Earle R. Williams

Sprites, ELF Transients, and Positive Ground Strokes

In two summertime mesoscale convective systems (MCSs), mesospheric optical sprite phenomena were often coincident with both large-amplitude positive cloud-to-ground lightning and transient Schumann resonance excitations of the entire Earth-ionosphere cavity. These observations, together with earlier studies of MCS electrification, suggest that sprites are triggered when the rapid removal of large quantities of positive charge from an areally extensive charge layer stresses the mesosphere to dielectric breakdown.

The Schumann Resonance: A Global Tropical Thermometer

The Schumann resonance, a global electromagnetic phenomenon, is shown to be a sensitive measure of temperature fluctuations in the tropical atmosphere. The link between Schumann resonance and temperature is lightning flash rate, which increases nonlinearly with temperature in the interaction between deep convection and ice microphysics.

An analysis of the conditional instability of the tropical atmosphere

Abstract The ice phase is included in thermodynamic calculations of convective available potential energy (CAPE) for a large number of soundings in the tropical atmosphere, at both land and ocean stations. It is found that the positive-buoyancy contribution to CAPE resulting from the latent heat of fusion more than offsets the negative-buoyancy contribution due to water loading in the reversible thermodynamic process. The departure from moist neutrality in much of the tropical atmosphere exhibits a threshold in boundary-layer wet-bulb potential temperature of 22°–23°C. The corresponding sea surface temperature is approximately 26°C, close to the empirical threshold for hurricane formation, which suggests that conditional instability plays an important role in the latter phenomenon. The simultaneous presence of finite CAPE and infrequent deep convection in the tropics is tentatively attributed to the convective inhibition energy (CINE) and to the mixing process that destroys positive buoyancy in incipient ...

A Radar and Electrical Study of Tropical “Hot Towers”

Abstract Radar and electrical measurements for deep tropical convection are examined for both “break period” and “monsoonal” regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the “break period” regime are associated with 10–20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°–3°C) in wet-bulb potential temperature between land and s...

Criteria for sprites and elves based on Schumann resonance observations

Ground flashes with positive polarity associated with both sprites and elves excite the Earths Schumann resonances to amplitudes several times greater than the background resonances. Theoretical predictions for dielectric breakdown in the mesosphere are tested using ELF methods to evaluate vertical charge moments of positive ground flashes. Comparisons of the measured time constants for lightning charge transfer with the electrostatic relaxation time at altitudes of nighttime sprite initiation (50–70 km) generally validate the electrostatic assumption in predictions made initially by Wilson [1925]. The measured charge moments (Q dS = 200–2000 C-km) are large in comparison with ordinary negative lightning but are generally insufficient to account for conventional air breakdown at sprite altitudes. The measured charge moments, however, are sufficient to account for electron runaway breakdown, and the long avalanche length in this mechanism also accounts for the exclusive association of sprites with ground flashes of positive polarity. The association of elves with large peak currents (50–200 kA) measured by the National Lightning Detection Network in a band pass beyond the Schumann resonance range is consistent with an electromagnetic pulse mechanism for these events.

The Down Under Doppler and Electricity Experiment (DUNDEE): Overview and Preliminary Results

Abstract DUNDEE (Down Under Doppler and Electricity Experiment) is described. DUNDEE was carded out in the vicinity of Darwin, Northern Territory, Australia, during the wet seasons of November 1988 through February 1989, and November 1989 through February 1990. The general goal of DUNDEE was to investigate the dynamical and electrical properties of tropical mesoscale convective systems and isolated deep convective storms. Darwin, situated at the southern tip of the “maritime continent,” experiences both monsoon and “break” period conditions during the wet season. We discuss the observational network deployed for DUNDEE and present preliminary scientific results. One particularly interesting observation is a large contrast in the frequency of total lightning between break period convection (high lightning rates) and convection in the monsoon trough (low lightning rates). A relationship between CAPE (convective available potentional energy) and total flash rate is presented and discussed to explain this obs...

Global circuit response to seasonal variations in global surface air temperature

Comparisons are made between the seasonal behavior of the global electrical circuit and the surface air temperature for the Tropics and for the globe. Positive correlations between global circuit parameters and temperature are identified on both semiannual and annual timescales. Lightning is the global circuit quantity found most responsive to temperature, with a sensitivity of the order of 10% per 1 C. These findings lend further validity to the use of global circuit measurements as a diagnostic for global change.

ENSO and the natural variability in the flow of tropical rivers

Abstract This paper examines the relationship between the annual discharges of the Amazon, Congo, Paran a, and Nile rivers and the sea surface temperature (SST) anomalies of the eastern and central equatorial Pacific Ocean, an index of El Nino-Southern Oscillation (ENSO). Since river systems are comprehensive integrators of rainfall over large areas, accurate characterization of the flow regimes in major rivers will increase our understanding of large-scale global atmospheric dynamics. Results of this study reveal that the annual discharges of two large equatorial tropical rivers, the Amazon and the Congo, are weakly and negatively correlated with the equatorial Pacific SST anomalies with 10% of the variance in annual discharge explained by ENSO. Two smaller subtropical rivers, the Nile and the Parana, show a correlation that is stronger by about a factor of 2. The Nile discharge is negatively correlated with the SST anomaly, whereas the Parana river discharge shows a positive relation. The tendency for reduced rainfall/discharge over large tropical convection zones in the ENSO warm phase is attributed to global scale subsidence associated with major upwelling in the eastern Pacific Ocean.

Mixed-Phase Microphysics and Cloud Electrification

Abstract A number of experimental studies have shown that sublimating ice acquires negative charge and ice undergoing vapor deposition acquires positive charge. Microphysical calculations are performed to determine the diffusional state (i.e., sublimation versus deposition) of riming graupel particles. Comparisons with earlier laboratory measurements of charge transfer to a rotating rimer in a cloud of supercooled water droplets and ice crystals again suggest that sublimating graupel particles charge negatively and graupel undergoing deposition charge positively. Implications for charge separation in thunderstorms are discussed.

ELF propagation parameters for uniform models of the Earth–ionosphere waveguide

Abstract Uniform models for the Earth–ionosphere cavity are considered with particular attention to the physical properties of the ionosphere for the extremely low frequency (ELF) range. Two consistent features have long been recognized for the range: the presence of two distinct altitude layers of maximum energy dissipation within the lower ionosphere, and a “knee”-like change in the vertical conductivity profile representing a transition in dominance from ion-dominated to electron-dominated conductivity. A simplified two-exponential version of the Greifinger and Greifinger (1978) technique widely used in ELF work identifies two slopes in the conductivity profile and, providing accurate results in the ELF communication band (45– 75 Hz ), simulates too flat a frequency dependence of the quality factor within the Schumann resonance frequency range (5– 40 Hz ). The problem is traced to the upward migration, with frequency increasing, of the lower dissipation layer through the “knee” region resulting in a pronounced decrease of the effective scale height for conductivity. To overcome this shortcoming of the two-exponential approximation and still retain valuable model analyticity, a more general approach (but still based on the Greifinger and Greifinger formalism) is presented in the form of a “knee” model whose predictions for the modal frequencies, the wave phase velocities and the quality factors reasonably represent observations in the Schumann resonance frequency range.