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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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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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Stiring A. Colgate
,
Zev Levin
, and
Albert G. Petschek

Abstract

The numerical calculations of the combined stochastic growth and induction charging due to drop interactions by Scott and Levin (1975) are analyzed in terms of a phenomenological model. The assumed initial drop size distribution which is concentrated around 20 μm radius evolves at one point in time to a two–peaked distribution at 20 and 150–200 μm, respectively. We show that when this two–peaked distribution occurs, the charging by the polarizationndash;induction mechanism is powerful enough to overcome the several charge reduction mechanisms and to make the actual charge a significant fraction (>1/3) of the saturated charge for a wide range of parameters. The saturation charge is defined as the charge carried on the particle so that no charge will be separated on the average in subsequent interactions as long as the field remains the same. Also, using the actual charge, one predicts in agreement with the numerical calculations what range of parameters permits a full 7–8 e–folds (e 7e 8) of electric field growth to take place before the small droplets are depleted.

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Lorraine A. Remer
,
Yoram J. Kaufman
,
Zev Levin
, and
Steven Ghan

Abstract

The new generation of satellite sensors such as the moderate resolution imaging spectroradiometer (MODIS) will be able to detect and characterize global aerosols with an unprecedented accuracy. The question remains whether this accuracy will be sufficient to narrow the uncertainties in estimates of aerosol radiative forcing at the top of the atmosphere. The discussion is narrowed to cloud-free direct forcing. Satellite remote sensing detects aerosol with the least amount of relative error when aerosol loading is high. Satellites are less effective when aerosol loading is low. The monthly mean results of two global aerosol transport models are used to simulate the spatial distribution of smoke aerosol in the Southern Hemisphere during the tropical biomass burning season. This spatial distribution allows us to determine that 87%–94% of the smoke aerosol forcing at the top of the atmosphere occurs in grid squares with sufficient signal-to-noise ratio to be detectable from space. The uncertainty of quantifying the smoke aerosol forcing in the Southern Hemisphere depends on the uncertainty introduced by errors in estimating the background aerosol, errors resulting from uncertainties in surface properties, and errors resulting from uncertainties in assumptions of aerosol properties. These three errors combine to give overall uncertainties of 1.2 to 2.2 W m−2 (16%–60%) in determining the Southern Hemisphere smoke aerosol forcing at the top of the atmosphere. Residual cloud contamination uncertainty is not included in these estimates. Strategies that use the satellite data to derive flux directly or use the data in conjunction with ground-based remote sensing and aerosol transport models can reduce these uncertainties.

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