Search Results

You are looking at 11 - 20 of 30 items for

  • Author or Editor: Zev Levin x
  • Refine by Access: All Content x
Clear All Modify Search
Zev Levin
and
Saul A. Yankofsky

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.

Full access
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.

Full access
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.

Full access
W. D. Scott
and
Zev Levin

Abstract

No abstract available.

Full access
Zev Levin
,
Morris Neiburger
, and
Lawrence Rodriguez

Abstract

Collection efficiencies of drops of radius A less than 120 µm falling through clouds of drop radius a for which the size ratio p=a/A was between 0.18 and 0.23 were found experimentally to be considerably lower than the corresponding theoretically computed collision efficiency. Assuming the difference to be due to failure of some of the colliding drops to coalesce, the coalescence efficiency was computed and found to decrease with increasing p, in qualitative agreement with the results of Whelpdale and List obtained for much larger collector drops.

Full access
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.

Full access
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.

Full access
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.

Full access
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).

Full access
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.

Full access