Charge Advection Research Articles

Electroviscous drag on squeezing motion in sphere-plane geometry.

Theoretically and experimentally, we study electroviscous phenomena resulting from charge-flow coupling in a nanoscale capillary. Our theoretical approach relies on Poisson-Boltzmann mean-field theory and on coupled linear relations for charge and hydrodynamic flows, including electro-osmosis and charge advection. With respect to the unperturbed Poiseuille flow, we define an electroviscous coupling parameter ξ, which turns out to be maximum where the film height h_{0} is comparable to the Debye screening length λ. We also present dynamic atomic force microscopy data for the viscoelastic response of a confined water film in sphere-plane geometry; our theory provides a quantitative description for the electroviscous drag coefficient and the electrostatic repulsion as a function of the film height, with the surface charge density as the only free parameter. Charge regulation sets in at even smaller distances.

Investigating the Relative Contributions of Charge Deposition and Turbulence in Organizing Charge within a Thunderstorm

Abstract Large-eddy-resolving simulations using the Collaborative Model for Multiscale Atmospheric Simulation (COMMAS), which contains microphysical charging and branched-lightning parameterizations, produce much more complex net charge structures than conventionally visualized from previous observations, simulations, and conceptual diagrams. Many processes contribute to the hydrometeor charge budget within a thunderstorm, including advection, hydrometeor differential sedimentation, subgrid turbulent mixing and diffusion, ion drift, microphysical separation, and the attachment of ion charge deposited by the lightning channel. The lightning deposition, sedimentation, and noninductive charging tendencies contribute the most overall charge at relatively large scales, while the advection tendency, from resolved turbulence, provides the most “texture” at small scales to the net charge density near the updraft region of the storm. The scale separation increases for stronger storm simulations. In aggregate, lightning deposition and sedimentation resemble the smoother distribution of the electric potential, while evidence suggests individual flashes could be responding to the fine texture in the net charge. The clear scale separation between the advection and other net charge tendencies suggest the charge advection is most capable of providing net charge texture; however, a clear-cut causality is not obtained from this study.

Scalable and Continuous Water Deionization by Shock Electrodialysis

Rising global demand for potable water is driving innovation in water treatment methods. Shock electrodialysis is a recently proposed technique that exploits deionization shock waves in porous media to purify water. In this letter, we present the first continuous and scalable shock electrodialysis system and demonstrate the separation of sodium, chloride, and other ions from a feed stream. Our prototype continuously removes over 99% (and up to 99.99%) of salt from diverse electrolytes over a range of concentrations (1, 10, and 100 mM). The desalination data collapse with dimensionless current, scaled to charge advection in the feed stream. Enhanced water recovery with increasing current (up to 79%) is a fortuitous discovery, which we attribute to electro-osmotic pumping. These results suggest the feasibility of using shock electrodialysis for practical water purification applications.

Evolution of radar reflectivity and total lightning characteristics of the 21 April 2006 mesoscale convective system over Texas

On 21 April 2006 a mesoscale convective system (MCS) passed within range of the Houston (KHGX) operational Weather Surveillance Radar — 1988 Doppler (WSR-88D, S-band) and the Houston Lightning Detection and Ranging (LDAR) network, which measures the time and three-dimensional location of total, or both intracloud (IC) and cloud-to-ground (CG), lightning. This study documents the evolution of total lightning and radar reflectivity for the 21 April 2006 MCS over Texas, with emphasis on the stratiform region and those processes in the convection region that likely influence stratiform region development. As the MCS traverses the LDAR network, the system slowly matures with a weakening convective line and a developing stratiform region and radar bright band. The area of stratiform precipitation increases by an order of magnitude and mean stratiform radar reflectivity increases by 7–8 dB in the radar bright band and mixed-phase zone (0° to − 10 °C) just above it. As the stratiform region matures, the total lightning pathway slopes rearward and downward from the back of the convective line and into the stratiform region. At early times, the pathway extends horizontally rearward 40 to 50 km into the stratiform region at an altitude of 10 to 12 km. Near the end of the analysis time period, the total lightning pathway slopes rearward 40 km and downward 6 km through the transition zone before extending 40 to 50 km in the stratiform region at an altitude of 5 to 7 km. The sloping pathway likely results from charged ice particles advected from the convective line by storm relative front to rear flow while the level pathway extending further into the stratiform region is likely caused by both charge advection and local in-situ charging. As the stratiform region matures, the stratiform region total lightning flash rate increases and total lightning heights decrease. The percentage of stratiform total lightning flashes originating in the stratiform region increases significantly from 10% to 45%. The number of positive CG flashes in the stratiform region also increases with 73% originating in the convective or transition regions. Both in-situ charging mechanisms created by the development of the mesoscale updraft and charge advection by the front to rear flow likely contribute to the increased electrification and total lightning production of the stratiform region.

Total Lightning Signatures of Thunderstorm Intensity over North Texas. Part II: Mesoscale Convective Systems

Abstract Total lightning data from the Lightning Detection and Ranging (LDAR II) research network in addition to cloud-to-ground flash data from the National Lightning Detection Network (NLDN) and data from the Dallas–Fort Worth, Texas, Weather Surveillance Radar-1988 Doppler (WSR-88D) station (KFWS) were examined from individual cells within mesoscale convective systems that crossed the Dallas–Fort Worth region on 13 October 2001, 27 May 2002, and 16 June 2002. LDAR II source density contours were comma shaped, in association with severe wind events within mesoscale convective systems (MCSs) on 13 October 2001 and 27 May 2002. This signature is similar to the radar reflectivity bow echo. The source density comma shape was apparent 15 min prior to a severe wind report and lasted more than 20 min during the 13 October storm. Consistent relationships between severe straight-line winds, radar, and lightning storm cell characteristics (e.g., lightning heights) were not found for cells within MCSs as was the case for severe weather in supercells in Part I of this study. Cell interactions within MCSs are believed to weaken these relationships as reflectivity and lightning from nearby storms contaminate the cells of interest. Another hypothesis for these weak relations is that system, not individual cell, processes are responsible for severe straight-line winds at the surface. Analysis of the total lightning structure of the 13 October 2001 MCS showed downward-sloping source density contours behind the main convective line into the stratiform region. This further supports a charge advection mechanism in developing the stratiform charge structure. Bimodal vertical source density distributions were observed within MCS convection close to the center of the LDAR II network, while the lower mode was not detected at increasing range.

Electric fields, cloud microphysics, and reflectivity in anvils of Florida thunderstorms

A coordinated aircraft–radar project that investigated the electric fields, cloud microphysics, and radar reflectivity of thunderstorm anvils near Kennedy Space Center is described. Measurements from two cases illustrate the extensive nature of the microphysics and electric field observations. As the aircraft flew from the edges of anvils into the interior, electric fields very frequently increased abruptly from ∼1 to >10 kV m1 even though the particle concentrations and radar reflectivity increased smoothly. The abrupt increase in field usually occurred when the aircraft entered regions with a reflectivity of 10–15 dBZ. We suggest that the abrupt increase in electric field was because the charge advection from the convective core did not occur across the entire breadth of the anvil and because the advection of charge was not constant in time. Also, some long‐lived anvils showed enhancement of electric field and reflectivity far downwind of the convective core. Screening layers were not detected near the edges of the anvils. Comparisons of electric field magnitude with particle concentration or reflectivity for a combined data set that included all anvil measurements showed a threshold behavior. When the average reflectivity, such as in a 3‐km cube, was less than approximately 5 dBZ, the electric field magnitude was <3 kV m1. Based on these findings, the Volume Averaged Height Integrated Radar Reflectivity (VAHIRR) is now being used by the NASA, the Air Force, the and Federal Aviation Administration in new Lightning Launch Commit Criteria as a diagnostic for high electric fields in anvils.

Lightning location relative to storm structure in a leading‐line, trailing‐stratiform mesoscale convective system

Horizontal and line‐normal, vertical cross‐sections and composite images of Dallas‐Fort Worth Lightning Detection and Ranging (LDAR II) VHF radiation sources and radar reflectivity over a 30‐min period provide a unique perspective on lightning pathways within a leading‐line, trailing‐stratiform (LLTS) mesoscale convective system (MCS) on 16 June 2002. The overwhelming majority of VHF lightning sources occurred within the leading convective line in a bimodal pattern in the vertical. Assuming that VHF source density maxima were most likely associated with positive charge, then the LDAR II observations suggest that the gross charge structure of the convective region of the MCS was characterized by a tripole with net positive charge centered at 4.5 km AGL (3°C) and 9.5 km AGL (−35°C) and net negative charge centered roughly in the relative minimum of the VHF source density maximum at 7 km AGL (−17°C). A persistent lightning pathway and inferred positive charge zone sloped rearward (by 40–50 km) and downward (by 4–5 km) from the upper VHF source maximum in the convective line, through the transition zone, and into the radar bright band of the stratiform region. In the stratiform region, VHF lightning sources and inferred positive charge were concentrated in three layers centered at 4.5, 6, and 9 km AGL (2°C, −11°C, and −31°C, respectively), consistent with past electric field studies of symmetric LLTS MCSs. Positive cloud‐to‐ground lightning flashes in the stratiform region were initiated in the convective line and followed the slanting pathway from the top of convective cores to the stratiform precipitation, where they were horizontally extensive, layered, and highly branched. The sloping lightning pathway was identical to hypothetical trajectories taken by snow particles. These observations provide further support for the advection of charge on snow along the sloping pathway and the in situ generation of charge in the horizontal lightning layers as primary contributors to electrification and positive lightning production rearward of the convective line.

Electrification of Stratiform Regions in Mesoscale Convective Systems. Part II: Two-Dimensional Numerical Model Simulations of a Symmetric MCS

Abstract Model simulations of a symmetric mesoscale convective system (MCS; observations discussed in Part I) were conducted using a 2D, time-dependent numerical model with bulk microphysics. A number of charging mechanisms were considered based on various laboratory studies. The simulations suggest that noninductive ice–ice charge transfer in the low liquid water content regime, characteristic of MCS stratiform regions, is sufficient to account for observed charge density magnitudes, and as much as 70% of the total charge in the stratiform region. The remaining 30% is contributed by charge advection from the convective region. The strong role of in situ charging is consistent with previous water budget studies, which indicate that roughly 70% of the stratiform precipitation results from condensation in the mesoscale updraft. Thus both in situ charging and charge advection (the two previously identified hypotheses) appear to be important contributors to the electrical budget of the stratiform region. The ...

A pseudotransient approach to steady state solution of electric field-space charge coupled problems

A hybrid technique is presented for the general solution of the governing equations of coupled space-charge and electrical-field problems, including terms for charge advection and diffusion. The governing equations are presented in a general theoretical framework. The methodology involves writing the governing equations in pseudotransient form and discretising using the finite volume method on a structured grid. The discretised equations are relaxed to a steady state by advancing in time using the approximate factorisation methodology. The Additive Correction Multigrid (ACM) method is used to accelerate iterative convergence. The technique is demonstrated by focusing on the problem of monopolar corona in an electrostatic precipitator (ESP). The key features of the methodology are speed, stability and modelling flexibility, and it is expected to be applicable to a range of charge-coupled problems.

Some theoretical estimations of spectral densities of short‐period electric noises generated near the ground

The paper presents a theoretical analysis of spectral densities of noises generated by charge advection in a vertical electric field and air‐Earth current measured on the ground by point and linear sensors. The analysis is restricted only to short time‐scales so that the turbulence transporting the charge is close to its inertial subrange and can be considered as homogeneous and locally isotropic. Hence the Kolmogorov scaling law is applied. Next the frozen turbulence approximation is used to obtain frequency spectra. In comparison with point sensors for long antennas, electric noises are considerably damped, especially at high frequencies.

Horizontal Distribution of Electrical and Meteorological Conditions across the Stratiform Region of a Mesoscale Convective System

Five soundings of the electric field and thermodynamic properties were made in a mesoscale convective system (MCS) that occurred in Oklahoma and Texas on 2–3 June 1991. Airborne Doppler radar data were obtained from three passes through the stratiform echo. From these electrical, kinematical, and reflectivity measurements, a conceptual model of the electrical structure of an MCS is developed. Low-level reflectivity data from the storm's mature and dissipating stages show typical MCS characteristics. The leading convective region is convex forward, and the back edge of the stratiform echo is notched inward. The maximum areal extent of the low-level echo is about 250 km × 550 km, and the radar bright band is intense (reflectivity 45–50 dBZ) through an area of at least 50 km × 100 km. The reflectivity above the bright band is horizontally stratified with decreasing intensity and echo-top height toward the rear of the system. Analyses of the velocity data reveal a convective-line-relative flow structure of front-to-rear flow and mesoscale ascent aloft, and weak rear inflow and descent below about 5 km. The electric field soundings are similar over a period of 3 h and a horizontal scale of 100 km across the stratiform region, suggesting that the charge structure is nearly steady state and the charge regions are horizontally extensive and layered. The basic charge structure consists of four layers: a 1–3-km-deep region of positive charge (density ρ ≈ +0.2 nC m−3) between 6 and 10 km, negative charge (ρ ≈ −1.0–2.5 nC m−3) between 5 and 6 km, positive charge (ρ ≈ +1.0–3.0 nC m−3) near 0°C, and negative charge (ρ ≈ −0.5 nC m−3) near cloud base. The upper positive and densest negative charge layers could result from advection of charge from the convective region. The negative charge layer may be augmented by noninductive collisional charging in the stratiform region. The positive charge near 0°C is probably caused by one or more in situ charging mechanisms. The negative charge near cloud base is likely the result of screening layer formation. In addition to the basic four charge layers, positive charge is found below the cloud in each sounding, and in the two soundings closest to the convection (70–100 km distant) there is a low-density negative charge region near echo top.

Lightning and electrical structure of mesoscale convective systems

Recent observations indicate that cloud-to-ground lightning discharges are common in the stratiform portions of Mesoscale Convective Systems. A bipolar lightning pattern is often found, with negative flashes preferred in the convective region and positive flashes preferred in the stratiform region. We first review a charge advection process offered to explain lightning bipoles. In this process, positive charge (contained on the surface of small ice particles) is transported into the stratiform region by storm-relative winds. Calculations for the expansion rate of a spherical volume of charge by Coulombic forces indicate slow expansion, thus maintaining high charge concentrations in the stratiform region. However, when turbulent diffusion of the charge volume is considered, expansion rates through mixing are large compared to expansion rates by Coulombic forces. We therefore argue that the charge advection process is not a plausible mechanism for explaining lightning bipoles. The charge structures observed in stratiform clouds are reviewed and an in-situ mechanism, where the stratiform clouds electrify independent from the deep convection, is offered as an explanation for lightning bipoles. New observations from a tropical field program are presented in support of this in-situ charging mechanism.

An effect of space charge advection on vertical air-earth current measurements

The measured values of electrical parameters of the atmosphere are known to be affected by wind, and days with strong winds (v x >6 m s−1) are normally classified as electrically disturbed. While the effect of horizontal wind on the electric field is readily explained in terms of changes in the spacecharge density, it is shown from electrodynamical considerations that the observed effect on the air-earth current cannot always be attributed solely to changes in the local conductivity and electric field. The existence of a vertical current of advection of magnitude ɛ0χ e υ x (∂E z /∂x) is revealed, and shown by calculations to be important (for instance during dust-storms and pointdischarge) when large space-charge concentrations are advected by wind. Since the current of advection is proportional tov x (∂E z /∂x), it is suggested that this parameter rather thanv x be used as a criterion for deciding wind-disturbed measurements of routine fair-weather atmospheric electric parameters.