Application of holography and automated image processing for laboratory experiments on mass and fall speed of small cloud ice crystals

• Main Authors: M. Weitzel, S. K. Mitra, M. Szakáll, J. P. Fugal, S. Borrmann
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-12-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/20/14889/2020/acp-20-14889-2020.pdf

<p>An ice cloud chamber was developed at the Johannes Gutenberg University of Mainz for generating several thousand data points for mass and sedimentation velocity measurements of ice crystals with sizes less than 150&thinsp;<span class="inline-formula">µ</span>m. Ice nucleation was initiated from a cloud of supercooled droplets by local cooling using a liquid nitrogen cold finger. Three-dimensional tracks of ice crystals falling through the slightly supersaturated environment were obtained from the reconstruction of sequential holographic images, automated detection of the crystals in the hologram reconstructions, and particle tracking. Through collection of the crystals and investigation under a microscope before and after melting, crystal mass was determined as a function of size. The experimentally obtained mass versus diameter (<span class="inline-formula"><i>m</i>(<i>D</i>)</span>) power law relationship resulted in lower masses for small ice crystals than from commonly adopted parameterizations. Thus, they did not support the currently accepted extrapolation of relationships measured for larger crystal sizes. The relationship between Best (<span class="inline-formula"><i>X</i></span>) and Reynolds (<span class="inline-formula"><i>Re</i></span>) numbers for columnar crystals was found to be <span class="inline-formula"><i>X</i>=15.3 <i>Re</i>^1.2</span>, which is in general agreement with literature parameterizations.</p>