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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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Raymond A. Shaw
,
Dennis Lamb
, and
Alfred M. Moyle

Abstract

A laboratory system composed of an electrodynamic levitation cell within an environmental control chamber has been designed and built. The system is ideal for studies of individual particles, such as pure water droplets, aqueous solution droplets, solid salt particles, and ice crystals, under mid- and upper-tropospheric conditions.

The experimental system has several features that make it particularly useful for studies of cloud physics. The levitation cell has a cubic geometry with transparent electrodes, thus allowing for full, three-axis positioning of a levitated particle, as well as a large range of viewing angles for optical access and light-scattering measurements. Particles in the approximate diameter range of 10 to 100 μm can be suspended indefinitely with minimal wall influences. The levitation cell is housed within an environmental control chamber capable of operating at temperatures (T), pressures (p), vertical velocities (w), and saturation ratios (with respect to ice, S i ) in the ranges −70 ⩽ T ⩽ −20°C, 200 ⩽ p ⩽ 1000 hPa, 0 ⩽ w ⩽ 0.2 m s−1, and 0 ⩽ S i ⩽ 1. The design allows for a continuous flow of gas vertically through the levitation cell during experiments, thereby maintaining a constant and well-characterized environment around the levitated particle. A grid-injection technique, whereby two flows with different trace gas (including water vapor) concentrations are mixed upstream at small spatial scales, allows for independent and rapid control of trace gas concentration near the levitated particle. Finally, for liquid droplets, particle size is continuously monitored by measuring scattered laser light from the particle. The light scattering measurements also allow droplet freezing to be clearly observed.

The system has been used for studies of homogeneous freezing nucleation of liquid water and surface kinetic properties on water droplets. The nucleation data are well described by the standard statistical description of homogeneous nucleation and are consistent with previously reported measurements. Droplet evaporation data obtained at low pressures illustrate the utility of the system in studying mass and energy transfer in the transition regime. The evaporation data derived from this system are consistent with a condensation coefficient of 0.06 if the thermal accommodation coefficient is assumed to be unity.

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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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Jielun Sun
,
Steven P. Oncley
,
Sean P. Burns
,
Britton B. Stephens
,
Donald H. Lenschow
,
Teresa Campos
,
Russell K. Monson
,
David S. Schimel
,
William J. Sacks
,
Stephan F. J. De Wekker
,
Chun-Ta Lai
,
Brian Lamb
,
Dennis Ojima
,
Patrick Z. Ellsworth
,
Leonel S. L. Sternberg
,
Sharon Zhong
,
Craig Clements
,
David J. P. Moore
,
Dean E. Anderson
,
Andrew S. Watt
,
Jia Hu
,
Mark Tschudi
,
Steven Aulenbach
,
Eugene Allwine
, and
Teresa Coons

A significant fraction of Earth consists of mountainous terrain. However, the question of how to monitor the surface–atmosphere carbon exchange over complex terrain has not been fully explored. This article reports on studies by a team of investigators from U.S. universities and research institutes who carried out a multiscale and multidisciplinary field and modeling investigation of the CO2 exchange between ecosystems and the atmosphere and of CO2 transport over complex mountainous terrain in the Rocky Mountain region of Colorado. The goals of the field campaign, which included ground and airborne in situ and remote-sensing measurements, were to characterize unique features of the local CO2 exchange and to find effective methods to measure regional ecosystem–atmosphere CO2 exchange over complex terrain. The modeling effort included atmospheric and ecological numerical modeling and data assimilation to investigate regional CO2 transport and biological processes involved in ecosystem–atmosphere carbon exchange. In this report, we document our approaches, demonstrate some preliminary results, and discuss principal patterns and conclusions concerning ecosystem–atmosphere carbon exchange over complex terrain and its relation to past studies that have considered these processes over much simpler terrain.

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