Eugene A. Mareev

ON THE MODELING OF SPRITES AND SPRITEPRODUCING CLOUDS IN THE GLOBAL ELECTRIC CIRCUIT

Quasi-stationary and fast transient processes connected with powerful lightning discharges and large-scale thunderstorm systems are analyzed. The main physical ideas serving as the foundation for sprite and sprite-producing cloud modeling are discussed with simple examples. Special attention is paid to the adequate description of the field sources and appropriate set of boundary conditions. The features of electron heating and ionization in the middle atmosphere are briefly discussed. The importance of the global circuit for the modeling of sprites and sprite-producing clouds is recognized. Along with the setting of boundary conditions, its role is connected with the importance of large thunderstorm complexes, including mesoscale convective systems (MCSs). It was shown recently that MCS stratiform regions make an especially large current contribution to the global circuit, serving either as an effective generator or as a discharger of the circuit depending on the polarity, magnitude and thickness of the MCS layers. On the other hand, stratiform regions of MCS are characterized by an enhanced rate of positive flashes, which are known to correlate with sprites. In the case of MCS the big narrow layers, generated near the 0◦ C isotherm serve as the source of electric charges for positive CG flashes. We suggest a model of a positive charge layer near the 0◦ C isotherm, based on the hypothesis that the melting-charging mechanism plays a principal role in the formation of the layer. We illustrate how microphysical considerations result in electric currents for the use in modeling of the global circuit and discharge processes. Further we address two aspects of the global electric circuit conception, particularly important from the viewpoint of sprites and sprite-producing cloud research. First is a classical aspect of the global circuit as the quasi-stationary current contour supported by the operation of thunderstorm generators over the globe. Another aspect is connected to the energy deposition and dissipation into the circuit, treated as an open dissipative system. Simple energetic estimates of sprite occurrence are presented. Nonlinear aspects of modeling throughout the chapter are emphasized.

Mechanisms for the formation of electric-field pulsation spectra in the near-surface atmosphere

We present the results of detailed measurements of the spectrum of short-term (f≃ 0.001-1 Hz) pulsations of the electric field of the near-surface atmosphere under the fair-weather and fog conditions. It is shown that the electric-field pulsations at frequencies 10-2-10-1 Hz have a power-law spectrum under both fair-weather and fog conditions. The spectral index varies in a range of from -1.23 to -3.36 depending on the experimental conditions, but the most probable values of the index fall in a range of from -2.25 to -3.0. The spectra corresponding to long time intervals of about a few hours are more steep. The relation of the spectral characteristics to the formation of aeroelectric structures (AESs) is studied. The distribution obtained for the structured spectra is bimodal, i.e., it exhibits two maxima in the ranges of spectral indices from -2.75 to -3.0 and from -2.25 to -2.5. The nonstructured-spectrum distribution is asymmetric and has a pronounced maximum corresponding to hard spectra with indices from -2.5 to -3.3. The intensity of the electric-field pulsations under fog conditions increases by about an order of magnitude compared to the case of fair-weather conditions. The mechanisms of spectrum formation of electric-field pulsations and their relations to the pulsation spectra of the electric-charge density with allowance for the neutral-gas turbulence and the presence of AESs are analyzed. We point out the key role of the nonlocal relation between the electric-field intensity and the space-charge density under conditions of spatially inhomogeneous turbulence. Model problems of the spectrum of electric-field fluctuations generated by a homogeneous and “structured” turbulence in the presence of charge-density fluctuations, considered as a passive tracer, are solved.

Simulating indirect effects that thunderstorm activity has on atmospheric temperature

A coupled chemistry-climate model of both lower and middle atmospheres is used to study variations in the temperature of the atmosphere when its chemical composition is disturbed due to thunderstorm activity, which results in variations in its local heating and cooling and in atmospheric heat and mass transfer. The results of model calculations showed that, due to variations in the lightning production of nitrogen oxides and resulting variations in the concentrations of atmospheric gases, the temperature varies mostly in the lower and middle stratospheres over both tropical and polar regions. On average, over a period of several decades, this effect quantitatively amounts to a few tenths of a degree; however, it can reach a few degrees at heights of the lower stratosphere over Polar regions. The level of the statistical significance of estimates exceeds 0.95 almost within all height ranges for the global lightning production (exceeding 6 TgN/year).

An upward connecting leader at tests of large-scale lightning-rod models

Estimations of connecting leader length for grounded objects of various geometries have been performed. The data of experimental studies of protection zones for large-scale lightning-rod models of different types are presented. In experiments the connecting leaders developing towards a spark discharge from a high-voltage electrode were observed. Experimental results on protection zones are compared to calculations under Russian standards of lightning protection

Peculiarities of the disturbance in the mesosphere composition and optical emissions caused by high-altitude discharges

The suggested equation system, including 267 chemical reactions and corresponding parametrizations of disturbances of the electric field and electron temperature, describes the dynamics of the mesosphere composition under the influence of high-altitude discharges (sprites and halos). Based on this system, the ionic disturbance, neutral components, and optical emissions of the night mesosphere caused by the sprites were modeled for a height of 77–85 km. Most attention was paid to the dynamics of disturbances of concentrations of electrons and O2+, NO+, H3O+, H5O2+, and N2+ typical of the studied heights. The major chemical reactions leading to the disturbance of ionic contents are determined and the relaxation dynamics of the chemical components is reviewed. The account of the excited atoms and molecules of nitrogen and oxygen allowed us to model the radiation of the sprite flash, calculate the volumetric velocity of the photon emission, and study the influence of the sprite on the neutral components of the mesosphere.