Droplet Growth Model Research Articles

The relative importance of atmospheric sulfates and nitrates in visibility reduction

Atmospheric visibility is largely determined by the concentration, size distribution and chemical composition of the ambient aerosol. In the present paper, total extinction coefficients were calculated for mixed NH 4NO 3-(NH 4) 2SO 4 aerosols of two different compositions under various humidity conditions. Pertinent thermodynamic and optical properties were measured at 25° C and incorporated into a computer model for droplet growth and Mie calculations. Log-normal particle size distributions with mass median diameters characteristic of the Los Angeles aerosol were assumed throughout the study. The results show that the extent of light extinction by aerosol particles depends on whether the aerosol is present as a multicomponent solution droplets of identical composition (internal mixture) or as a mixture of single-salt aerosols (external mixture|. The extinction coefficients calculated for externally mixed sulfate — nitrate aerosols compare remarkably well with published statistical correlations derived from the Los Angeles visibility data. In addition, it is shown that, for externally mixed sulfate-nitrate aerosols, the sulfate indeed contributes overwhelmingly to visibility reduction, a finding in agreement with general observations for atmospheric aerosols.

Water droplet growth measurements in a continuous flow parallel plate thermal diffusion cloud chamber

A continuous flow thermal diffusion cloud chamber has been used to make real-time measurements of water droplet growth on a natural aerosol. Droplet size distributions were monitored with an optical particle detector. Droplet trajectories through the chamber were calculated in order to account for variations in the environmental conditions experienced by the growing droplets. The Fukuta—Walter droplet growth model has been used to predict droplet sizes along these trajectories. The results of the measurements and the predictions are compared and found to agree reasonably well using the commonly accepted values of 0.036 for the condensation coefficient and 1.00 for the thermal accommodation coefficient.

The ice phase and the evolution of cloud droplet spectra

Abstract The effect of the introduction of the ice phase in a cloud droplet growth model is examined. The ice particles are introduced by freezing cloud droplets at rates consistent with observations of natural ice nucleus concentrations. In moderate updrafts the production of large particles is retarded owing to the glaciation of the cloud. In stronger updrafts where insufficient time is available for precipitation to form by condensation and coalescence, the production of small precipitation particles is increased.

Discussion of Mason and Jonas' model of droplet growth in cumulus clouds

Discussion of Mason and Jonas' model of droplet growth in cumulus clouds

Discussion of Mason and Jonas' model of droplet growth in cumulus clouds

Discussion of Mason and Jonas' model of droplet growth in cumulus clouds

The Stochastic Coalescence Model for Cloud Droplet Growth

Abstract The stochastic coalescence model for droplet growth in warm clouds is analyzed, with a view to clarifying the theoretical foundations and significance of the well-known stochastic coalescence equation. It is suggested that the analysis of the model is most logically carried out in terms of a function P (n, m; t which is defined as the probability that the number of cloud droplets consisting of m molecules at time t will be n. A time-evolution equation for P (n, m; t is derived, and under certain stated assumptions it is deduced that: 1) the mean value of P (n, m; t with respect to n satisfies the stochastic coalescence equation; and 2) regardless of the initial conditions, the graph of P (n, m; t vs n will approach the Poisson shape as t →∞ to with an estimable “relaxation tirne.” The implications of these results for the stochastic fluctuations in the number of cloud droplets are examined. It is found that a distinction must be made between fluctuations in droplet concentration arising from the ...

A Model for Cloud Droplet Growth by Condensation in an Inhomogeneous Medium

Abstract A new model for droplet growth by condensation is proposed which can produce a broad droplet size spectrum as well as local fluctuations in droplet number concentration and liquid water content. The model depends neither on mixing nor on the type of condensation nuclei present. It is based on small fluctuations in relative humidity coupled with droplet sedimentation. During the early stages of condensation, small local variations in relative humidity can produce significant variations in droplet sizes. The resulting non-uniformities in droplet sedimentation rate create inhomogeneities in droplet number concentration, which, in turn, cause variations in vapor pressure and differences in droplet growth rates. Numerical experiments show that the initial conditions required for this model to produce a broad droplet size spectrum are sufficiently general to be of common occurrence. Although the model does not include any turbulence effects, it is argued that the process modeled does not require the co...

Nonadiabatic Compressible Flow Of Humid AirAround Airfoils

The heat addition by the phase transition of water vapor in humid air, which changes the properties of compressible flows around airfoils, is investigated by numerical simulations. The computations are based on a recent small-disturbance approach by Rusak and Lee [11]. This approach describes the heat release by the nonequilibrium and homogeneous condensation process using the classical nucleation and droplet growth models. The nonlinear interaction between the heat transfer by the condensation, the kinetic energy of the compressible flow, and the thermal properties of the flow is explored and the similarity parameters which govern the problem are presented. The results of the calculations show good agreement with available simulations using the Euler equations. The similarity rules are used to suggest various flow cases which have a sufficiently close behavior of both the flow dynamics and the condensation properties. It is shown that nonadiabatic compressible flows of humid air may be applied to augment the aerodynamic performances of airfoils used in fluid machinery.