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Dennis Lamb

Abstract

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Naihui Song
and
Dennis Lamb

Abstract

A continuous flow cloud chamber system was constructed for studies of microphysical and chemical processes in supercooled clouds. An important feature of the cloud chamber was the generation of the components of the supercooled clouds external to the main wind tunnel where crystal growth took place. A population of ice crystals was allowed to grow to relatively large sizes in a steady-state environment with specially imposed flow gradients. Thus, microphysical and chemical processes in supercooled clouds could be simulated under realistic and controlled conditions.

The cloud chamber was utilized here to study ice crystal growth by vapor deposition over a broad range of supercooled cloud conditions. The crystal habit, size, and mass were measured for growth times up to 4 min, temperatures between −6° and −16°C, and liquid water contents from 0.3 to 6 g m−3. The data indicate that the liquid water content enhances the crystal vapor growth rates from less than 2% to almost 20% per unit increase in liquid water content (g m−3), depending on the crystal habit. The growth enhancement that arises from the presence of supercooled liquid water is explained in terms of the transient “vapor flush” effect from the repeated close passage of supercooled water droplets during crystal sedimentation.

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Naihui Song
and
Dennis Lamb

Abstract

An experimental study of aerosol scavenging by ice growing in supercooled clouds was conducted with a continuous flow cloud chamber. Techniques for detecting insoluble (latex) submicron particles in individual ice crystals were developed. The effects of microphysical parameters on the scavenging process were examined quantitatively. Measurements of the aerosol scavenging rates were documented as functions of cloud temperature, liquid water content, and the diameters (0.109 μm and 0.551 μm) of the nearly monodisperse aerosol particles. Scavenging data were acquired at temperatures of −6°, −8°, −11.5°, and −14°C. The liquid water contents of the supercooled clouds were varied from ∼0.3 to 6 g m−3, while the maximum dimensions of the ice crystals ranged from about 50 to 300 μm.

The scavenging data agree with some previously published theoretical and experimental results and expand the empirical database available for understanding the mechanisms of scavenging. It was found that the presence of liquid water reduced the aerosol removal rates, particularly for crystals growing in the habit transition region near −8°C. It is hypothesized that the retardation effect is due to enhancement of the thermophoretic forces arising from more rapid vapor deposition and latent heat release at higher liquid water contents. The scavenging efficiency at a given liquid water content, however, was not found to depend significantly on the growth habit of the ice crystal. The data, particularly regarding the dependence of the scavenging rates on liquid water content, appear to resolve an important conflict in the literature regarding the relative roles of thermophoresis and diffusiophoresis in the scavenging of submicron particles by ice crystals growing in supercooled clouds.

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Dennis Lamb
and
William D. Scott

Abstract

The formation of multiple layers of adsorbed water molecules on the basal and prism faces of ice may be responsible for the remarkable temperature dependence of all growth variables (linear growth rate, step velocity, and mean migration distance). This effect results from an increased residence time of molecules in the adsorbed state as the melting point is approached. A quantitative treatment based on the Brunauer, Emmett and Teller model of multi-layer adsorption exemplifies these concepts and appears to explain the measured trends with temperature. When the theoretical treatment is used in conjunction with a growth model based on the propagation of spiral steps, reasonable values for the condensation coefficient emerge. The alternation of the primary habit of ice crystals with temperature is explained when the theoretical treatment is applied to the basal and prism faces, respectively.

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Dennis Lamb
and
Peter V. Hobbs

Abstract

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Jen-Ping Chen
and
Dennis Lamb

Abstract

A detailed microphysical and chemical cloud model has been developed to investigate the redistribution of atmospheric trace substances through cloud processes. A multicomponent categorization scheme is used to group cloud particles into different bins according to their various properties. Cloud drops are categorized simultaneously and independently in both their water mass and solute mass components. Ice phase particles are additionally categorized according to their “shapes,” special effort having been paid to the parameterization of their growth and habit changes. The hybrid bin method used conserves the mass and number of particles while at the same time performing fast and accurate calculations for transferring various properties between categories within the multicomponent framework. With a minimum of parameterization, this model is capable of simulating detailed microphysical and chemical processes that occur during cloud and precipitation formation.

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Jen-Ping Chen
and
Dennis Lamb

Abstract

A theoretical analysis of surface kinetic and gas-phase diffusional effects permits the growth rates and habits of ice crystals to be specified in a self-consistent way. The analysis makes use of the fact that the difference between the condensation coefficients of the prism and basal faces determines the primary crystal habits, whereas the spatial variations of the vapor density contribute to the secondary habits. The parameterization scheme that results from the theoretical analysis yields a power law relationship between the a and c axial lengths that matches earlier empirical formulas derived from observational data for the temperature range of −30° to 0°C. Through application of this adaptive parameterization in a microphysical model that categorizes ice particles according to both their masses and shapes, it is shown that deviations from the power law relationship may develop if the crystals experience significant variations in the air temperature and in their inherent growth habits. A mass-dimension relationship is also derived through the theoretical analysis that can be used as a less detailed parameterization scheme for the growth of ice crystals by vapor deposition.

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Jen-Ping Chen
and
Dennis Lamb

Abstract

A detailed microphysical model is used to simulate the formation of wintertime orographic clouds in a two-dimensional domain under steady-state conditions. Mass contents and number concentrations of both liquid- and ice-phase cloud particles are calculated to be in reasonable agreement with observations. The ice particles in the cloud, as well as those precipitated to the surface, are classified into small cloud ice, planar crystals, columnar crystals, heavily rimed crystals, and crystal aggregates. Detailed examination of the results reveals that contact nucleation and rime splintering are the major ice-production mechanisms functioning in the warmer part of the cloud, whereas deposition/condensation-freezing nucleation is dominant at the upper levels. Surface precipitation, either in the form of rain or snow, develops mainly through riming and aggregation, removing over 17% of the total water vapor that entered the cloud.

The spectral distributions of cloud particles in a multicomponent framework provide information not only on particle sizes but also on their solute contents and, for ice particles, their shapes. Examination of these multicomponent distributions reveals the mechanisms of particle formation and interaction, as well as the adaptation of crystal habits to the ambient conditions. Additional simulations were done to test the sensitivity of cloud and precipitation formation to the size distribution of aerosol particles. It is found that the size distribution of aerosol particles has significant influence on not only the warm-cloud processes, but also the cold-cloud processes. A reduction in aerosol particle concentration not only causes an earlier precipitation development but also an increase in the amount of total precipitation from the orographic clouds.

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Lindsay M. Sheridan
,
Jerry Y. Harrington
,
Dennis Lamb
, and
Kara Sulia

Abstract

The relationship among aspect ratio, initial size, and the evolution of the size spectrum is explored for ice crystals growing by vapor deposition. Ice crystal evolution is modeled based on the growth of spheroids, and the ice size spectrum is predicted using a model that is Lagrangian in crystal size and aspect ratio. A dependence of crystal aspect ratio on initial size is discerned: more exaggerated shapes are shown to result when the initial crystals are small, whereas more isometric shapes are found to result from initially large crystals. This result is due to the nature of the vapor gradients in the vicinity of the crystal surface. The more rapid growth of the smaller crystals is shown to produce a period during which the size distribution narrows, followed by a broadening led by the initially smallest crystals. The degree of broadening is shown to depend strongly on the primary habit and hence temperature.

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Huiwen Xue
,
Alfred M. Moyle
,
Nathan Magee
,
Jerry Y. Harrington
, and
Dennis Lamb

Abstract

Experiments were conducted with an electrodynamic levitation system to study the kinetics of droplet evaporation under chemically rich conditions. Single solution droplets of known composition (HNO3/H2O or H2SO4/HNO3/H2O) were introduced into an environmentally controlled cubic levitation cell. The gaseous environment was set intentionally out of equilibrium with the droplet properties, thus permitting the HNO3 mass accommodation coefficient to be determined. Measurements were performed at room temperature and various pressures (200–1000 hPa). Droplet sizes (initial radii in the range 12–26 μm) were measured versus time to high precision (±0.03 μm) via Mie scattering and compared with sizes computed by different models for mass and heat transfer in the transition regime. The best agreement between the theoretical calculations and experimental results was obtained for an HNO3 mass accommodation coefficient of 0.11 ± 0.03 at atmospheric pressure, 0.17 ± 0.05 at 500 hPa, and 0.33 ± 0.08 at 200 hPa. The determination of the mass accommodation coefficient was not sensitive to the transport model used. The results show that droplet evaporation is strongly limited by HNO3 and occurs in two stages, one characterized by rapid H2O mass transfer and the other by HNO3 mass transfer. The presence of a nonvolatile solute (SO2− 4) affects the activities of the volatile components (HNO3 and H2O) and prevents complete evaporation of the solution droplets. These findings validate recent attempts to include the effects of soluble trace gases in cloud models, as long as suitable model parameters are used.

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