Abstract
Abstract. The air bubble structure is an important parameter to determine the radiation properties of graupel and hailstones. For 3-D imaging of this structure at micron resolution, a cryo-stage was developed. This stage was used at the tomography beamline of the Swiss Light Source (SLS) synchrotron facility. The cryo-stage setup provides for the first time 3-D-data on the individual pore morphology of ice particles down to infrared wavelength resolution. In the present study, both sub-mm size natural and artificial ice particles rimed in a wind tunnel were investigated. In the natural rimed ice particles, Y-shaped air-filled closed pores were found. When kept for half an hour at −8 °C, this morphology transformed into smaller and more rounded voids well known from literature. Therefore, these round structures seem to represent an artificial rather than in situ pore structure, in contrast to the observed y-shaped structures found in the natural ice particles. Hence, for morphological studies on natural ice samples, special care must be taken to minimize any thermal cycling between sampling and measurement, with least artifact production at liquid nitrogen temperatures.
Highlights
The evolution of hydrometeors in the atmosphere is coupled to a variety of microphysical characteristics
The density of ice particles depends on the amount of air bubbles in the ice, which in turn depends on the regime which is dominant during growth (Pruppacher and Klett, 1997)
Extrapolating the decreasing pore volume vs. increasing voxel length relationship down to a fictive voxel length of 10 nm (i.e. ≥0), we found that approximately 97 % of the total pore volume was detected at the chosen spatial resolution of 1.4 μm
Summary
The evolution of hydrometeors in the atmosphere is coupled to a variety of microphysical characteristics Ice particles such as graupel and hailstones exhibit wide variations of their densities which are affected by cloud temperatures, cloud drop size distributions, and liquid water contents (Pruppacher and Klett, 1997). Graupel particles appear more or less opaque because of air bubbles trapped during freezing of the riming droplets Both the opacity due to the inner void structure and enhanced surface roughness due to polycrystallinity have an impact on light scattering. It varies with the number, size distribution, locations, and shapes of the air bubbles. Such a ray-tracing approach will work in scenarios where the size of the individual pores is at least two orders of magnitude larger than the wavelength of the scattered light
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