AE43A-01
Lightning Activity Relative to the Microphysical and Kinematic Structure of Storms during a Thunder-Snow Episode on 29-30 November 2006
We have examined lightning activity relative to the microphysical and kinematic structure of a winter thunderstorm complex (a thunder-snow episode) observed east of Norman, Oklahoma during the evening of 29-30 November 2006. Polarimetric radar provided information about the type of particles present in various regions of the storms. The Lightning Mapping Array (LMA) recorded VHF signals produced by developing lightning channels. The times of arrival of these lightning signals across the array were then used to reconstruct the location and structure of lightning, and these reconstructions were overlaid with radar data to examine the relationship between lightning properties and storm particle types. Four storms in this winter complex have been examined. It was inferred from lightning structure that, in their mature stage, all cells we examined had a positive tripole electrical structure (an upper positive charge center, a midlevel negative charge center, and a lower positive charge center). The storms began with lightning activity in the lower dipole (lower positive and midlevel negative regions), but this evolved into lightning activity throughout the tripole structure within approximately 15-20 minutes. In the longer lived storms, the mature stage lasted for approximately 1.5-2 hours. During this stage, the lower positive charge region was situated less than 5 km above ground, the midlevel negative charge region was typically above 5 km, and the upper positive charge region was located at an altitude of less than 10 km in all the storm cells analyzed. The charge regions descended over approximately the last 30 minutes of lightning activity, the lower charge regions eventually reaching ground. This resulted in the loss of the lower positive charge center and the subsequent diminishment of the lower negative charge center. Lightning initiation usually coincided with the edges of regions of high reflectivity and was coincident with the presence of graupel and ice crystals in the lower dipole. Radar data suggest that ice crystals were the dominant charge carriers in the upper positive region.
AE43A-02
On the Use of Ice Mass Fluxes to Estimate Total Lightning in Cloud Resolving Models
In the last years, radar and total lightning data have been used to show that there is a linear relationship between the total lightning flash rate and the precipitation and non-precipitation ice mass flux product. This relationship is referred to as the flux hypothesis. The flux hypothesis is based on the assumption that the non-inductive charging mechanism that involves rebounding collisions of riming graupel pellets with ice crystals in the presence of supercooled liquid water plays a dominant role in thunderstorm electrification. Three different storms have been simulated using the Weather and Research Forecasting (WRF) model to show that this relationship can be used in cloud resolving models to diagnose the total lightning flash rate. Two STERAO storms (10 July and 12 July 1996) and the 13 July 2005 storm that occurs in Northern Alabama have been simulated. The model results are compared to radar and lightning data. Sensitivity tests about the microphysics scheme and the horizontal resolution of the domain have also been performed.
AE43A-03
Storm Characteristics Determining Dominant Cloud-to-Ground Lightning Polarity
Detailed analyses of storms from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in 2000 have established relationships between radar observables and lightning flash rate and polarity. We combine visual and in situ aircraft microphysical observations with these earlier radar analyses to establish additional relationships. In particular, we show that storms forming in an environment with the right balance between vertical shear of the horizontal wind, and convective available potential energy, develop in such a way that initial convective development occurs in relatively isolated flanking cells. When these cells develop vigorously without entraining precipitation from more mature neighboring cells, precipitation formation in the new cells is delayed. This means that precipitation finally develops at higher altitudes, higher cloud liquid water concentrations, and lower temperatures, compared to precipitation formation in storms in which new cells are less vigorous and/or entrain precipitation debris from earlier cells. The storms in which initial convective development occurs in isolated flanking cells go through an extended stage of almost purely intracloud lightning production, and then into a phase where there is a mixture of intracloud and cloud-to-ground lightning lowering positive charge. Storms in which precipitation develops earlier and lower in new cells, due to entrainment of precipitation debris from older neighboring portions of the storm, tend to produce predominantly negative cloud- to-ground lightning. The relationship between microphysical conditions in these different types of storms, and results of laboratory microphysical experiments concerning charge separation during particle collisions, are not straightforward.
AE43A-04 INVITED
Non-classical Generators in the Global Electric Circuit
The electrodynamics of the Earth's atmosphere and ionosphere has been a topic of considerable interests for many decades. There are three main sources of current in this circuit, thunderstorms, the ionospheric dynamo, and the magnetospheric dynamo. This presentation will begin with a review of the both the ionospheric and magnetospheric dynamos. The ionospheric dynamo primarily operates at equatorial latitudes and is driven by tidal motions of the neutral atmosphere. The magnetospheric dynamo is driven by the coupling between the Earth's magnetic field and the solar wind. Current modeling of these systems is primarily global scale and does not consider coupling with the thunderstorm sources, but we will discuss some of the early work in this area and pathways for including this modeling in global scale models.
AE43A-05 INVITED
Atmospheric Dusty Plasma as a Factor of the Global Electric Circuit
Natural radioactivity is an important factor of the Global Electric Circuit (GEC). The ions produced by the different sources of ionization (mainly by radon and its progeny) become the centers of the water vapor condensation through the ion hydration process. Attachment of the water molecules to the ions produced by ionization (up to few hundred molecules attached to one ion) lead to formation of the large ion clusters. Theoretical estimations and experimental measurements show that particles grow up to ~10 nanometers size. These heavy ion clusters replace the light ions in the near ground layer of atmosphere what leads to increase of the near ground layer conductivity. The latent heat of evaporation released by water molecules attached to the ions create the upward convection flux spreading the heavy ions through the boundary layer of atmosphere. Taking into account that the resistivity of the boundary layer is near 70% of the total columnar atmosphere resistance, this process contributes essentially to the local modification of the GEC parameters. The increased level of ionization over the tectonic plates borders and over the active tectonic faults creates the large areas over the Earth's surface with the high resistivity. One may expect the increase of the ionosphere potential over these areas or decrease of the vertical electric current. The paper presents the possible corrections to the GEC electric current balance due to the natural radioactivity and formation of the atmospheric dusty plasma.
AE43A-06 INVITED
Venus Lightning
While the atmosphere of Venus is relatively dry, it has an extensive and dynamic cloud cover consisting of sulfuric acid droplets that can and do support lightning. Unambiguous signatures of transient electromagnetic pulses were seen by Venera 11-14 landers and pulses of whistler mode waves were observed propagating out of the Venus atmosphere into the ionosphere by the Pioneer Venus Orbiter. Flashes were observed by the Venera 9 visible spectrometer and by a 61" terrestrial telescope. Still because some studies either returned null results or found properties different than those of terrestrial lightning, Venus lightning became controversial. The signal strength observed by the electric antennas at 100Hz on Pioneer Venus was sufficiently strong that such signals were predicted to be detectable by the Venus Express fluxgate magnetometer. Thus a high rate (128Hz) sampling mode was included in the design of the magnetometer. This system has successfully identified strong electromagnetic signals leaving the atmosphere guided by the magnetic field. When the ionospheric magnetic field is nearly horizontal no signals are seen but when the magnetic field dips into the ionosphere signals are observed nearly 50% of the time. While the orbit of the Venus Express mission is not ideally suited for mapping the entire surface of Venus, we expect to obtain some information on the occurrence rate as a function of local time and latitude over much of the northern hemisphere.
AE43A-07 INVITED
Martian Atmospheric Electricity and Global Circuit Driven By Dust Storms
It is well-known that small particles of differing composition and size will exchange charge via contact electrification or "tribo-electrification". On Earth, mixing grains in dust devils have been found to generate electricity via tribo-electric processes. When in collision with the ground or surface, smaller grains tend to obtain a negative charge polarity while heavy grains/surface contain an equal and opposite positive charge. Due to vertical winds, the small negative grains are lofted high into the storm thereby generating a charge separation into a dipole and thus creating macroscopic electric dipole moment within the storm. Electric fields in terrestrial dust devils have been measured to exceed 100 kV/m in a coherent, dipolar configuration that is consistent with a dipole moment oriented downward (negative charge at higher altitudes). It is believed that similar processes occur in Martian dust devils and storms, giving rise to large dust storm E-fields. Since electrical breakdown in CO2 gas is ~ 20 kV/m, the storm E-fields are limited in values to levels well below that of terrestrial dust devils/storms. However, the low levels of electrostatic energy density are compensated by the relatively large volumes of the storms that can then create a global atmospheric electric circuit. Previous analysis suggest the circuit is highly variable, becoming most intense during dust storm season. The storms may also modify atmospheric chemistry, creating oxidants and becoming a sink for atmospheric methane. We will review models of the Martian dust storm electrostatic system, global circuit, and introduce the new harsh chemistry that can occur. We will also show lab and desert studies that are consistent with these models.
AE43A-08 INVITED
Paschen Curves for Solar System Atmospheres
An understanding of the nature of electrical discharges in planetary atmospheres is required to understand recent observations of lightning on Jupiter and Saturn, as well as to study electrical breakdown processes in planetary atmospheres in general. The atmospheric composition of bodies in the solar system falls into four major classes: 1. Nitrogen/Oxygen (Earth); 2. Carbon Dioxide (Venus/Mars); 3. Hydrogen/Helium (Jupiter/Saturn/Uranus/Neptune); 4. Nitrogen/Methane (Titan/Triton). Conventional electrical breakdown varies significantly among these composition classes on account of the differing electron impact ionization cross sections and electronegative properties of the respective constituent species. The avalanche stage of electrical breakdown in a homogeneous gas is the simplest, linear stage of breakdown. Its characteristics may be computed from solutions to the electron Boltzmann equation, yielding Townsend's first ionization coefficient α. Breakdown between two closely spaced material surfaces also involves numerous electrode effects such as photoionization, and secondary ionization arising from electron impact and ion bombardment of the electrodes. These secondary effects are empirically described by Townsend's second ionization coefficient γ, whose value depends on the composition of the electrodes. For a given gas and electrode composition, the first and second coefficients may be combined into Paschen's curve, which gives the minimum voltage required for breakdown to occur between two plane parallel electrodes separated by a distance d in a gas at pressure p, expressed as a function of pd. Knowledge of Paschen breakdown could be useful in environments such as on the surface of Mars, or in free floating balloon platforms at Venus or Titan, where differential electrostatic charging between different system elements may occur, potentially resulting in electrical discharges. In this talk Paschen curves are presented for the various composition classes of solar system atmospheres.