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Dennis Lamb

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Dennis Lamb

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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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Naihui Song and Dennis Lamb

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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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Dennis Lamb and Peter V. Hobbs

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Dennis Lamb and William D. Scott

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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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Jen-Ping Chen and Dennis Lamb

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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

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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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Jen-Ping Chen and Dennis Lamb

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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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Dennis Lamb and Raymond A. Shaw

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Water phase transitions are central to climate and weather. Yet it is a common experience that the principles of phase equilibrium are challenging to understand and teach. A simple mechanical analogy has been developed to demonstrate key principles of liquid evaporation and the temperature dependence of equilibrium vapor pressure. The system is composed of a circular plate with a central depression and several hundred metal balls. Mechanical agitation of the plate causes the balls to bounce and interact in much the same statistical way that molecules do in real liquid–vapor systems. The data, consisting of the number of balls escaping the central well at different forcing energies, exhibit a logarithmic dependence on the reciprocal of the applied energy (analogous to thermal energy k B T) that is similar to that given by Boltzmann statistics and the Clausius–Clapeyron equation. These results demonstrate that the enthalpy (i.e., latent heat) of evaporation is well interpreted as the potential energy difference between molecules in the vapor and liquid phases, and it is the fundamental driver of vapor pressure increase with temperature. Consideration of the uncertainties in the measurements shows that the mechanical system is described well by Poisson statistics. The system is simple enough that it can be duplicated for qualitative use in atmospheric science teaching, and an interactive animation based on the mechanical system is available online for instructional use (http://phy.mtu.edu/vpt/).

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