed to output data products similar to that of the FORTE PDD and future satellite-based optical waveform detectors, thereby facilitating comparisons to real transient event data.Section 2 details the results of earlier work in this area, while section 3 describes this new model. In section 4 we examine some of the simplest cases to test the model's validity, and in section 6 we compare the simulations to real PDD data. Previous StudiesRadiative transfer in clouds has been studied extensively, but typically with applications to the stea...

Revisiting "Narrow Bipolar Event" intracloud lightning using the FORTE satellite

“…7 in Kirkland et al, 2001). A typical slant range from the sensor to the cloud top within the field-of-view is ∼900 km, implying that the optical ERP at the cloud top corresponding to the trigger threshold is ∼ 3 × 10 8 W. We do not know what the peak power is at the actual source within the cloud, other than that it is brighter; all that matters is the cloud-top ERP (Koshak et al, 1994;Light et al, 2001b). Relative to the triggerthreshold irradiance of 3 × 10 −5 W m −2 , PDD recorded a broad distribution of events distributed upward in irradiance to > 3 × 10 −4 W m −2 .…”

“…This is a physical delay for an electrical transient to develop into a region where the emitted photons have a higher chance of being seen by a satellite. On the other hand, the "propagation delay" is due to the diffusion of light outward through intervening cloud before reaching the cloud surface (Koshak et al, 1994;Light et al, 2001b). Each of these types of delay in turn has great variability, so the distribution of sferic-minus-light time differences is not only grossly displaced, but also broadened.…”

Joint radio and optical observations of the most radio-powerful intracloud lightning discharges

“…The measured optical signal's delay (relative to the VHF signal) has previously been shown to consist of two additive contributions (Light and Hamlin, 2008;Suszcynsky et al, 2000): First, there is a "scattering" delay (of the light relative to the VHF) due to the multiple Mie scatterings during transport through the cloud to the cloud's top, where the light can be seen from orbit (Koshak et al, 1994;Light et al, 2001b). For an ensemble containing hundreds of examples of joint radio and optical recordings, the temporal broadening due to scattering was estimated as 138 µs (see Fig.…”

Lightning detection on Venus: a critical review

“…As noted by Russell (2011), a low circular polar orbit is desirable for radar mapping, and a modest lightning instrument would be a valuable but relatively inexpensive augmentation to a radar mapping mission. We may note that such combined optical and RF surveys of lighting (e.g., Light et al 2001) at Earth have been performed on a rather small satellite platform (e.g., FORTE, 210 kg, flown in 1997).…”