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

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 snowcovered 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 .

Anthropogenic Aerosol Influences on Mixed-Phase Clouds

“…Stratiform MPCs are prevalent in the Arctic [67,89] and in orographic terrain [28,50,57]. In situ observations and surface remote sensing suggest that clouds in the mixedphase temperature range in mid-latitudes tend to consist almost entirely of ice or of liquid water [10,11,48].…”

Polarimetric radar and in situ observations of riming and snowfall microphysics during CLACE 2014

“…A first possible explanation would involve collisional mechanisms (e.g., Vardiman, 1978;Yano and Phillips, 2011) that require only earlier stages of riming and the presence of supercooled liquid water, ice crystals, and turbulence (all conditions that are met during EV3). A second explanation, recently proposed for the same measurement campaign of CLACE 2014 (Lloyd et al, 2015), would be the lifting and transport of hoarfrost crystals generated at ground level. During EV3 the environmental conditions are favorable to this process: wind and turbulence are significant (Fig.…”

“…On the other hand, the formation of secondary ice can also follow the mechanisms of Vardiman (1978) or Yano and Phillips (2011): less efficient and not constrained by temperature but favored by riming. In addition, the recently proposed mechanism of hoarfrost crystal lifting and transport (Lloyd et al, 2015) could be a secondary source of ice. Secondary ice would increase K dp by enhancing the number concentration of ice crystals.…”

“…Non-collisional mechanisms have been hypothesized as well. For instance, Rangno (2008) described the process of fragmentation of freezing supercooled droplets in mixed-phase clouds, and recently Lloyd et al (2015) hypothesized that the lifting and transport of hoarfrost crystals from the ground to the clouds could explain the very high concentration of small crystals frequently observed in low level winter clouds.…”

HoloGondel: in situ cloud observations on a cable car in the Swiss Alps using a holographic imager

“…A spatial resolution comparable to a tethered balloon system can be achieved because wind speeds are lower than the traveling speed of aircraft. However, ground-based measurements are limited to single-point observations of the cloud and may be influenced by surface-based processes (Rogers and Vali, 1987;Geerts et al, 2015;Lloyd et al, 2015).…”