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- Author or Editor: Zev Levin x
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Abstract
The stochastic electrical numerical model of cloud growth and precipitation development of Scott and Levin (1975) has been refined to include a distribution of charge within each size class. Each size class is separated into three subclasses containing negative, neutral and positive charge, respectively. The results indicate that the electric field reaches values of around 4 kV cm−1 within about 1000 s and that both positive and negative charges are carried on the particles. In agreement with the previous model, most precipitation size particles carry negative charges while most smaller cloud particles carry positive charges. However, the electrification shows an enhancement in precipitation in the early stages of cloud development. The effect reverses when the field approaches its maximum value. At that point the electrical forces affect the particle interactions through their fallspeed, and the precipitation rate falls below the corresponding rate in the unelectrified case.
Abstract
The stochastic electrical numerical model of cloud growth and precipitation development of Scott and Levin (1975) has been refined to include a distribution of charge within each size class. Each size class is separated into three subclasses containing negative, neutral and positive charge, respectively. The results indicate that the electric field reaches values of around 4 kV cm−1 within about 1000 s and that both positive and negative charges are carried on the particles. In agreement with the previous model, most precipitation size particles carry negative charges while most smaller cloud particles carry positive charges. However, the electrification shows an enhancement in precipitation in the early stages of cloud development. The effect reverses when the field approaches its maximum value. At that point the electrical forces affect the particle interactions through their fallspeed, and the precipitation rate falls below the corresponding rate in the unelectrified case.
Abstract
The charges carried on failing raindrops were measured simultaneously with the charges separated by splashing on solid metal surfaces. It was found that the ejected fragments carry predominantly negative charges leaving the solid surface positively charged. This agreed well with previous results from laboratory experiments, although the magnitude of the charges separated by natural raindrops was found to be smaller than those separated by freshly prepared water samples. The application of these results to the space charge near the ground during rainfall and to the electrification of thunderclouds are discussed.
Abstract
The charges carried on failing raindrops were measured simultaneously with the charges separated by splashing on solid metal surfaces. It was found that the ejected fragments carry predominantly negative charges leaving the solid surface positively charged. This agreed well with previous results from laboratory experiments, although the magnitude of the charges separated by natural raindrops was found to be smaller than those separated by freshly prepared water samples. The application of these results to the space charge near the ground during rainfall and to the electrification of thunderclouds are discussed.
Abstract
A time-dependent numerical model is used to simulate the growth of the electric field in thunderclouds by the polarization mechanism, including both the growth of hydrometeors and the growth of the electric charge centers. The results demonstrate a direct coupling between the hydrometeor growth and the electric field. Different types of cloud are discussed with reference to their electrical behavior.
It is found that clouds containing large ice particles and small supercooled water drops and fully glaciated clouds can produce electric fields sufficient for lightning to occur. Electrical forces in the clouds tend to slow down the relative fall velocities of the precipitation particles, and reduce their interaction rate. The net effect is a slowing down of the growth of the hydrometeors and the rate of buildup of the electric field.
Abstract
A time-dependent numerical model is used to simulate the growth of the electric field in thunderclouds by the polarization mechanism, including both the growth of hydrometeors and the growth of the electric charge centers. The results demonstrate a direct coupling between the hydrometeor growth and the electric field. Different types of cloud are discussed with reference to their electrical behavior.
It is found that clouds containing large ice particles and small supercooled water drops and fully glaciated clouds can produce electric fields sufficient for lightning to occur. Electrical forces in the clouds tend to slow down the relative fall velocities of the precipitation particles, and reduce their interaction rate. The net effect is a slowing down of the growth of the hydrometeors and the rate of buildup of the electric field.
Abstract
Measurements of rain drop size spectra in Israel were carried out over a period of two years. It is shown that the size distribution can be best described by a lognormal distribution. With its parameters weighted by a certain choice of moments, this distribution has a better squared-error fit to the observed data than the gamma or the exponential distributions. Furthermore, this distribution is well suited for explaining drop size distribution effects in the dual-parameter remote measurement of rainfall. The lognormal distribution has the advantage that all its moments are also lognormally distributed. Its parameters, in their form presented here, have physical meaning (NT =drop concentration, Dg =the geometric mean diameter, and σ=standard geometric deviation). This facilitates direct interpretation of variations in the drop size spectrum. The different moments can easily be integrated to obtain simple expressions for the various rainfall parameters. The observed values of Dg and NT are found to depend more strongly than σ on rainfall rate (R). At high R (>45 mm h−1) the distribution tends to a steady state form (Dg and σ constant). These results suggest that the lognormal representation is suitable for a broad range of applications and can facilitate interpretation of the physical processes which control the shaping of the distribution.
Abstract
Measurements of rain drop size spectra in Israel were carried out over a period of two years. It is shown that the size distribution can be best described by a lognormal distribution. With its parameters weighted by a certain choice of moments, this distribution has a better squared-error fit to the observed data than the gamma or the exponential distributions. Furthermore, this distribution is well suited for explaining drop size distribution effects in the dual-parameter remote measurement of rainfall. The lognormal distribution has the advantage that all its moments are also lognormally distributed. Its parameters, in their form presented here, have physical meaning (NT =drop concentration, Dg =the geometric mean diameter, and σ=standard geometric deviation). This facilitates direct interpretation of variations in the drop size spectrum. The different moments can easily be integrated to obtain simple expressions for the various rainfall parameters. The observed values of Dg and NT are found to depend more strongly than σ on rainfall rate (R). At high R (>45 mm h−1) the distribution tends to a steady state form (Dg and σ constant). These results suggest that the lognormal representation is suitable for a broad range of applications and can facilitate interpretation of the physical processes which control the shaping of the distribution.
Abstract
Use of the lognormal form of raindrop size distributions in simulations of differential reflectivity (ZDR ) measurements is investigated. Using two remotely measured variables and an empirical relation, the three parameters of the lognormal distribution can be deduced and the spectrum integrated to obtain rain rate. This is demonstrated by a simulation of the ZDR method using ground-based drop size distributions. Drop axis ratio and sampling time effects are also investigated and results compared to those obtained using a gamma distribution. It is shown that the lognormal representation is easily adaptable for use in the ZDR method. Using our dataset, we show that the lognormal size distribution provides lower average absolute deviations of theoretically determined rain rates from actual ones (10.7%) than those obtained using either the exponential (41.0%) or gamma distributions (11.8%).
Abstract
Use of the lognormal form of raindrop size distributions in simulations of differential reflectivity (ZDR ) measurements is investigated. Using two remotely measured variables and an empirical relation, the three parameters of the lognormal distribution can be deduced and the spectrum integrated to obtain rain rate. This is demonstrated by a simulation of the ZDR method using ground-based drop size distributions. Drop axis ratio and sampling time effects are also investigated and results compared to those obtained using a gamma distribution. It is shown that the lognormal representation is easily adaptable for use in the ZDR method. Using our dataset, we show that the lognormal size distribution provides lower average absolute deviations of theoretically determined rain rates from actual ones (10.7%) than those obtained using either the exponential (41.0%) or gamma distributions (11.8%).
Abstract
A stochastic numerical cloud model is used to investigate simultaneously growth of precipitation, the formation of electrical charges on the particles, and the development of the ambient electric field utilizing the polarization charging mechanism. The results indicate a close coupling between precipitation growth and electrification. Precipitation is reduced when the electric field reaches magnitudes of kilovolts per centimeter. The distributions of charge on the particles show charges of a realistic magnitude. Simple restraints on the coalescence efficiency based on electric charge show that, indeed particle charges can have a profound effect on rain development through coalescence. The overall results qualitatively agree with the results from the continuous collection model of Ziv and Levin, i.e., the partial levitation of the particles due to electrical forces and the termination of electric field growth can occur at electric field strengths large enough for lightning.
Abstract
A stochastic numerical cloud model is used to investigate simultaneously growth of precipitation, the formation of electrical charges on the particles, and the development of the ambient electric field utilizing the polarization charging mechanism. The results indicate a close coupling between precipitation growth and electrification. Precipitation is reduced when the electric field reaches magnitudes of kilovolts per centimeter. The distributions of charge on the particles show charges of a realistic magnitude. Simple restraints on the coalescence efficiency based on electric charge show that, indeed particle charges can have a profound effect on rain development through coalescence. The overall results qualitatively agree with the results from the continuous collection model of Ziv and Levin, i.e., the partial levitation of the particles due to electrical forces and the termination of electric field growth can occur at electric field strengths large enough for lightning.
Abstract
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Abstract
Droplets freely suspended in the air stream of a wind tunnel were nucleated with dedicated bacterial cells in either the contact or immersion mode. Immersion freezing seemed to give a noncontinuous frequency distribution of freezing with temperature whereas the corresponding curve for contact was monotonic. Although the latter nucleation mode was more efficient by ∼2°C, the temperature ranges over which droplets froze by either mode of nucleation were closer to 0°C than those so far published for nonbiogenic ice nuclei of natural origin.
Abstract
Droplets freely suspended in the air stream of a wind tunnel were nucleated with dedicated bacterial cells in either the contact or immersion mode. Immersion freezing seemed to give a noncontinuous frequency distribution of freezing with temperature whereas the corresponding curve for contact was monotonic. Although the latter nucleation mode was more efficient by ∼2°C, the temperature ranges over which droplets froze by either mode of nucleation were closer to 0°C than those so far published for nonbiogenic ice nuclei of natural origin.
Abstract
Charge separation which occurs when polarized ice particles collide in a potential gradient has been found to be an extremely important charge generating mechanism. The fair weather potential gradient is sufficient to initiate considerable charge separation (3 × 10−5 esu per collision). Then positive feedback effects inherent in this polarization charging mechanism can readily explain the strong charging found in glaciated clouds or thunderclouds in general. This theoretical prediction is well corroborated by the present experimental results obtained during simulated experiments in the field with potential gradients <5000 V m−1. However, higher potential gradients produced even more charge than predicted by theory. Also shown are distributions of the original charges carried by the ice particles, the charges transferred to the ice sphere, and the charges carried off after separation. The distributions also support the theory of polarization charging which predicts charging in proportion to the square of the ice particle radius.
Abstract
Charge separation which occurs when polarized ice particles collide in a potential gradient has been found to be an extremely important charge generating mechanism. The fair weather potential gradient is sufficient to initiate considerable charge separation (3 × 10−5 esu per collision). Then positive feedback effects inherent in this polarization charging mechanism can readily explain the strong charging found in glaciated clouds or thunderclouds in general. This theoretical prediction is well corroborated by the present experimental results obtained during simulated experiments in the field with potential gradients <5000 V m−1. However, higher potential gradients produced even more charge than predicted by theory. Also shown are distributions of the original charges carried by the ice particles, the charges transferred to the ice sphere, and the charges carried off after separation. The distributions also support the theory of polarization charging which predicts charging in proportion to the square of the ice particle radius.
Abstract
A hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops. ice crystals, graupel, and snow particles. The size distribution function for each type of particle is divided into 34 spectral bins. In each spectral category two physical moments of the distribution function (number and mass concentrations are independently calculated using the method of moments. The following physical processes are computed: nucleation of drops and ice crystals, freezing of drops, diffusional growth/evaporation of drops and ice particles, collisional coalescence of drops and ice particles, binary breakup of drops, melting of ice particles, and sedimentation. The model describes the different stages of cloud development, the formation of ice, its growth by deposition and riming, the formation of graupel, and the precipitation stage. Analysis of the distribution functions for the different species provides insight into the different microphysical processes active in rain formation in mixed clouds. As an illustration of the capability of the model, the simulation of a mixed-phase continental cloud is presented.
Abstract
A hydrodynamic nonhydrostatic anelastic numerical model of an axisymmetric convective cloud is described in which the microphysical processes are treated in detail for different species of hydrometeors: drops. ice crystals, graupel, and snow particles. The size distribution function for each type of particle is divided into 34 spectral bins. In each spectral category two physical moments of the distribution function (number and mass concentrations are independently calculated using the method of moments. The following physical processes are computed: nucleation of drops and ice crystals, freezing of drops, diffusional growth/evaporation of drops and ice particles, collisional coalescence of drops and ice particles, binary breakup of drops, melting of ice particles, and sedimentation. The model describes the different stages of cloud development, the formation of ice, its growth by deposition and riming, the formation of graupel, and the precipitation stage. Analysis of the distribution functions for the different species provides insight into the different microphysical processes active in rain formation in mixed clouds. As an illustration of the capability of the model, the simulation of a mixed-phase continental cloud is presented.
