Articles | Volume 15, issue 22
Atmos. Chem. Phys., 15, 12953–12969, 2015
Atmos. Chem. Phys., 15, 12953–12969, 2015

Research article 24 Nov 2015

Research article | 24 Nov 2015

The origins of ice crystals measured in mixed-phase clouds at the high-alpine site Jungfraujoch

G. Lloyd1, T. W. Choularton1, K. N. Bower1, M. W. Gallagher1, P. J. Connolly1, M. Flynn1, R. Farrington1, J. Crosier1,2, O. Schlenczek3,4, J. Fugal3,4, and J. Henneberger5 G. Lloyd et al.
  • 1Centre for Atmospheric Science, University of Manchester, Manchester, UK
  • 2NERC National Centre for Atmospheric Science (NCAS), Manchester, UK
  • 3Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 4Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany
  • 5ETH, Zurich, Institute for Atmospheric and Climate Science, Universitätsstraße 16, 8092, Zurich, Switzerland

Abstract. During the winter of 2013 and 2014 measurements of cloud microphysical properties over a 5-week period at the high-alpine site Jungfraujoch, Switzerland, were carried out as part of the Cloud Aerosol Characterisation Experiments (CLACE) and the Ice Nucleation Process Investigation and Quantification project (INUPIAQ). Measurements of aerosol properties at a second, lower site, Schilthorn, Switzerland, were used as input for a primary ice nucleation scheme to predict ice nuclei concentrations at Jungfraujoch. Frequent, rapid transitions in the ice and liquid properties of the clouds at Jungfraujoch were identified that led to large fluctuations in ice mass fractions over temporal scales of seconds to hours. During the measurement period we observed high concentrations of ice particles that exceeded 1000 L−1 at temperatures around −15 °C, verified by multiple instruments. These concentrations could not be explained using the usual primary ice nucleation schemes, which predicted ice nucleus concentrations several orders of magnitude smaller than the peak ice crystal number concentrations. Secondary ice production through the Hallett–Mossop process as a possible explanation was ruled out, as the cloud was rarely within the active temperature range for this process. It is shown that other mechanisms of secondary ice particle production cannot explain the highest ice particle concentrations. We describe four possible mechanisms that could lead to high cloud ice concentrations generated from the snow-covered surfaces surrounding the measurement site. Of these we show that hoar frost crystals generated at the cloud enveloped snow surface could be the most important source of cloud ice concentrations. Blowing snow was also observed to make significant contributions at higher wind speeds when ice crystal concentrations were < 100 L−1.

Short summary
The paper explores the microphysical structure of clouds at the high-alpine measurement site Jungfraujoch, Switzerland. High concentrations of ice crystals were measured by a range of instruments. The presence of these high concentrations could not be explained through conventional understanding of ice formation processes in clouds and the possibility that the surface provides a significant source of ice crystals is investigated.
Final-revised paper