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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
,
Graham Feingold
,
Shalva Tzivion
, and
Albert Waldvogel

Abstract

A comparison is made between the evolution of raindrop spectra as measured at stations in the Swiss Alps separated by vertical distances of the order of 600 m, with that modeled in an axisymmetrical model including detailed microphysics. Results show that under steady rain, weak advective conditions, and rain rates greater than 2 mm h−1, the model satisfactorily reproduces the features of the observed drop spectrum. Results deteriorate for low rain rates (of the order of 1 mm h−1) since drop collisions are too few to modify the spectrum significantly. The general agreement between modeled and observed spectra suggests that further considerations of this kind are justified.

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Yan Yin
,
Zev Levin
,
Tamir Reisin
, and
Shalva Tzivion

Abstract

Numerical experiments were conducted to evaluate the role of hygroscopic flare seeding on enhancement of precipitation in convective clouds. The spectra of seeding particles were based on measurements of the particles produced by hygroscopic flares used in field experiments in South Africa. The seeding effects were investigated by comparing the development of precipitation particles and rain production between the seeded and unseeded cases for clouds with different cloud condensation nuclei (CCN) concentrations and spectra.

The South African hypothesis that the introduction of larger and more efficient artificial CCN below cloud base at the early stage of cloud development would influence the initial condensation process in the cloud, resulting in a broader droplet spectrum and in acceleration of the precipitation growth by coalescence, was tested. The results show that the largest seeding particles broaden the cloud droplet distribution near cloud base, leading to an earlier formation of raindrops, graupel particles, and, therefore, stronger radar echoes at a lower altitude. The results also show that the large artificial CCN prevent some of the natural CCN from becoming activated. It was found that seeding with the full particle spectrum from the flares could increase rainfall amount in continental clouds having CCN concentrations of more than about 500 cm−3 (active at 1% supersaturation). Seeding more maritime clouds resulted in reducing the integrated rain amount, although in some cases rain formation was accelerated. The physical mechanisms responsible for these results were explored by investigating the relative importance of different segments of the size spectrum of the seeding particles to precipitation development. It was found that, out of the full spectrum, the most effective particles were those with radii larger than 1 μm, especially those larger than 10 μm; the particles smaller than 1 μm always had a negative effect on the rain development.

The sensitivity of seeding effects to seeding time, seeding height, and seeding amounts also was tested. The biggest precipitation enhancement was obtained when seeding was conducted a few minutes after cloud initiation and above cloud base. The radar reflectivity at that time period was lower than 0 dBZ. Rain enhancement also increased with the increase in the concentration of the large seeding particles in the spectrum (at least for the amounts tested here).

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

Abstract

A new, accurate, efficient method for solving the stochastic collection equation (SCE) is proposed. The SCE is converted to a set of moment equations in categories using a new analytical form of Bleck&'s approach. The equations are written in a form amenable to solution and to a category-by-category analysis of drop formation and removal. This method is unique in that closure of the equations is achieved using an expression relating high-order moments to any two lower order moments, thereby restricting the need for approximation of the category distribution function only to integrals over incomplete categories. Moments in categories are then expressed in terms of complete moments with the aid of linear or cubic polynomials. The method is checked for the case of the constant kernel and a linear polynomial kernel. Results show that excellent approximation to the analytical solutions for these kernels are obtained. This is achieved without the use of weighting functions and with modest computing time requirements. The method conserves two or more moments of the spectrum (as required) and successfully alleviates the artificial enhancement of the collection process which is a feature of many schemes.

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Zev Levin
,
Shimon O. Krichak
, and
Tamir Reisin

Abstract

A mesoscale model RAMS (the Regional Atmospheric Modeling System) was used to investigate the effectiveness of the broadcast static seeding method for dispersing particles into clouds, as it is used in Israel. The model was run using three nested grids, with 500 m × 500 m horizontal resolution in the finest grid. In this paper, the particles were assumed to be inert; namely, only the wind field controlled the dispersal of the tracer particles, and no interaction with cloud or precipitation particles was considered. Although the resolution of the model is good for mesoscale studies, it could not resolve individual plumes. The results, therefore, present average values of the concentrations at each level. The simulations showed that seeding particles reach altitudes at which they could become effective as ice nuclei. These cases were primarily the ones in which the updrafts developed over the seeding lines when the seeding plane was just passing underneath. In these cases only, seeding at about 1-km level (∼4°C) with 500 g h−1 of inert material (simulating AgI particles) resulted in about 1 × 103–2 × 103 L−1 being lifted to the −10°C level. Based on previous laboratory studies of the seeding agent used in Israel, out of these total concentrations, only 1–2 L−1 could form ice at −10°C. The simulations also suggest that in most other cases the horizontal advection diluted the particles in the air and only very low concentrations (<10−3 L−1, active at −10°C) reached the −10°C level. Most other released particles were transported horizontally with the winds and were later on forced down by downdrafts. Although these simulations await some experimental verification, they suggest that the broadcast seeding method used in Israel is not so effective for widespread rain enhancement operations.

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Joachim P. Kuettner
,
J. Doyne Sartor
, and
Zev Levin

Abstract

Most of the precipitation related theories on charge generation in thunderstorms fall into one of two categories: the inductive or polarization mechanism initiated by the ambient fair-weather field, and the non-inductive mechanism connected with certain electrochemical or thermoelectric particle characteristics. Our numerical study addresses the question of which mechanism gives more realistic results with regard to charge distribution and hold strength and what effect a combination of the two processes produces. The investigation is a first attempt using a simplified model.

In this model the microphysical processes of particle growth and simultaneous electrification are embedded in a steady state two-dimensional vortex circulation with and without vertical wind shear. The net space charge and potential are obtained everywhere in the cloud and the resulting electric fields are calculated. Computations are made for the collisions of growing solid precipitation (graupel) particles with either supercooled droplets (ice-water case) or with ice crystals (ice-ice case).

The results indicate that the non-inductive mechanism produces a rapid growth of the electric field in the early stages but tends to level out at a stable value considerably below the breakdown field strength. The inductive mechanism in turn shows a slow initial field growth with quickly varying charge distributions of often inverted polarity; however, it will reach breakdown field strength eventually due to its quasi-exponential growth character. Only the combination of the two processes achieves realistic thunderstorm conditions. It appears that the non-inductive mechanism controls the charge distribution and its polarity, and the inductive mechanism the field strength. both ice-water and ice-ice collisions give similar results, the only difference being a higher elevation of the charge dipole. in the ice-ice case. The opposite precipitation and cloud charges are always strongly masked.

The results permit some interesting conclusions on the origin of the fair-weather field.

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