Ice Crystal Measurements with the New Particle Spectrometer NIXE-CAPS

Beschreibung

Mixed-phase clouds consist of liquid droplets and ice crystals and appear in the temperature range between 0 ◦C and 40 ◦C. They are in the focus of recent research because model studies indicate that their degree of glaciation have an impact on the cloud radiative properties. Up to now, mainly the measurement of bulk liquid and ice water content is used to investigate the mixed-phase cloud glaciation process. This study, for the first time, presents extensive size resolved laboratory and aircraft based in-situ mixed-phase cloud observations. For this purpose, the Novel Ice EXpEriment - Cloud and Aerosol Particle Spectrometer (NIXE-CAPS), an established cloud particle instrument, but equipped with an additional depolarization detector to distinguish ice crystals and liquid droplets, is used. The complete set of measured parameters includes concentration and phase of cloud particles in the size range of 0.61 μm to 937.5 μm. Here, the dependence of mixed-phase cloud glaciation on the initial number of ice active aerosol, relative humidity and temperature is investigated for clouds generated in the AIDA cloud chamber and for natural clouds observed on board of the British aircraft BAE146 during the COALESC campaign over the UK in 2011. A significant difference in the degree of glaciation is found for AIDA mixed-phase clouds evolved in either sub- or supersaturated humidity conditions with respect to water (RHw). The droplet concentration in supersaturated RHw regimes is constantly high (around 500 cm−3) over the whole temperature range, since the droplets do not evaporate (dropletice coexisting regime). Under subsaturated conditions where evaporation of droplets occurs (Wegener-Bergeron-Findeisen regime), their concentrations decrease with temperature from about 100 cm−3 at 270 K to 1 cm−3 at 235 K. This decrease in droplet concentration is most likely caused by the increasing difference of the water vapor saturation pressure with respect to liquid and ice. Hence, the droplet concentration in mixed phase clouds seems to be mainly driven by the dynamic situation. The number of ice nuclei in the AIDA chamber and thus the ice crystal concentration of the AIDA clouds was constantly high. In the droplet-ice coexistence regime, where also high liquid droplet concentrations are observed, the resultant number fraction of frozen cloud particles is only about 10 % for all temperatures. In contrast, in the subsaturatedWegener-Bergeron-Findeisen regime, the ice number fraction increases with decreasing droplet concentration from about 20 % at 270 K up to 80 % at 235 K. Thus, the colder AIDA clouds in the Wegener-Bergeron-Findeisen regions show a high degree of glaciation which is expected for Wegener-Bergeron-Findeisen conditions, but complete glaciation does not occur. Nevertheless, the ice mass fraction is very close to 100 %, since the remaining particles classified as droplets are only small.

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Jessica Meyer

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