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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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Graham Feingold
and
Zev Levin

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.

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Graham Feingold
and
Zev Levin

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%).

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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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Orit Altaratz
,
Zev Levin
, and
Yoav Yair

Abstract

A study of the morphology and evolution of winter thunderstorms in Israel and over the eastern Mediterranean was conducted during the 1995–96 winter season. Electrically active cells were analyzed by combining data from weather radar and an operational lightning positioning and tracking system. This enabled the identification of reflectivity features of electrically active cells, and tracing of the spatial and temporal evolution of thunderstorms. The results show that, in winter, rain clouds became thunderclouds if their echo top was higher than 6500 m (at a temperature level colder than −30°C), provided that the reflectivity at the level of the −10°C isotherm was larger than 35 dBZ. The period between the first radar echo and the first detected lightning flash (probably a ground flash) was found to be 10–15 min, a period at which the top of the 40-dBZ echo was located higher than the −8°C level.

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Shalva Tzivion
,
Tamir Reisin
, and
Zev Levin

Abstract

An axisymmetrical nonhydrostatic convective cloud mode with detailed treatment of warm cloud microphysics is presented. Ale microphysical processes considered aere nucleation on cloud condensation nuclei, condensation/evaporation, collisional coalescene/breakup (Low and List kernel), and sedimentation. An accurate multi-moment treatment is implemented in the calculations of the microphysical processes. The results indicate that the collisional breakup process is very important in warm clouds and inhibits the growth of drops to large sizes where spontaneous breakup is significant. This diminishes the importance of the Langmuir chain-reaction mechanism for rain formation. The effect of salt seeding are examined for three different cases: one maritime case and two continental cloud cases. No significant effect followed the injection of up to half a ton of salt particles for the maritime cue, while the effect was very significant for the continental clouds. The sensitivity to various seeding parameters was also investigated, including size of seeding particles, quantity of seeding material, timing and duration of seeding and location of seeding The size of the seeded particles and the timing of seeding were found to be crucial parameters. Premature seeding could have a negative effect. Up to 71% increase in total rainfall was obtained under optimal seeding conditions.

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Tamir Reisin
,
Shalva Tzivion
, and
Zev Levin

Abstract

Simulations of seeding clouds for rain enhancement with ice nuclei (IN) or hygroscopic particles were conducted using a numerical model of an axisymmetric convective cloud with detailed treatment of both warm and cold microphysical processes. The simulations were performed for three clouds that differed in their cloud condensation nuclei (CCN) concentrations and spectra. Tests were carried out on clouds characterized as maritime (100 CCN cm−3), moderate continental (600 CCN cm−3), and extreme continental (1100 CCN cm−3) using two different initial conditions in which cloud tops reached −20° and −12°C.

The seeding time was found to be a critical parameter for obtaining positive results. The optimal “time window” for IN seeding was found to be very short and to correspond to the time at which the natural ice began to form. Seeding after this time reduced the rain. The optimal concentration of seeding material was about 75–125 L−1. In the maritime clouds rain formation processes were very efficient, and seeding did not produce any significant increase in rain amounts. In the moderate and extreme continental clouds with tops at −20°C, seeding with IN at the optimal time and location increased the precipitation by 9% and 35%, respectively. Ice nuclei seeding of a warmer cloud with a top temperature of −12°C did not change the rainfall when seeding took place in the optimal time window.

Seeding with hygroscopic particles had a dramatic effect on the rainfall. In the moderate and extreme continental clouds increases of 65% and 109% in rain amounts were obtained. In these cases, the optimal time window was longer, and even clouds with tops at −12°C doubled their rain amounts.

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Zev Levin
,
Eliezer Ganor
, and
Victor Gladstein

Abstract

Measurements of aerosol composition in the eastern Mediterranean reveal that sulfate is found in most aerosol particles. Some of the large particles contain mixtures of chemicals such as sulfate and sea salt. The most striking observation is the discovery that mineral dust particles often get coated with sulfate and other soluble materials. The amount of soluble material on these particles is found to be related to their surface area, suggesting that the deposition process could be surface dependent. The mechanism by which sulfate is found on some of the mineral dust particles is believed to originate from evaporating cloud drops, which were originally nucleated on sulfate cloud condensation nuclei (CCN) and subsequently collected dry interstitial mineral dust particles. The presence of soluble material on mineral dust particles, converts the latter into effective giant CCN. This is further corroborated by the fact that the few large drops near the bases of convective clouds near the coast of Israel sometimes contain both dust and sulfate. Calculations show that the presence of such large CCN could be instrumental in producing large drops (20–40 µ m), which would accelerate precipitation development through drop growth by collection. Ice crystal concentration in these types of clouds was found to be much higher than expected based on previously reported measurements of ice nuclei. These high concentrations are believed to be produced by one or more of the ice multiplication processes such as the Hallett–Messop mechanism and/or the enhanced nucleation of ice under supersaturation conditions. Both ice multiplication mechanisms are possible when large drops are formed. These findings paint out that many of the rain clouds in this region have mixed characteristics (between maritime and continental).

Since cloud seeding with ice nuclei attempts to increase ice crystal content in the clouds in order to enhance rain, the use of such seeding techniques in these clouds seems to be fruitless since any additional ice would not enhance rain or may even reduce it.

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Morris Neiburger
,
Zev Levin
, and
Lawrence Rodriguez

Abstract

A method of measuring drop size distribution in clouds, especially adapted for wind tunnels and other cloud chambers in which the cloud is carried in an air stream, is presented. It consists of photographing the cloud illuminated both by a continuous source and a stroboscopic light. The fall velocities of the drops determined from the photographs are used with tables of terminal velocities to give the size distributions. The advantages of the method are discussed.

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