A modeling study of the time-averaged electric currents in the vicinity of isolated thunderstorms (original) (raw)
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Electric fields and current densities under small Florida thunderstorms
Journal of Geophysical Research, 1991
The surface electric field E and Maxwell current density JM have been measured simultaneously under and near small Florida thunderstorms. These records show that the amplitude of JM is of the order of 1 nA/m 2 or less in the absence of precipitation and that there are regular time variations in JM during the intervals between lightning discharges that tend to have the same shapes after different discharges in different storms. Negative cloud-to-ground (CG) lightning produces an abrupt negative change in E and a corresponding negative (or bipolar) transient in JM that is followed by a positive overshoot. Under a storm, this overshoot peaks about 1 nA/m2 above background and then decays in a quasi-exponential or linear fashion until the next discharge occurs. Nearby cloud discharges produce a lightning transient and then either a small change in JM or a negative change that subsequently relaxes back to the predischarge level in 5 to 20 s. CG flashes at a range of about 20 km produce a fast transient in JM and then a positive overshoot that subsequently relaxes back to the predischarge level in 5 to 20 s. Distant cloud discharges produce overshoots and subsequent decays that are very similar to CG flashes but of opposite (i.e., negative) polarity. We believe that the major causes of the aforementioned time variations in JM between lightning discharges are currents that flow in the finitely conducting atmosphere in response to the field changes rather than rapid time variations in the strength of the cloud current sources. The displacement current densities that are computed from the E records dominate JM except when there is precipitation, when E is large and steady, or when E is unusually noisy. 1. INTRODUCTION Until recently, most observations of the electrical environment Ufider and near thunderstorms have focused on the cloud electric fields or the field changes that are caused by lightning.. Such data can be used to infer the cloud charge distrlbutiofi and the chafiges tn this distribution that are caused by lightning. These results, in turn, provide information about the electrification processes and how the cloud electrical structure evolves throughout the storm fLatham,
Maxwell currents under thunderstorms
Journal of Geophysical Research, 1982
We point out that recent observations of the time variations in thunderstorm electric fields, both aloft and at the ground, can be interpreted in terms of a total Maxwell current density that varies slowly with time in the intervals between lightning discharges. We utilize this quasi-static behavior to estimate and map the Maxwell current densities under a small Florida thunderstorm using data provided by a large field mill network. An area integral of these current densities gives a total Maxwell current just above the ground of about 0.5 A, a value which is a reasonable lower limit for the total Maxwell current produced by the cloud, and an upper limit for the rate of charge transport to ground between lightning flashes. Using the quasi-static behavior of the Maxwell current density, we derive an expression for the field-dependent current density under a thunderstorm during the field recovery following a lightning discharge, and we infer values of air conductivity under the small storm which range from 2 to 6 x 10-• 3 mho/m. Finally, we present data that indicate that the area-average Maxwell current is not usually affected by lightning, but instead varies slowly throughout the evolution of the storm. Therefore, we suggest that cloud electrification processes probably do not depend on the cloud electric field, which exhibits large and rapid time variations, as much as they do on more slowly varying quantities, such as the meteorological structure of the storm and/or the storm dynamics. 1. INTRODUCTION Recent tethered-balloon measurements by Winn and Byerley [-1975], Standlet and Winn [-1979], and Winn et al. [1980] show that, under thunderstorms, the electric field at an altitude of a few hundred meters tends to increase linearly with time between lightning discharges, whereas the field at the ground is not linear due to the space charge produced by corona processes. Standler [1980] has applied these results and has shown that, when the field at the ground is steady, then the spatially averaged corona current density at this time can be estimated from the slope of the electric field recovery following a lightning discharge at an earlier time when the field at the ground was not steady but was crossing zero. In this note we point out that the experimental and theoretical results of Winn, Standler, and associates can be interpreted simply in terms of a total Maxwell current density which varies slowly with time between lightning discharges. We show that measurements of the displacement current density when the field is close to zero can be used to estimate values of the Maxwell current density; and we compute and map Maxwell current densities under a small Florida storm, using data provided by a large field mill network. We show that, if the Maxwell current is quasi-static and if convection currents are steady, then the local field-dependent current density, which includes the corona current, can be derived from changes in the displacement current during a lightning field recovery. We apply this method and find reasonable values for the fielddependent current densities and atmospheric conductivities under the small storm. Finally, we present evidence that shows that the average Maxwell current density is usually not affected by lightning discharges and varies slowly throughout the evolution of the storm. Since the Maxwell current is steady at times when the field, both at the ground and aloft, undergoes large
A global model of thunderstorm electricity
Journal of Geophysical Research, 1993
An axisymmetric numerical model in an Earth-centered spherical coordinate system is created to calculate the electric field distribution and current distribution from a thunderstorm source in the global electrical circuit. The model includes a hemisphere in which the thunderstorm is located, an atmosphere and ionosphere with anisotropic height-variable conductivities, and a passive magnetic conjugate hemisphere. Both single-cell thunderstorms and symmetric multicell thunderstorm complexes can be modeled. The current output from the thunderstorm spreads out in the ionosphere and flows along the magnetic field lines into the conjugate hemisphere. Approximately half of the current that reaches the ionosphere flows into the conjugate hemisphere, and the rest is redirected to the fair-weather portion of the storm hemisphere. Two examples of this general model are discussed, a single-cell severe storm and a multicell severe storm. Results of this study show that it is important to include a realistic model of the ionosphere to evaluate the spread of current in the ionosphere and the mechanism of thunderstorm charging of the global electric circuit.
Modeling the electric structures of two thunderstorms and their contributions to the global circuit
Atmospheric Research, 2009
This study examines the electricity in two thunderstorms, typical for their respective locales (the Great Plains and the New Mexico mountains), by modeling them as a set of steady-state horizontal layers of external currents. The model electric sources, corresponding to the charge separation processes in the thundercloud, are embedded in an exponential conducting atmosphere. The source parameters are determined by fitting the model electric field to measured profiles. The resulting currents to the ionosphere (i.e., the Wilson current) from the two storms are 0.53 A and 0.16 A, while the calculated electrical energies of the storms are 2.3 × 10 10 J and 2.8 × 10 9 J, respectively. The more vigorous storm is estimated to transfer 16 000 C in the global circuit during 8.5 h of its lifetime, while the weaker mountain storm transferred about 1200 C in its entire 2-h lifetime. Removal of the screening charge layer from above the updraft region in one modeled storm leads to only a small increase in the net Wilson current of less than 3%, while it provides a substantial local disturbance of the electric field. Overall, the model findings indicate that differences in the Wilson currents and electrical energies of the two storms result from differences in their internal dynamical and electrical structures as well as their geographical locations.
Transient currents in the global electric circuit due to cloud-to-ground and intracloud lightning
Atmospheric Research, 2009
Intracloud (IC) and cloud-to-ground (CG) lightning flashes produce transient changes in the electric field (E) above a thundercloud which drive transient currents in the global electric circuit (GEC). Using in-cloud and above-cloud E data from balloons, ground-based E data, and Lightning Mapping Array data, the above-cloud charge transfers due to lightning transients are estimated for five IC and five CG flashes from four thunderstorms that occurred above the mountains in New Mexico, USA, in 1999. For the five CG flashes (which transferred −4 to −13 C to the ground), the transient currents moved +1 to +5 C of charge upward from cloudtop toward the ionosphere, with an average transient charge transfer of about 35% of the charge transferred to ground. For the five IC flashes (which neutralized 6 to 21 C inside the cloud), the transient currents moved −0.7 to −3 C upward, with an average transient charge transfer of about 12% of the lightning charge. Estimates for three thunderstorms indicate that the transient currents made only a small GEC contribution compared to the quasi-stationary Wilson currents because of the offsetting effects of IC and CG flashes in these storms. However, storms with extreme characteristics, such as high flash rates or predominance of one flash type, may make a significant GEC contribution via lightning transients.
Electrical Environment in a Storm Cloud
2012
This article gives an overview of the electrical characteristics of the thundercloud and the predominant mechanisms that are at the origin. The specific cloud that can produce lightning is described and the parameters that control its development and its organization are discussed. According to the variety of the scales of time and space associated with the mechanisms that occur within the thundercloud, it is difficult to simulate them both experimentally and numerically. Thus, the advances in the knowledge of the thunderstorm electricity have been sometimes relatively slow and have raised a lot of debates. Furthermore, in-situ observation remains difficult because of the hostility of the thundercloud medium for instrumentation, sensors, aircraft or other carriers of sensors. The responses to the questions in the domain of thundercloud electricity can sometimes remain speculative. However, recent detection techniques and laboratory experiments allow a better knowledge of the cloud e...
Modelling and observations of thundercloud electrification and lightning
Atmospheric Research, 2001
A two-dimensional lightning frequency model was applied to two case studies of thunderclouds: 9th July 1981, the CCOPE case, and 19th July 1991, the CaPE case. Factors influencing lightning activity were discussed. Although there was a strong link between updraft and lightning frequency, the relationship was not distinct; the initial environmental conditions, the updraft speed and more significantly the graupel number concentration were shown to have a large effect. The model computations suggested the importance of the 2-mm diameter graupel particles. No definite relationship between lightning frequency and cloud ice content could be established in this study. The results demonstrated the limitations of the current charge transfer parameterisation scheme used, with a more detailed structure being required. However, the model was capable of reproducing a realistic cloud structure and lightning activity using a relatively simple dynamical framework. q address: a.gadian@umist.ac.uk A. Gadian . 0169-8095r01r$ -see front matter q 2001 Elsevier Science B.V. All rights reserved.
Microphysical and electrical evolution of a Florida thunderstorm: 1. Observations
Journal of Geophysical Research, 1996
This study deals with the microphysical and electrical evolution of a thunderstorm that occurred on August 9, 1991, during the Convection and Precipitationf Electrification (CAPE) Experiment in eastern Florida. During its approximately 1-hour lifetime, the storm was penetrated several times by the Institute of Atmospheric Sciences' T-28 aircraft at midlevels. It was also penetrated at low and middle-levels by a National Oceanographic and Atmospheric Administration (NOAA) P-3 and scanned by three radars, one of which had multiparameter capabilities, operated by the National Center for Atmospheric Research. Two stages of the storm's evolution are analyzed herein during which the storm grew to produce precipitation and lightning. The first stage, sampled during the first T-28 penetration at 5.25 km (-3øC) and the P-3 at 6.4 km (-10øC), was characterized by a 2-to 3-kin wide updraft (maximum 14 m s '•) with cloud liquid water contents up to 4 g m -3, low concentrations of graupel at -10øC, and small to medium raindrops in concentrations of less than 200 m -3 at-3øC. A downdraft region also existed that was devoid of cloud liquid water, but contained graupel up to 2 mm. Radar data (Zr)•) are consistent with a coalescence-dominated precipitation generation mechanism followed by transport of drops in the updraft to heights with temperatures colder than -7øC, where freezing formed graupel that continued to grow by riming. Electrification during this stage remained weak. The second stage, sampled during the second and third T-28 penetrations and the second P-3 penetration, was characterized at midlevels by a narrower updraft and a more diffuse, broad downdraft separated by a 1-to 2-km wide transition zone. The updraft continued to show significant cloud liquid water (•2 g m -3) with few precipitation particles, while the downdraft had very little cloud liquid with graupel in concentrations >1 e -•. The transition zone shared both updraft and downdraft characteristics. The increase in ice concentration was accompanied by a rapid increase in the electrification of the cloud with peak electric fields reaching-20 kV m -• at T-28 altitude and the detection of lightning by ground-based sensors and pilot report. As time progressed, precipitation particle concentrations reached several per liter at midlevels in both updrafts and downdrafts. The observations are consistent with electrification through a precipitation-based mechanism involving the development of the ice phase. um'esolved. 1978a; Jayaratne et al., 1983' Saunders et al., 1991] that charge
Journal of The Atmospheric Sciences, 2010
The long-standing mainstay of support for C. T. R. Wilson's global circuit hypothesis is the similarity between the diurnal variation of thunderstorm days in universal time and the Carnegie curve of electrical potential gradient. This rough agreement has sustained the widespread view that thunderstorms are the ''batteries'' for the global electrical circuit. This study utilizes 10 years of Tropical Rainfall Measuring Mission (TRMM) observations to quantify the global occurrence of thunderstorms with much better accuracy and to validate the comparison by F. J. W. Whipple 80 years ago. The results support Wilson's original ideas that both thunderstorms and electrified shower clouds contribute to the DC global circuit by virtue of negative charge carried downward by precipitation. First, the precipitation features (PFs) are defined by grouping the pixels with rain using 10 years of TRMM observations. Thunderstorms are identified from these PFs with lightning flashes observed by the Lightning Imaging Sensor. PFs without lightning flashes but with a 30-dBZ radar echotop temperature lower than 2108C over land and 2178C over ocean are selected as possibly electrified shower clouds. The universal diurnal variation of rainfall, the raining area from the thunderstorms, and possibly electrified shower clouds in different seasons are derived and compared with the diurnal variations of the electric field observed at Vostok, Antarctica. The result shows a substantially better match from the updated diurnal variations of the thunderstorm area to the Carnegie curve than Whipple showed. However, to fully understand and quantify the amount of negative charge carried downward by precipitation in electrified storms, more observations of precipitation current in different types of electrified shower clouds are required.
Current Problems of Electrodynamics of a Thunderstorm Cloud
Radiophysics and Quantum Electronics, 2005
Electrodynamics of a thunderstorm cloud is considered with allowance for recirculation and multiflow motion of charged intracloud particles. In this simulation, the large-scale electric-field emerges due to the charge separation at the process of air convection and develops through the oscillation regime in the initial and final stages of the thunderstorm evolution. These oscillations qualitatively explain the observed behavior of the electric field of a thunderstorm. On the other hand, the multiflow convection is unstable and leads to generation of small-scale electrostatic waves (wavelength from 1 to 100 m) with amplitude reaching the conventional breakdown value. Such an instability can initiate microdischarge intracloud activity at the preliminary stage of the lightning discharge and between individual return strokes. We propose a three-dimensional cellular automata model which describes the main features of the preliminary stage of the lightning.