Fine Structure of Thunderstorm Electric Field: Spectra from Soundings and Significance for Charge Generation Mechanisms

Several electric field soundings through stratiform clouds and convective regions of a mid-latitude mesoscale convective system, made with balloon-borne electric field meters and radiosondes, have been examined. All these soundings demonstrate the presence of fine structures in the electric field distribution, with characteristic spatial scales of irregularities ranging from hundreds to tens of meters. Fourier analyses of the measured in-cloud electric fields give power-law spectra with the spectral index close to –2. The mechanisms of spectra formation are briefly discussed. INTRODUCTION It is well known that turbulence in the atmosphere leads to Kolmogorov-type spectra of wind and temperature fluctuations. More recently, spectra of fluctuations in electric field have been investigated. Short-term pulsations of the electric field in the surface atmospheric layer at the frequencies ∆f ≅ 10–1 Hz were found to obey universal power-law spectra under fair-weather and fog conditions with the most probable values of the spectral index to vary from -2 to -3 depending on particular atmospheric state [Anisimov et al., 2002]. It is of great importance to investigate the fine structure of electric field and space charge density in thunderstorm clouds, where intensive multi-flow motion of charged particles (cloud and precipitation particles) can lead to the mutual transformation of gas mechanical energy and electric field energy. Our theoretical studies to date have shown that a thundercloud has the ability of self-organization manifested as small-scale electrical stratification [Trakhtengerts, 1989; Mareev et al., 1999]. As a result, electric cells are generated, which is of particular interest for understanding intra-cloud, cloud-to-ground and high-altitude discharge inception. In the present paper several electric field soundings, made with balloon-borne electric field meters and radiosondes, are examined. Fourier and wavelet analyses of the measured in-cloud electric fields clearly demonstrate the presence of fine structures in the electric field distribution. DATA DESCRIPTION AND RESULTS Five electric field soundings examined in this paper were collected in the same mesoscale convective system (MCS), moved through Arkansas on June 5, 1998. Data gathered in an updraft and outside updrafts within the convective region, in the transition zone, and in the stratiform cloud region, were first pubished by Stolzenburg et al. [2001]. The details and characteristics of the electric field meter data can be found in [Stolzenburg et al., 1998]. Electric field data files included: Emin – the minimum measured E value during 1 second (2.5 spins of the instrument about the horizontal axis); Ez a 5-point smoothed Emin (smoothed over one rotation about the vertical axis), which serve as our “best estimate” of the vertical electric field. The pressure, temperature, relative humidity and altitude data were available as well. The altitude was determined from data collected with a meteorological radiosonde using the hypsometric equation. The respective electricfield data files were subjected to Fourier transform 0 4 8 0.8 1.6 2.4 3.2 4.8 5.6 6.4 7.2 8.8 9.6 10.4 11.2