Our group,in collaboration with Prof E.J. Davis (Chem. Eng.), uses the technique of electrodynamic levitation to study the evolution of microscopic ice particles found in atmospheric clouds. The apparatus consists of an electrodynamic balance with an internal thermal diffusion chamber in which, by controlling temperatures and illumination levels, we can cause the ice particles to grow or sublimate. Endcap electrodes supply the levitating field, and a stabilizing ac field is applied to ring electrodes. Images can be grabbed digitally from video telemicroscopes with a resolution of about one micrometer. We have observed growth and sublimation of ice particles at temperatures between 0 and -30 C. We have witnessed the breakup of sublimating frost particles, a process that may be related to rapid increases in ice particle concentrations observed in clouds under certain conditions. We can produce a range of crystal habits by sublimating frost particles to small seeds and then growing them under controlled conditions. This results in platelike or columnar depending on the temperature.

Growth rates can be measured both optically or through analysis of the levitating fields, and the two methods agree quite well as shown in this graph . In the future we plan to grow ice particles between -30 and -60 C (temperatures characteristic of cirrus clouds) and to compare the growth rates with theoretical models.

We have begun to gather data on the light-scattering properties of hexagonal ice particles using a He-Ne laser and photodiode array. Scattered intensities have been measured over a 50+ degree window from crystals of known orientation. We plan to compare our results with ray-tracing models

ICE NUCLEATION

We have developed a droplet free-fall tube to observe the freezing of supercooled droplets in free-fall. Our initial studies looked at nucleation rates and the microphysics of droplet deformation during the freezing of homogeneous solutions. Comparisions were made with observations of the freezing of heterogeneous solution droplets containing bionucleants.

CHARGE TRANSFER

It has been established ( Baker et al, 1987) that under certain temperature and humidity conditions collisions between uncharged ice particles, in the absence of external electric fields, result in reproducible charging of both colliding particles. The collisional charge transfer process is probably the dominant one creating in-cloud electric fields in convective thunderstorms (Link to Vicki's work. Solomon and Baker, 1994, 1998; Baker et al, 1995). Theoretical analysis of the collisional charge transfer mechanism (Baker and Dash, 1989, 1991) suggests that electrically charged fluid may be transferred from one particle to the other during the collisions. Simulations of the collisions ( Mason et al, 1996; 1998) appear to bear out this hypothesis. The simulations are continuing in a collaborative effort.

NUMERICAL MODELS

(a) Growth/sublimation model

The temperature- and humidity-dependent growth and sublimation rates and habit variations that are observed in the laboratory depend on surface kinetic processes that have not been explained theoretically. We have developed a theoretical model of the diffusional growth/sublimation of a three-dimensional ice crystal, in which the surface boundary condition specifying the relationship linking surface humidity to molecular incorporation rate can be varied. The model is based on ideas developed by Jon Nelson but the numerical method solves the exterior Neumann problem directly using a boundary integral representation of the solution on the piecewise smooth 3D ice crystal. We plan to utilize the results to analyze the laboratory data and to insert the single particle model into a cirrus cloud model.

(b) Uptake model

The uptake of environmental gases onto ice may be an important process in determining the chemical composition of the troposphere. In collaboration with T. Peter and his colleagues at the Max Planck Institut fur Chemie we have analyzed laboratory experiments of SO_2 on ice. These reveal an anomalous temperature dependence and appear to demonstrate some bulk diffusion of the gas into the ice. We suggest that the uptake mechanism involves fluid at the ice/vapor interface and in grain boundaries (Huthwelker et al, 2001). This work is part of an ongoing collaborative effort with the group in Mainz.

References

Aardahl, C.L., R. Vehring, E.J. Davis, G. Schweiger and B.D. Swanson, Characterization of the electric field and particle trapping in a double-ring electrodynamic balance, J. Aerosol Sci. Vol. 28, 1491-1505 1997.

Bacon, N.J., M.B. Baker and B.D. Swanson, Initial Stages in the Morphological Evolution of Vapor Grown Ice Crystals: A Laboratory Investigation, In press Q. J. Roy. Meteor. Soc. 2003.

Bacon, N.J. and B.D. Swanson, Laboratory Measurements of Light Scattering by Single Levitated Ice Crystals, J. Atmos. Sci. 57 2094-2107 2000.

Bacon,N.J., B.D. Swanson, M.B. Baker and E.J. Davis, Laboratory Measurements of Light Scattering by Single Ice Particles, J. Aerosol Sci. Vol. 29, S1317--S1318 1998.

Bacon, N.J., B.D. Swanson, M.B. Baker, and E.J. Davis, The Breakup of Levitated Frost Particles, J. Geophys. Res. Vol. 103, 13,763-13,775 1998.

Baker, M. B. Cloud Microphysics and Climate. Science 276 1072-1078, 1997.

Baker, M. B. Ice Particles in the Atmosphere. In Ice in the Natural and endangered Environment, NATO ASI, ed., J. Wettlaufer, 1998.

Baker, M. B. and J. G. Dash, Charge transfer in thunderstorms and the surface melting of ice, J. Cryst. Growth 97, 770-776, 1989.

Baker, M. B. and J. G. Dash, A Mechanism of Charge Transfer Between Colliding Ice Particles in Thunderstorms. J. Geophys. Res. 99, 10,621-10,626, 1994.

Baker, M., H. Christian and J. Latham. A Computational Study of the Relationships Linking Lightning Frequency and other Thundercloud Parameters. Q. J. Roy. Met. Soc. 121, 1525-1548, 1995.

Dash, J.G.; Mason, B.L.; Wettlaufer, J.S., Theory of charge and mass transfer in ice-ice collisions, J. Geophys. Res. 106 20395-402 2001.

Huthwelker, T., Th. Peter, D. Lamb, M. Baker and B. Swanson, 1996: A Theoretical Model for SO2 Uptake into Ice Particles, Proceedings, ICCP, Zurich 1996.

Huthwelker, T., D. Lamb, M. Baker, B. Swanson and Th. Peter, 2001: Uptake of SO2 by Polycrystalline Water Ice, J. Colloid and Int. Sci., 238 149-59, 2001.

Mason, B.L.; Dash, J.G., Charge and mass transfer in ice-ice collisions: experimental observations of a mechanism in thunderstorm electrification, J. Geophys. Res. 105, 10185-92 2000.

Mason, B. L., J. G. Dash, M. B. Baker and J. Wettlaufer. Charge and Mass Transfer during Ice-Ice collisions. In Ice in the Natural and Endangered Environment, NATO ASI, ed. J. Wettlaufer, 1998.

Nelson, J.T.; Baker, M.B., New theoretical framework for studies of vapor growth and sublimation of small ice crystals in the atmosphere, J. Geophys. Res. 101, 7033-47 1996.

Baker, M. and Nelson, J., A new model of charge transfer during ice-ice collisions, Atmos. Chem. and Phys. 3 and available on-line at http://www.copernicus.org/EGU/acp/acp.htm 2003.

Peter, Th. and M. Baker, Lifetimes of Small Ice Particles in the Atmosphere, in Clouds, Chemistry and Climate, eds. Crutzen and Ramanathan. 1994.

Qu, X., E.J. Davis and B.D. Swanson, Non-isothermal Droplet Evaporation and Condensation in the Near-Continuum Regime, J. Aerosol Sci. 32 1315-39 2001.

Solomon, R. and M. Baker, Electrification of New Mexico Thunderstorms. Mon. Wea. Rev. 122, 1878-1886. 1994

Solomon, R.; Baker, M., A one-dimensional lightning parameterization, J. Geophys. Res. 101, 14983-90 1996.

Solomon, R.; Baker, M., Lightning flash rate and type in convective storms, J. Geophys. Res. 103, 14041-57 1998.

Solomon, R.; Schroeder, V.; Baker, M.B., Lightning initiation-conventional and runaway-breakdown hypotheses, Q. J. Roy. Meteoro. Soc. 127, 2683-704 2001

Swanson, B.D., N.J. Bacon, E.J. Davis and M.B. Baker, Electrodynamic Trapping and Manipulation of Ice Crystals, Q. J. Roy. Meteor. Soc. 125, 1039-58 1999

Wood, S.E.; Baker, M.B.; Calhoun, D., New model for the vapor growth of hexagonal ice crystals in the atmosphere, J. Geophys. Res. 106, 4845-70 2001.

Wood, S.E., M.B. Baker and B.D. Swanson, New Instrument for Studies of Homogeneous and Heterogeneous Ice Nucleation in Free-falling Supercooled Water Droplets, Rev. Sci. Inst. 73, 3988 2002.

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