Abstract. Preliminary observations of simultaneous VHF and optical emissions from lightning as seen by the Fast on-Orbit Recording of Transient Events (FORTE) spacecraft are presented. VHF/optical waveform pairs are routinely collected both as individual lightning events and as sequences of events associated with cloud-to-ground (CG) and intracloud (IC) flashes. CG pulses can be distinguished from IC pulses on the basis of the properties of the VHF and optical waveforms but mostly on the basis of the associated VHF spectrograms. The VHF spectrograms are very similar to previous ground-based HF and VHF observations of lightning and show signatures associated with return strokes, stepped and dart leaders, attachment processes, and intracloud activity. For a typical IC flash, the FORTE-detected VHF is generally characterized by impulsive broadband bursts of emission, and the associated optical emissions are often highly structured. For a typical initial return stroke, the FORTE-detected VHF is generated by the stepped leader, the attachment process, and the actual return stroke. For a typical subsequent return stroke, the FORTE-detected VHF is mainly generated by dart leader processes. In remedy of this situation the Fast on-Orbit Recording of Transient Events (FORTE) satellite was launched on August 29, 1997. FORTE is a joint Los Alamos National Laboratory and Sandia National Laboratories satellite experiment that was primarily designed to address technology issues associated with treaty verification and the monitoring of nuclear tests from space. The satellite carries VHF broadband radio receivers and an Optical Lightning System (OLS) which are optimally designed for the detection of lightning transients. The design of this instrumentation and its availability for continuous scientific use makes FORTE an ideal space platform from which to monitor and study the simultaneous emission of VHF and optical radiation from lightning. This paper reports on the preliminary phenomenology and analysis of the correlated FORTE VHF and optical data sets.The goals of this study are twofold: (1) to demonstrate the utility of using a dual phenomenology approach for the remote 2191

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...

Abstract. We review data from observations of terrestrial lightning obtained by the FORTE satellite between September 1997 and January 2000. A silicon photodiode detector (PDD) records the intensity-time history of transient optical events occurring within its 80 ø circular field of view. This field of view corresponds to a circle on the Earth's surface having an approximate diameter of 1200 km. We describe the instrument, present examples of the data, explain how the data are screened for false triggers, and review, within the context of previous measurements, the general statistics of peak irradiance, pulse width, and energy associated with the data. We compare the FORTE data with National Lightning Detection Network (NLDN) reported cloud-to-ground (CG) strokes and find that the PDD detection efficiency for these CG strokes is -6%. Moreover, we infer that FORTE preferentially detects the in-cloud portion of optical lightning signals. Events having inferred peak powers between l0 s and 10 TM W and optical 3 9 energy outputs between 10 and 10 J are observed. From a population of nearly 700,000 events we find that the median peak power and median detected optical energy at the source are estimated to be -1 x 109W and 4.5 x l0 s J, respectively. These values of source peak power and energy are comparable to previous space-based measurements and consistent with aircraft-based and ground-based measurements. The observed median effective pulse width is about 590 microseconds. Further, the pulse widths for CG strokes, reported by NLDN, are inversely proportional to pulse peak power.

Evidence has accumulated for the past two decades demonstrating a correlation between Earth transits of the heliospheric current sheet (HCS) and changes in winter tropospheric vorticity. These correlations persisted for a few years following the Agung and E1 Chich6n volcanic eruptions, but were significantly weaker at other times. This suggests that the missing link in a physical mechanism explaining the correlation may involve volcanic aerosols and their effect on cloud microphysics, via atmospheric electricity. An analysis of 500-rnbar northern hemispheric vorticity for the 1991-1994 winter periods following the Pinatubo eruption shows a similar correlation between tropospheric vorticity and Earth transits of the HCS, supporting the previous interpretation. Paper number 96JD01554. 0148-0227/96/96JD-0 l 554509.00 riodically sweeping over the Earth. This HCS passage event has historically been referred to as a magnetic sector boundary crossing, which is defined by a persistent large-scale reversal in the interplanetary magnetic field (IMF) spiral polarity. The HCS provides a physical basis for the observed reversal in the IMF polarity, and thus its passage can be thought of as the more fundamental event. The correlation between HCS structure and terrestrial atmospheric vorticity has been observed only when significant stratospheric aerosol loading occurs as a resuit of volcanic injections. An analysis of the 500-mbar vorticity for the Pinatubo aerosol period has been performed, and the results show a recurrence of the correlation between tropospheric vorticity and HCS structure. A mechanism invoking a solar wind modulation of the global electric circuit, the currents of which appear to affect cloud microphysical processes and thus meteorological processes, provides an explanation for the observed correlations. Background Wilcoz et al. [1973,1974] analyzed twice daily grids of geopotential height, at various standard pressure levels, using the geostrophic approximation to calculate the vorticity at each grid point. For each grid, Wilcox then calculated the vorticity area index (VAI), defined by Roberts and Olsen [1973] as the surface area north of 200 N over which the absolute vorticity exceeded +20 x 10 -5 s -1 plus the surface area over which the absolute vorticity exceeded +24 x 10 -5 s -•. 29,689 29,690 KIRKLAND ET AL.: STRATOSPHERIC AEROSOLS AND THE WILCOX EFFECT

The Semi-Arid Land-Surface-Atmosphere Program (SALSA) is a multi-agency, multinational research effort that seeks to evaluate the consequences of natural and human-induced environmental change in semi-arid regions. The ultimate goal of SALSA is to advance scientific understanding of the semi-arid portion of the hydrosphere-biosphere interface in order to provide reliable information for environmental decision making. SALSA approaches this goal through a program of long-term, integrated observations, process research, modeling, assessment, and information management that is sustained by cooperation among scientists and information users. In this preface to the SALSA special issue, general program background information and the critical nature of semi-arid regions is presented. A brief description of the Upper San Pedro River Basin, the initial location for focused SALSA research follows. Several overarching research objectives under which much of the interdisciplinary research contained in the special issue was undertaken are discussed. Principal methods, primary research sites and data collection used by numerous investigators during 1997-1999 are then presented. Scientists from about 20 US, five European (four French and one Dutch), and three Mexican agencies and institutions have collaborated closely to make the research leading to this special issue a reality. The SALSA Program has served as a model of interagency cooperation by breaking new ground in the approach to large scale interdisciplinary science with relatively limited resources. Published by Elsevier Science B.V.