Mixed-phase Cloud Measurements in a High Alpine Environment using Digital In-line Holography

Mixed-phase Clouds (MPCs), consisting of a unstable mixture of ice particles and supercooled liquid droplets, are found in many seasons of the year over broad stretches of the earth, and because of their complicated structure and dynamics, the level of understanding of MPCs is low.

In-situ measurements of MPCs at the high altitude research station Jungfraujoch, Switzerland were taken with the digital holographic imager HOLIMO II (Henneberger et. al, 2013) during the winters of 2012 and 2013 (within the CLACE campaign). Each HOLIMO II hologram yields a three-dimensional position, size and shadowgraph for each particle within a well-defined sample volume. Advancements in data processing software now offer phase-resolved size distributions, concentrations, and water contents, with a sampling rate that sees variations in these parameters on a 25m length-scale in a MPC.

The field data reveal the unstable co-existence of water droplets and ice crystals, i.e. the presence of an only partially-glaciated MPC maintained at the Jungfraujoch for over several hours. In-situ aircraft observations in MPCs associated with frontal systems by Korolev et al. (2003) show a large preference for MPCs either being fully glaciated or with almost no glaciation. The analysis of the CLACE 2013 data from HOLIMO II shows however a much larger frequency of intermediate glaciation conditions in these MPCs. The higher longevity of these intermediate conditions of MPCs at the Jungfraujoch than in the Canadian frontal MPCs suggests either higher updraft velocities, and therefore higher water-vapor supersaturations, or the absence of sufficiently high ice nuclei to quickly glaciate the MPC. The Jungfraujoch location has a steeper topography for northerly winds meaning higher updraft velocities than for southerly winds. And the measurements show more intermediate values of glaciation from the North with the higher updraft velocities than from the South which has consistently either fully-glaciated or almost no glaciation in its MPCs.

Henneberger, J., Fugal, J. P., Stetzer, O., and Lohmann, U.: HOLIMO II: a digital holographic instrument for ground-based in-situ observations of microphysical properties of mixed-phase clouds, Atmos. Meas. Tech., 6, 2975–2987, doi: 10.5194/amt-6-2975-2013, 2013.

Korolev, A. V., Isaac, G. A., Cober, S. G., Strapp, J. W., and Hallett, J.: Microphysical characterization of mixed-phase clouds, Q. J. Roy. Meteor. Soc., 129, 39–65, doi: 10.1256/gj.01.204, 2003.