Atmos. Chem. Phys. Discuss., 9, 20739-20763, 2009
www.atmos-chem-phys-discuss.net/9/20739/2009/
doi:10.5194/acpd-9-20739-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Scanning electron microscopy and molecular dynamics of surfaces of growing and ablating hexagonal ice crystals
W. C. Pfalzgraff, R. M. Hulscher, and S. P. Neshyba
University of Puget Sound, Tacoma, Washington, 98416, USA

Abstract. Optical properties of cirrus ice clouds play an important role in regulating Earth's radiative balance. It has been hypothesized that the surfaces of cirrus ice crystals may be characterized by mesoscopic (micrometer-scale) texturing, or roughness, in order to explain discrepancies between theoretical and observed light-scattering properties. Here, we present the first clearly resolved observations of surfaces of hexagonal ice crystals, using variable-pressure scanning electron microscopy. During growth conditions, the ice surface develops trans-prismatic strands, separated from one another by distances of 5–10 μm. These strands become more pronounced during ablation, and exhibit a wider range of separations. Under re-growth conditions, faceting is re-established initially at prismatic edges. Molecular dynamics studies of a free-standing ice Ih nanocolumn showed no trans-prismatic strands at the atomistic level, suggesting that these strands originate at a spatial scale greater than 10 nm. The observed surface roughness could be used to construct more realistic representations of cirrus clouds in climate models, and constrain theories of ice crystal growth and ablation.

Citation: Pfalzgraff, W. C., Hulscher, R. M., and Neshyba, S. P.: Scanning electron microscopy and molecular dynamics of surfaces of growing and ablating hexagonal ice crystals, Atmos. Chem. Phys. Discuss., 9, 20739-20763, doi:10.5194/acpd-9-20739-2009, 2009.
 
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