Zev Levin

The Effects of Desert Particles Coated with Sulfate on Rain Formation in the Eastern Mediterranean

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 Isr...

The Lognormal Fit to Raindrop Spectra from Frontal Convective Clouds in Israel

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 d...

Modification of mineral dust particles by cloud processing and subsequent effects on drop size distributions

Dust particles coated with soluble materials, such as sulfate, are frequently observed in the Mediterranean. Thus far, the processes responsible for the sulfate coating of dust particles have still not been identified. One possible explanation is that the formation of the sulfate-coated aerosols is related to cloud processing of dust particles. In this process the scavenging of aerosol particles and gases, such as SO2, O3, and H2O2, by the droplets and the subsequent impaction scavenging of mineral dust particles followed by evaporation could release into the atmosphere dust particles coated with soluble materials. These modified particles can serve as giant cloud condensation nuclei and thus can have a significant impact on the microphysical development of other clouds. Using an air parcel model with detailed microphysics, it is shown that cloud processing of dust particles is a possible effective pathway to form soluble coatings on these particles. Furthermore, the simulations show that after one or two cycles of particles through convective clouds the contribution of gas uptake by drops and subsequent liquid phase oxidation add considerable mass of soluble material to particles in the size range of 0.05 μm. On the other hand, this process adds about 1 order of magnitude less mass to the larger particles as compared to the contribution made by coagulation of drops containing soluble aerosols.

An Efficient Numerical Solution to the Stochastic Collection Equation

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 tim...

The effects of giant cloud condensation nuclei on the development of precipitation in convective clouds — a numerical study

Numerical experiments are conducted to investigate the effects of giant cloud condensation nuclei (CCN) on the development of precipitation in mixed-phase convective clouds. The results show that the strongest effects of introducing giant CCN occur when the background concentration of small nuclei is high, as that in continental clouds. Under these conditions, the coalescence between water drops is enhanced due to the inclusion of giant CCN, resulting in an early development of large drops at the lower parts of the clouds. It also leads to the formation of larger graupel particles and to more intensive radar reflectivities. When the background concentration of small nuclei is low, as in maritime clouds, the effect of the giant CCN is smaller and the development of precipitation is dominated by the droplets formed on large nuclei.

The effects of aerosols on precipitation and dimensions of subtropical clouds: a sensitivity study using a numerical cloud model

Numerical experiments were carried out using the Tel-Aviv University 2-D cloud model to investigate the effects of increased concentrations of Cloud Condensation Nuclei (CCN), giant CCN (GCCN) and Ice Nuclei (IN) on the development of precipitation and cloud structure in mixedphase sub-tropical convective clouds. In order to differentiate between the contribution of the aerosols and the meteorology, all simulations were conducted with the same meteorological conditions. The results show that under the same meteorological conditions, polluted clouds (with high CCN concentrations) produce less precipitation than clean clouds (with low CCN concentrations), the initiation of precipitation is delayed and the lifetimes of the clouds are longer. GCCN enhance the total precipitation on the ground in polluted clouds but they have no noticeable effect on cleaner clouds. The increased rainfall due to GCCN is mainly a result of the increased graupel mass in the cloud, but it only partially offsets the decrease in rainfall due to pollution (increased CCN). The addition of more effective IN, such as mineral dust particles, reduces the total amount of precipitation on the ground. This reduction is more pronounced in clean clouds than in polluted ones. Polluted clouds reach higher altitudes and are wider than clean clouds and both produce wider clouds (anvils) when more IN are introduced. Since under the same vertical sounding the polluted clouds produce less rain, more water vapor is left aloft after the rain stops. In our simulations about 3.5 times more water evaporates after the rain stops from the polluted cloud as compared to the clean cloud. The implication is that much more water vapor is transported from lower levels to the mid troposphere under polluted conditions, something that should be considered in climate models. Correspondence to: A. Teller (amit@storm.tau.ac.il)

Rain Production in Convective Clouds As Simulated in an Axisymmetric Model with Detailed Microphysics. Part I: Description of the Model

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 ...

Resurgence in Ice Nuclei Measurement Research

Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ic...

Desert aerosol transport in the Mediterranean region as inferred from the TOMS aerosol index

[1] We proposed to identify the sources of desert dust aerosols with local maxima of the TOMS aerosol index distribution averaged for the long period. Being simpler than the approach based on a dusty days occurrence, our method gives the same results. It was first shown that in spring-summer, the flux of dust from the sources located at latitude � 16� N and longitude � 16� E and around latitude � 19� N and longitude � 6� W exceed the sinks due to settling and transport. As a result the atmosphere over North Africa is almost permanently loaded with a significant amount of mineral desert dust in spring and in summer. It is also shown that the Chad basin source located around latitude 16� N and longitude 16� E is relatively more stable with a maximum activity around April. The region around latitude 19� N and longitude 6� Wappears as a more variable source with maximum in July. Low pressure systems, called Sharav cyclones, mobilize the already suspended mineral dust and transport it eastward and northward along the Mediterranean basin. A new method for description of dust plumes propagation was applied to the study of dust events in the Mediterranean Sea and enabled us to follow their dynamics. Identifiable dust plumes appear first in the western sector of the sea and then move eastward with a speed of about 7 to 8 degrees per day. In spring, this motion continues at least up to the eastern coast of the Mediterranean. In summer the dustplume is prevented from penetrating further east of about 15� E. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: Aerosols, desert dust, dust storms, dust sources, Mediterranean Citation: Israelevich, P. L., Z. Levin, J. H. Joseph, and E. Ganor, Desert aerosol transport in the Mediterranean region as inferred from the TOMS aerosol index, J. Geophys. Res., 107(D21), 4572, doi:10.1029/2001JD002011, 2002.

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

Abstract The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes between 0 and 400 km. Ionization of methane and nitrogen due to galactic cosmic rays (GCR) is important at night where these ions are converted to ion clusters such as CH + 5 CH 4 , C 7 H + 7 , C 4 H + 7 , and H 4 C 7 N + . The ubiquitous aerosols observed also play an important role in determining the charge distribution in the atmosphere. Because polycyclic aromatic hydrocarbons (PAHs) are expected in Titans atmosphere and have been observed in the laboratory and found to be electrophilic, we consider the formation of negative ions. During the night, the very smallest molecular complexes accept free electrons to form negative ions. This results in a large reduction of the electron abundance both in the region between 150 and 350 km over that predicted when such aerosols are not considered. During the day time, ionization by photoemission from aerosols irradiated by solar ultraviolet (UV) radiation overwhelms the GCR-produced ionization. The presence of hydrocarbon and nitrile minor constituents substantially reduces the UV flux in the wavelength band from the cutoff of CH 4 at 155 to 200 nm. These aerosols have such a low ionization potential that the bulk of the solar radiation at longer wavelengths is energetic enough to produce a photoionization rate sufficient to create an ionosphere even without galactic cosmic ray (GCR) bombardment. At altitudes below 60 km, the electron and positive ion abundances are influenced by the three-body recombination of ions and electrons. The addition of this reaction significantly reduces the predicted electron abundance over that previously predicted. Our calculations for the dayside show that the peaks of the charge distributions move to larger values as the altitude increases. This variation is the result of the increased UV flux present at the highest altitudes. Clearly, the situation is quite different than that for the night where the peak of the distribution for a particular size is nearly constant with altitude when negative ions are not present. The presence of very small aerosol particles (embryos) may cause the peak of the distribution to decrease from about 8 negative charges to as little as one negative charge or even zero charge. This dependence on altitude will require models of the aerosol formation to change their algorithms to better represent the effect of charged aerosols as a function of altitude. In particular, the charge state will be much higher than previously predicted and it will not be constant with altitude during the day time. Charging of aerosol particles, whether on the dayside or nightside, has a major influence on both the electron abundance and electrical conductivity. The predicted conductivities are within the measurement range of the HASI PWA instrument over most but not all, of the altitude range sampled.