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Schlagwörter:
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Zusammenfassung:
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 degrees 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.
