Search Results

You are looking at 1 - 7 of 7 items for :

  • Author or Editor: Zev Levin x
  • Journal of Applied Meteorology and Climatology x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
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.

Full access
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.

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
,
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.

Full access