Determination of Mass-Fall Speed Relation for Cold Room Laboratory-Grown Ice Crystals Using Digital Holography

The mass and sedimentation velocity of cloud ice crystals are crucial parameters in cloud models. While the mass of the crystals determines the clouds’ ice water content, sedimentation velocity directly governs their spatial and temporal evolution. Those properties are frequently represented in models by parameterizations relating them to ice crystal size, which is more easily measured by in-situ cloud probes or by remote sensing and for which a large amount of measurement data is available. However, many of those parameterizations have been calculated using very sparse experimental data, yielding insufficient statistical significance. In particular, for small crystal sizes (< 200 µm in maximum dimension), a very limited amount of experimental data is available, and often parameterizations are merely extrapolated from measurements obtained from experiments with larger crystals. With this in mind, a cloud ice generation chamber of 4 meters height and 60 cm diameter has been constructed in the cold room facility of Mainz University with the purpose of investigating small cloud ice particles in a well-controlled environment. Small ice crystals of 10 µm in diameter are generated in the chamber at temperatures between -10 and -30 °C, and grow at different supersaturations which can be adjusted to allow the production of different size ranges and habit distributions. A measurement system utilizing digital in-line holography and microscopic imaging has been developed for accurate determination of mass and fall speed of small ice crystals. The fall velocities, which are in the range of a few cm/s, have been measured by taking multiple holographic images of each crystal during its fall. The advantage of holography in this setting is that it is not limited to one plane of focus, yielding in-focus images of objects throughout a range of several centimeters along the optical axis. Mass data has been calculated by catching the ice crystals on a hydrophobic surface and taking digital images before and after melting them under an optical microscope. A large data set consisting of several thousand individual ice crystals with diameters between 10 and 150 µm and featuring various habits, e.g. hollow and solid columns or dendrites, has been generated and analyzed. From this data set, power law type parameterizations relating both mass and sedimentation velocity to size have been developed and compared to previous literature data.