Abstract

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.

Highlights

  • During January and February 2014 the Ice Nucleation Process Investigation and Quantification (INUPIAQ) project took place at three high-alpine sites in the Swiss Alps: Jungfraujoch (46.55◦ N, 7.98◦ E), Schilthorn (46.56◦ N, 7.84◦ E) and Kleine Scheidegg (46.59◦ N, 7.96◦ E), Switzerland, as part of the Cloud Aerosol Characterisation Experiment (CLACE) 2014

  • – Changes in the liquid water content (LWC) and IWC lead to significant changes in ice mass fraction values occurring over temporal scales of seconds to hours during both 2013 and 2014

  • – Calculated cloud ice mass fraction values at Jungfraujoch are influenced by a number of often competing changes in cloud microphysical properties involving the ice and liquid phases, where changes in the different phase loadings were often independent of one another

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Summary

Introduction

During January and February 2014 the Ice Nucleation Process Investigation and Quantification (INUPIAQ) project took place at three high-alpine sites in the Swiss Alps: Jungfraujoch (46.55◦ N, 7.98◦ E), Schilthorn (46.56◦ N, 7.84◦ E) and Kleine Scheidegg (46.59◦ N, 7.96◦ E), Switzerland, as part of the Cloud Aerosol Characterisation Experiment (CLACE) 2014. Choularton et al (2008) presented data from earlier CLACE experiments that took place at Jungfraujoch (JFJ) in which ice particle number concentrations and habit were measured with a Cloud Particle Imager (CPI, SPEC Inc., USA) probe, water droplets with a Forward Scattering Spectrometer Probe (FSSP-100, DMT, USA) and liquid water content with a Particulate Volume Monitor (PVM-100, Gerber Scientific). They observed rapid transitions between liquid and glaciated cloud that took place on spatial scales of just a few metres

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