Maxwell currents beneath thunderstorms (original) (raw)
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
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
A modeling study of the time-averaged electric currents in the vicinity of isolated thunderstorms
Journal of Geophysical Research, 1992
A thorough examination of the results of a time-dependent computer model of a dipole thunderstorm revealed that there are numerous similarities between the time-averaged electrical properties and the steady state properties of an active thunderstorm. Thus, the electrical behavior of the atmosphere in the vicinity of a thunderstorm be can be determined with a formulation similar to what was first described by Holzer and Saxon in 1952. From the Maxwell continuity equation of electric current, a simple analytical equation was derived that expresses a thunderstorm's average current contribution to the global electric circuit in terms of the generator current within the thundercloud, the intracloud lightning current, the cloud-to-ground lightning current, the altitudes of the charge centers, and the conductivity profile of the atmosphere. This equation was found to be nearly as accurate as the more computationally expensive numerical model, even when it is applied to a thunderstorm with a reduced conductivity thundercloud, a time-varying generator current, a varying flash rate, and a changing lightning mix.
Electric fields produced by Florida thunderstorms
Journal of Geophysical Research, 1978
Electric fields produced by air mass thunderstorms have been recorded at 25 field mill sites at the NASA Kennedy Space Center (KSC) during the summers of 1975 and 1976. Time-and area-averaged fields produced by individual storms are typically-0.8 to-2.1 kV m-• during periods of intense lightning activity and usually 2-4 times larger (-2.3 to-4.3 kV m-•) in the final, less active storm periods. The total number of lightning discharges generated by individual storms ranged from 8 to 1987. The average flashing rate per storm was 0.3-9.3 discharges per minute. Large storms tend to evolve through an initial, an active, and a final phase of electrical activity. The average point discharge computed during the active period is 3-4 times less than that computed during the final phase. In 1975, about 71% of all lightning discharges occurred during active storm periods, which in turn represented only about 27% of the total storm durations. In general, the structures of the electrostatic fields produced by lightning discharges in Florida were found to be similar to those in other geographical locations. During active storm periods, 42-52% of all lightning discharges were to ground,land during a final storm period only about 20% were to ground. The fraction of discharges to ground which contained continuing currents ranged from 29 to 46%; the mean duration of 239 ground flashes was about 420 ms. The average density of lightning flashes to ground d•uring 1974 and 1975 at KSC was about 6 km-•' month-• during the months of June and July. ' detect the electrified Clouds and lightning discharges which represent hazards to various ground and launch operations, KSC has constructed and operated an extensive network of instruments to measure the vertical electric field of the atmosphe•re. Recently, KSC has also provided facilities and support for a number of other coordinated measurements through the Thunderstorm Research International Program [Pierce, 1976]. Since the characteristics of the electric fields produced by thunderstorms and lightning at KSC are of great practical as well as scientific importance, it is appropriate to present examp!es of these fields under a variety of storm conditions and to survey their overall behavior. Most thunderstorms which occur in central Florida during the summer are air mass storms initiated by local heating and. sea breeze convergence [Byers and Rodebush, 1948]. The synoptic conditions which tend to produce storms over the KSC complex and Merritt Island have been analyzed in detail by N•umann [1971 ]. M0st storms ar.e produced under conditions of moderate westssOuthWesterly flow through the 500-mbar level. With these winds, convective'cells tend to,develop west of the Indian River in the early afternoon and later merge to form !a•rge organized systems which advance east-northeastwar d over KSC. The electrical activity produced by these lat. ge storms may affe ct KSC for periods of 3 hours or more. Under light wind conditions, 'one or more isolated cells may develop directly over KSC with little or no hOriZOntal movement nt. As we shall see, the durations of these small storms are usually less than I hour. In this paper we show examples of the electric fields produced by large and small storms at KSC, present time averages of these fields, estimate the magnitude O f point discharge currents, examine the time development of lightning activity in large storms, and, finally, examine the detailed structure of the changes in electrostatic field produced by Florida lightning.
Diurnal variation of the global electric circuit from clustered thunderstorms
Journal of Geophysical Research: Space Physics, 2014
The diurnal variation of the global electric circuit is investigated using the World Wide Lightning Location Network (WWLLN), which has been shown to identify nearly all thunderstorms (using WWLLN data from 2005). To create an estimate of global electric circuit activity, a clustering algorithm is applied to the WWLLN data set to identify global thunderstorms from 2010 to 2013. Annual, seasonal, and regional thunderstorm activity is investigated in this new WWLLN thunderstorm data set in order to estimate the source behavior of the global electric circuit. Through the clustering algorithm, the total number of active thunderstorms are counted every 30 min creating a measure of the global electric circuit source function. The thunderstorm clusters are compared to precipitation radar data from the Tropical Rainfall Measurement Mission satellite and with case studies of thunderstorm evolution. The clustering algorithm reveals an average of 660 ± 70 thunderstorms active at any given time with a peak-to-peak variation of 36%. The highest number of thunderstorms occurs in November (720 ± 90), and the lowest number occurs in January (610 ± 80). Thunderstorm cluster and electrified storm cloud activity are combined with thunderstorm overflight current measurements to estimate the global electric circuit thunderstorm contribution current to be 1090 ± 70 A with a variation of 24%. By utilizing the global coverage and high time resolution of WWLLN, the total active thunderstorm count and current is shown to be less than previous estimates based on compiled climatologies.
The behavior of total lightning activity in severe Florida thunderstorms
Atmospheric Research, 1999
The development of a new observational system called LISDAD Lightning Imaging Sensor . Demonstration and Display has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5-20 min. A distinguishing feature of severe storms is the presence of lightning 'jumps' -abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds -wind, hail, tornadoes -is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity. q 0169-8095r99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.
— The study of electric field is an important tool for lightning research. Electric Field Mills (EFMs) are electro-mechanical devices used to observe static atmospheric electric fields during fair weather and during storm conditions. EFM readings show significant changes in electric field due to an approaching thunderstorm and particularly, during lightning periods. Attempts to comprehend the variations prior to and after a strike has been done by observatories all over the world. This paper focuses on identifying characteristic changes in atmospheric electric field prior to a lightning strike. Both static and dynamic field changes are studied by means of literature survey and static field variations are also studied through analysis of EFM data from Sonnblick Observatory, Austria and National Centre for Earth Science Studies (NCESS), Trivandrum, Kerala. During thunderstorms and the few seconds prior to lightning, significant changes are observed in both static and dynamic electric fields. Static field changes include a magnitude rise and reversal of direction of electric field from the fair weather situation, and dynamic field changes are characterized by the occurrence of Preliminary Breakdown Pulses(PBP) which is the dynamic electrical activity inside the cloud before a strike.This study, by identifying the characteristic changes in atmospheric electric field prior to lightning strike by comparing multiple observations from different parts of the world ,points to great possibilities in the field of lightning prediction.
Lightning-Initiation Locations as a Remote Sensing Tool of Large Thunderstorm Electric Field Vectors
Journal of Atmospheric and Oceanic Technology, 2005
The lightning data that are recorded with a three-dimensional lightning mapping array (LMA) are compared with data from an electric field change sensor (in this case a flat-plate antenna operated both as a “slow” and a “fast” antenna). The goal of these comparisons is to quantify any time difference that may exist between the initial responses of the two instruments to a lightning flash. The data consist of 136 flashes from two New Mexico thunderstorms. It is found that the initial radiation source detected by the LMA usually precedes the initial response of both the slow and fast antennas. In a small number of cases, the flat-plate antenna response precedes the initial LMA source, but by no more than 2 ms. The observations of such a close time coincidence suggest that the first LMA radiation source of each flash was located at or very near the flash-initiation point. Thus, the first LMA radiation source and the initial sequence of sources from a lightning flash can be used as remot...
Lorentz Force Contribution to Thunderstorm's Electrical Characteristics
2021
In this paper, the exerted electric and geomagnetic forces on the electrified hydrometeors in thunderclouds are compared. The parameters of geomagnetic field are acquired from International Geomagnetic Reference Field (IGRF) model. First, the calculations showed that the magnitude of the electric force exerted on a charged hydrometeor dominated the magnitude of the geomagnetic force in troposphere. These results revealed the significance of electric force in the formation of thunderclouds’ charge structure. Moreover, as the electric field increases in thunderstorm conditions, (regarding the dependence of the induction mechanism of cloud electrification to the intensity of the electric field), the increased electric field strengthens the induction mechanism of cloud electrification and influences the electrical properties of thunderstorm. Second, using satellite-based/ground-based data and reports, an inverse relation has been revealed between the total geomagnetic field and the mean...