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Zev Levin

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.

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Zev Levin

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.

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Ami Ziv
and
Zev Levin

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.

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Tamir Reisin
,
Zev Levin
, and
Shalva Tzivion

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.

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William D. Scott
and
Zev Levin

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.

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W. D. Scott
and
Zev Levin

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.

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Tamir Reisin
,
Zev Levin
, and
Shalva Tzivion

Abstract

This paper presents an evaluation of the relative importance of the warm versus cold processes in convective clouds and their relative contribution to the development of rain. For this purpose, an axisymmetrical model of a cold convective cloud with detailed microphysics is used.

Five different types of clouds having characteristics from maritime to extreme continental are simulated. Identical initial conditions are used, leading to the formation of convective clouds of medium depth, with relatively strong updrafts. For these specific conditions, the effects of the different microphysical processes on the production of rain are tested by varying the cloud condensation nuclei (CCN) spectra and the spectra of the nucleated drops. The role of ice crystal concentrations and drop freezing is also reviewed.

The simulations showed that maritime clouds are efficient rain producers. In these clouds, large graupel mass contents develop by the freezing of large drops through their interaction with ice crystals. Rain efficiency decreases with increasing CCN concentration (or with the “continentality” of the clouds). For the same dynamics and liquid water content maritime clouds produce more rain with higher intensifies than continental clouds.

Reducing the ice nuclei concentrations generally produces less rain, especially near the cloud center. In moderate continental clouds, changing the concentration of ice crystals by a few orders of magnitude results in a change in the spatial distribution of the rain but only a small change in the total amount of precipitation.

Self-freezing of drops plays only a minor role in rain production because freezing due to interactions of supercooled drops with ice crystals takes precedent. In the simulated clouds snow is inefficiently produced, especially in maritime ones.

The Bergeron–Findeisen mechanism plays only a minor role in the depletion of supercooled water during the developing and mature stages of the cloud because of the presence of very low ice crystal concentrations as compared to that of the drops. During the dissipation stage of the clouds, however, the Bergeron–Findeisen mechanism helps to accelerate the glaciation.

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Zev Levin
,
Joachim H. Joseph
, and
Yuri Mekler

Abstract

Simultaneous measurements of optical depth and Size distribution in a dust storm are presented. The measured and derived properties of the aerosol are compared with each other and with other results published in the scientific literature. We observe some global commonality in the measured size spectra of desert aerosols especially for post-frontal conditions. On the other hand, during the passage of the front itself, high aerosol concentrations with a sharp peak in radius at −1 μm were observed. Generally, these were not similar to other size distributions reported in the literature.

The imaginary part of the refractive index in the spectral region 0.3-1.7 μm was found to be similar to that found in other deserts. Comparison of the optical measurements with the direct sampling data suggests that the general time trends of the size distributions, as measured in situ, are followed by the optical depth and its variations with wavelength. On the other hand, detailed short-term fluctuations detected by our direct measurements are not followed by the optical method. We have observed that a simple power law for the size distribution, in the range r>0.15μm is a reasonable approximation only during clear and calm conditions with small optical depth. During the dust storm itself, the deviations from a power law are lame as shown by both direct in situ and optical observations.

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Shalva Tzivion (Tzitzvashvili)
,
Graham Feingold
, and
Zev Levin

Abstract

The evolution of raindrop spectra with altitude through collisional collection/breakup sedimentation and evaporation is presented. Two-moment treatment of sedimentation and evaporation is developed to complement Part I (Feingold et al.) of this series. We have obtained an accurate, stable numerical scheme for evaporation that enables the investigation of the effect of evaporation on spectra subject to entrainment of strongly subsaturated air (including ventilation). The method includes provision for treatment of the variation of the sub/supersaturation within a time step in a dynamical framework. Results confirm that steady-state raindrop spectra are characterized by a bimodal or trimodal structure that becomes evident shortly after evolution commences. After sufficient evolution, peaks become clearly defined at 0.25 mm and 0.8 mm and further evolution with altitude affects only the relative magnitude of these peaks. It is shown that the evaporation process is not only dependent on the subsaturation of ambient air but is also strongly dependent on the shape of the drop spectrum. Evaporation tends to increase the number of the smallest raindrops (≤ 0.1 mm) at the expense of the larger drops but does not modify the position of the peaks. The effect of drop spectral evolution on radar reflectivity (Z) and scavenging (Λ) profiles is studied.

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Graham Feingold
,
Zev Levin
, and
Shalva Tzivion (Tzitzvashvili)

Abstract

The evolution of raindrop spectra below cloud base in subsaturated atmospheres is traced with the aid of an axisymmetrical rainshaft model which includes the detailed warm microphysical treatment presented in parts I and II of this series. As input to the model, a stationary cloud provides rainfall with a predetermined drop spectrum. Mass loading and evaporative cooling generate downdrafts below cloud base. For near-adiabatic lapse rates and moderate mass loading, microbursts develop. For a given liquid water content, the magnitude of these downdrafts depends primarily on the lapse rate of temperature, but also on the drop spectrum injected at cloud base. For a given liquid water content, spectra comprising a relatively large number of small drops tend to generate significantly stronger downdrafts than spectra with a greater component of large drops. It is shown that drop collection and breakup may also affect the magnitude of the generated downdrafts significantly. When spectra comprising mainly small drops evolve to create larger drops, or when spectra comprising mainly large drops evolve to create smaller drops, neglect of collection and breakup can modify the downdrafts by up to about 50%. It is shown that in a steady state situation the drop spectra evolve toward bi- or trimodal spectra as predicted by simple rainshaft models with fixed dynamics.

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