Recovery curves of the surface electric field after lightning discharges occurring between the positive charge pocket and negative charge centre in a thundercloud

Surface observations of the electric field recovery curves of the lightning discharges occurring between the positive charge pocket and negative main charge centre in an overhead thundercloud are reported. Such recovery curves are observed to have an additional step of very slow field-change observed at an after-discharge value of electric field equal to 5-6 kV m-1. The behavior of recovery curves is explained in terms of the coronae charge and the relative efficiencies of the charge generating processes responsible for growth of positive charge pocket and main negative charge centre in the thundercloud. The charging currents responsible for the growth of charge in positive charge pockets is computed to be 2-4 times larger than that for the growth of the main negative charge. However, the charge destroyed in such a discharge is found to be comparable to that in a discharge between the main charge centres of the thundercloud

Recovery curves of the lightning discharges occurring in the dissipation stage of thunderstorms

Measurements of atmospheric electric field made below two thunderstorms show that all lightning discharges occurring in the dissipating stage of a thunderstorm occur at almost the same value of the pre-discharge electric field at the ground surface. The observation is explained on the basis of the shielding of the electric fields generated by the positive charge in the downdrafts by the negative charge in the screening layers formed around them in the subcloud layer. Our observations suggest that in the dissipating stage of the thunderstorm, the charge generating mechanisms in cloud have ceased to operate and the charge being transported from the upper to lower regions of cloud by downdrafts is the only in-cloud process affecting the surface electric field and/or enhancing the electric field stress in and below the cloud base to cause yet another lightning discharge

Scavenging of aerosol particles by large water drops 2. The effect of electrical forces

The effect of electrical forces on the collection efficiency of millimeter-sized water drops collecting micron-sized aerosol particles has been investigated in a laboratory experiment. The observations show higher collection efficiencies for drops of 3.6- to 4.8-mm diameters than reported in some of the earlier studies for smaller drops. The limited and sparse data obtained in our experiments show that the collection efficiency of a drop is higher when it is charged or interacts with the aerosol in the presence of an electric field. The collection efficiency shows a maximum when the drop charge of either polarity is in the range of 10−12 to 10−11 C. The data show that the drop surface charge density required for this maximum decreases with the increase in drop size but is independent of the particle size. However, the peak value of collection efficiency is higher for larger particles. Moreover, the total charge on the drop required for this maximum remains almost constant at about 2–3×10−12C. The collection efficiency increases with the increase in the electric field, and the effect of the electric field is stronger for larger drops. In high fields, the drop collection efficiency shows a maximum for particles of diameter between 3.5 and 5 μm. The change in collection efficiency for the same change in particle size is larger for higher electric fields. Distortion of large drops and the consequent charge accumulation on the rim of the drop has been proposed to explain the results. The decrease in collection efficiency for large values of drop charge and electric field support the drop-to-particle charge transfer during their interaction

Changes in the concentration and size-distribution of the sub-micron particles associated with the sea- and land-breezes at a coastal station

Surface measurements of the size-distribution of submicron aerosol particles in the range of 0.003 to 1 μm diameter have been made on 6-8 January 1998, at Thiruvananthapuram during an inter-comparison campaign of the Indian Ocean experiment (INDOEX). The results are studied with respect to the setting-in of the sea- and land-breezes at the station. Observations show an increase of up to an order of magnitude in aerosol concentrations of all size categories with the setting-in of the land breeze at 1800-1900 IST. High concentrations of aerosol particles prevail throughout the period of the land breeze at night-time. Aerosol concentrations remarkably decrease at about 1000 IST with the arrival of much cleaner air with the sea breeze. During the land breeze at night-time, the size-distributions of aerosol particles is bimodal with the maximum at 0.075 and 0.024 μm diameters. During the sea breeze, especially in the afternoon, the maximum in the accumulation mode shifts to a slightly higher size (0.133 μm) and the maximum in the nucleation mode seems to shift to a smaller size (0.013 μm or smaller). The size-distribution curves during the daytime are mostly open-ended at the small particle-size end. The enhanced coagulation of aerosol particles and the gas-to-particle conversion processes have been proposed to explain the shift of maxima in the accumulation mode and the enhanced generation of small particles in the nucleation mode in the afternoon, respectively

Scavenging of aerosol particles by large water drops 3. Washout coefficients, half-lives, and rainfall depths

Using the average values of collection efficiencies obtained from our experiments [Pranesha and Kamra, 1996, this issue] washout coefficients for the drops in the diameter (D) range 3.6≤D≤5.0 mm collecting micron-sized aerosol particles have been calculated when the drops are neutral, charged, or falling in an electric field. Compared with the neutral case, the values of washout coefficients are higher in both electrical cases, the increase being more pronounced for smaller particles. Washout coefficients show a maximum for a drop charge of 10−12 to 10−11 C. With an increase in electric field, the washout coefficients increase linearly for ∼1-μm particles, show a maximum for ∼4-μm particles, and change insignificantly for ∼7-μm particles. Combining our experimental values of collection efficiencies with the theoretical collision efficiencies of McGann and Jennings [1991] for smaller drops, washout coefficients, half-lives, and rainfall depths have been computed for the raindrop size distribution extending from 0.1- to 5 mm-diameter. Results show that raindrops of diameter >1 mm contribute dominantly in removing particles of diameter 1–2 μm and their contributions increase with the rainfall rate. When the effect of the raindrops of diameter >1 mm is included, the values of washout coefficient increase by about 2 orders of magnitude for particles of diameter 1–2 μm and by about 1 order of magnitude for particles of diameter >2 μm. It can be concluded from the estimates of rainfall depth that a heavy rainfall over a short duration is more efficient in removing the particles of diameter 2.2 μ

Vertical profiles of atmospheric electric parameters close to ground

The ion-aerosol balance equations have been solved for different mixing strengths and two types of ionization profile to get vertical profiles of atmospheric electric parameters close to the Earth's surface. The inclusion of surface radioactivity in the model causes the decrease in electric field to be more intense at lower levels than at higher levels, an increase in the asymptotic value of the field, and an increase in the space charge density gradient. When the mixing of atmosphere is very weak, a reverse electrode effect is observed under the condition of enhanced ionization due to trapped radon, with the decrease in ion densities above 1 m. However, with an increase in eddy diffusivity, the positive ion concentration decreases, and the negative ion concentration increases with increasing height above the diffusion layer. Further increase, in eddy diffusivity causes the profiles to become almost similar to those for which the ionization is constant with height. Even with the inclusion of radon radioactivity, relatively higher diffusivity keeps the nature of the profile of the negative ions similar to that for the constant ionization case. On the contrary, there is a drastic change in the profile of positive ions because of the strong downward gradient of positive ions in the diffusion sublayer. The profile of positive ions is more sensitive to the radon radioactivity and that of negative ions to the strength of turbulence. Further, the asymptotic value of aerosol concentration is reached before that of small ions when mixing is very low. Our results are in reasonable agreement with the earlier theoretical and experimental studies

Evolution of lightning in an isolated hailstorm of moderate size in the tropics

Evolution of lightning activity in a tropical hailstorm of moderate size that developed in the premonsoon season at Pune (18°32′N, 73°51′E, 559 m above sea level) is studied from the measurements of surface electric field, the Maxwell current and thunder. Total flash rate is counted from the electric field record, and the cloud-to-ground (CG) flash rate is estimated from the visual observations. Precise timings of their occurrence were confirmed from the observations of overshoot in the Maxwell current records. The storm exhibited an almost constant rate of one CG flash every 1 to 2 min over the whole life time of the storm. The ratio of intracloud (IC) to CG flashes (IC/CG) increased with the increase in total flash rate. In the convective stage of the storm, field changes from consecutive flashes were generally found to alternate in polarity. Moreover, in this stage, field changes occur in pairs, the first field change of each pair being of negative polarity and the second one of positive polarity. The two field changes in a pair occur with an average time difference of 14.3 ± 8.4 s while two consecutive pairs appear after 29.3 ± 9.1 s. In between the convective and mature stages, our observations suggest the occurrence of the phenomenon of rain gush and the field excursion associated with falling precipitation. Development of the mature stage was marked with rapid transitions in the surface electric field and the Maxwell current polarities from negative to positive. Further, total flash rate and IC/CG ratio sharply increase, and the lightning-induced electric field changes become almost exclusively of negative polarity. Observations suggest possibly a lifting up of the charging region in mature stage of the storm. The dissipating stage of the storm witnessed hail and rain showers, sharp transition of electric field and the Maxwell current from positive to negative polarity and occurrence of a few positive CG discharges. Our observations are consistent with the general belief that that some lightning flashes, by neutralizing and depositing charge in the region of opposite polarity, change the charge distribution so as to trigger another discharge in the stor

Physical properties of the arctic summer aerosol particles in relation to sources at Ny-Alesund, Svalbard

Measurements of the number concentration and size distribution of aerosol particles in the size range of 0.5-20 μm diameter were made with an aerodynamic particle sizer at an Arctic site at Ny-Alesund, Svalbard in August-September 2007 during the International Polar Year 2007-2008. Data are analyzed to study the aerosol number concentration-wind speed relationships. The sea-salt particles of marine origin generated within the Arctic circle are identified as the main source of the Arctic summer aerosols. Total number concentration of aerosol particles increases with increase in wind speed, the increase being more when winds from open leads over the oceanic sector are reaching the station as compared to when winds from pack ice in other directions are reaching the station. The larger increase with winds from the oceanic sector is attributed to the enhanced bubble-breaking activity and increased entrainment of dimethyl sulphide particles at the sea surface. Although, the increase in total aerosol number concentration associated with the winds from the oceanic sector is spread over the whole range of particle sizes, the increase in coarse mode particles is more prominent than that in the accumulation mode particles. The age of airmass over pack ice is also an important factor to determine the aerosol concentration over the Arctic region. The process of rainout/washout of the aerosol particles due to drizzle/snowfall is an effective sink mechanism in the Arctic environment. The aerosol particle concentration starts decreasing within a few minutes from the start of these events but requires a few hours to restore to the normal background aerosol level after the end of event

Inadvertant modification of atmospheric electricity

Human activities of different scales of space and time cause a change in electrical state of the atmosphere or cloud electrification

Effect of relative humidity on the electrical conductivity of marine air

Measurements of the atmospheric electric conductivity made in the equatorial Indian Ocean and Arabian Sea in August and September 1991 show that the value of conductivity decreases from 2.3 × 10−14 mho m−1 in the equatorial Indian Ocean, where relative humidity of the surface air is 70-80%, to 1.1 × 10−14 mho m−1 in the region of the Somali current, where relative humidity of the surface air increases to 80-90%. the inverse relationship observed between conductivity and relative humidity, in spite of showing a large scatter, is stronger in the case of negative than positive conductivity. the sharp increase in the sizes of ions and marine aerosol particles when the relative humidity exceeds 75-80% is proposed as the cause of the observed decrease in conductivity in the region of the Somali curren