The modeling of the electrical conductivity of polymer composites reinforced with conductive fibers is investigated. Existing models generally can be divided into percolation theories and non‐percolation theories. The basis of the percolation theory is the fact that the conductivity of the composite increases dramatically at a certain fiber concentration called the percolation threshold. This theory can be used to model the behavior of the composite or to predict the percolation threshold itself. Non‐percolation theories include terms, which account for microstructural data such as fiber orientation, length, and packing arrangement. A comparison of experimental data with predictions from the various models reveals that only the percolation theory is able to accurately model the conductive behavior of an actual composite. Two alternative new models, which predict the volume resistivity of a composite using microstructural data, are evaluated. The first model relates resistivity to the concentration and orientation of the fibers, while assuming perfect fiber‐fiber contact. The relationship between resistivity and fiber concentration predicted by the model is in qualitative agreement with actual data, and predictions of the anisotropy in volume resistivity compare well with experimental results. The second model accounts for the effect of fiber‐fiber contact and fiber length on composite resistivity. Predictions are in excellent agreement with experimental data for polypropylene composites reinforced with nickel‐coated graphite fibers.

The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 flashes/rain, with some values reaching 500 flashes/min. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5-20 minutes. A distinguishing feature of severe storms is the presence of lightning "jumps"-abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds-wind, hail, tornadoes-is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity.

Thunderstorm case studies and earlier observations are described which illuminate the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning. A consistent temporal evolution starting with peak IC activity changing to predominant CG activity and concluding with strong outflow (microburst) suggests that ice is responsible for both the electrical (i.e., lightning) and dynamical (i.e., microburst) phenomena. The IC activity is attributed to the updraft-driven accumulation of graupel particles in the central dipole region, and the subsequent CG activity to the descent of ice particles beneath the height of the main negative charge. The subsequent descent and melting of ice particles beneath the height of the 0øC isotherm are associated with the acceleration of the downdraft and outflow. The IC lightning precursor can provide a valuable short-term (5-10 min) warning for microburst hazard at ground level. INTRODUCTIONConvective storms are well recognized to produce two common types of lightning: intracloud (IC) and cloud to ground (CG). Some convective storms are recognized to produce strong downdrafts, often referred to as microbursts, which have been demonstrated to pose a severe hazard to commercial aviation [Fujita, 1985]. This study was initially concerned with a search for a practical short-term precursor to the microburst hazard in the cloud electrical development. In the course of this investigation, consistent relationships between the stage of the convective activity and the lightning type became apparent, and tied in closely with earlier electrical observations of thunderstorms and with contemporary observations of microburst precursors in Doppler radar observations [Campbell, 1988]. This paper is concerned with a description of these relationships and with an interpretation based on ice-phase microphysics. CASE STUDIESThe microburst-producing clouds in this study were almost invariably air mass thunderstorms in weakly sheared environments and exhibited peak lightning rates greater than or equal to three flashes/min. The IC lightning dominated over CG lightning in the early stages of vertical development. The observed temporal relationship between the peak lightning rate and the peak outflow velocity was very systematic in the storms studied. The evolution of total lightning rate, CG lightning rates (all events located within 20 km of the Doppler radar), and maximum differential velocity at the ground are recorded for several case studies, which are discussed briefly in chronological order. The velocity differential across all outflows reported here exceeded 20 m/s. A headwind-tailwind change of 10 m/s is of concern to commercial aviation, while the largest microburst velocity differentials observed by Doppler radars have been roughly 40-50 m/s. All lightning rates were tabulated in 1-min time intervals. All microbursts studied were classified as "wet" 13,213

A three-stage Yb-fiber amplifier emitted 1.43 kW of single-mode power when seeded with a 25 GHz linewidth master oscillator (MO). The amplified output was polarization stabilized and phase locked using active heterodyne phase control. A low-power sample of the output beam was coherently combined to a second fiber amplifier with 90% visibility. The measured combining efficiency agreed with estimated decoherence effects from fiber nonlinearity, linewidth, and phase-locking accuracy. This is the highest-power fiber laser that has been coherently locked using any method that allows brightness scaling.