3-D imaging and quantification of graupel porosity by synchrotron-based micro-tomography

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.

Electric Effect during the Fast Dendritic Freezing of Supercooled Water Droplets

“…This means that, for example, ramifications of the dendrites cause a symmetry break which in turn should be responsible for an asymmetric distribution of the present (salt) ions during stage-two freezing which even could get solidified in the final compact ice. Here X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and synchrotron-based microtomography should be appropriate tools to verify this hypothesis. , To our knowledge, such a symmetry breaking effect of the first freezing stage has not yet been described in the literature up to now. We are of the opinion that this finding is of general importance for basic research as well as for atmospheric applications and plan to investigate it in detail in a separate study.…”

MASCDB, a database of images, descriptors and microphysical properties of individual snowflakes in free fall

“…Individual crystals may branch with each other and form aggregates 1 , 2 . They may collide with supercooled liquid water droplets that will freeze on them upon impact (riming 3 , 4 ), they may sublimate or generate secondary ice by breaking up or splintering 5 . Measuring, describing and understanding the interactions occurring at the microphysical scale are complex tasks and important limiting factors to represent these mechanisms in numerical weather prediction models or climate models 6 .…”

Measurement report: Mass and Density of Individual Frozen Hydrometeors

“…Where the exponents described in previous studies (Holroyd, 1971;Fabry and Szyrmer, 1999;Heymsfield, 2003;Brandes et al, 2007;Tiira et al, 2016) are close -1, those from our study are approximately -0.5, suggesting a weaker dependence of density on particle size than is generally assumed. Indeed, even for rimed snowflakes, high resolution electron microscope imagery reveals a highly porous internal structure (Rango et al, 2003;Enzmann et al, 2011). On the ground, the density of snowfall at a high-elevation location in the Wasatch Mountains in Utah is typically less than 100 kg m −3 (Alcott and Steenburgh, 2010), a location where mode snowflake diameters lie between 1 mm and 2 mm (Garrett and Yuter, 2014).…”