Abstract
Abstract. For the first time, a closure study of the relationship between the ice-nucleating particle concentration (INP; INPC) and ice crystal number concentration (ICNC) in altocumulus and cirrus layers, solely based on ground-based active remote sensing, is presented. Such aerosol–cloud closure experiments are required (a) to better understand aerosol–cloud interaction in the case of mixed-phase clouds, (b) to explore to what extent heterogeneous ice nucleation can contribute to cirrus formation, which is usually controlled by homogeneous freezing, and (c) to check the usefulness of available INPC parameterization schemes, applied to lidar profiles of aerosol optical and microphysical properties up to the tropopause level. The INPC–ICNC closure studies were conducted in Cyprus (Limassol and Nicosia) during a 6-week field campaign in March–April 2015 and during the 17-month CyCARE (Cyprus Clouds Aerosol and Rain Experiment) campaign. The focus was on altocumulus and cirrus layers which developed in pronounced Saharan dust layers at heights from 5 to 11 km. As a highlight, a long-lasting cirrus event was studied which was linked to the development of a very strong dust-infused baroclinic storm (DIBS) over Algeria. The DIBS was associated with strong convective cloud development and lifted large amounts of Saharan dust into the upper troposphere, where the dust influenced the evolution of an unusually large anvil cirrus shield and the subsequent transformation into an cirrus uncinus cloud system extending from the eastern Mediterranean to central Asia, and thus over more than 3500 km. Cloud top temperatures of the three discussed closure study cases ranged from −20 to −57 ∘C. The INPC was estimated from polarization/Raman lidar observations in combination with published INPC parameterization schemes, whereas the ICNC was retrieved from combined Doppler lidar, aerosol lidar, and cloud radar observations of the terminal velocity of falling ice crystals, radar reflectivity, and lidar backscatter in combination with the modeling of backscattering at the 532 and 8.5 mm wavelengths. A good-to-acceptable agreement between INPC (observed before and after the occurrence of the cloud layer under investigation) and ICNC values was found in the discussed three proof-of-concept closure experiments. In these case studies, INPC and ICNC values matched within an order of magnitude (i.e., within the uncertainty ranges of the INPC and ICNC estimates), and they ranged from 0.1 to 10 L−1 in the altocumulus layers and 1 to 50 L−1 in the cirrus layers observed between 8 and 11 km height. The successful closure experiments corroborate the important role of heterogeneous ice nucleation in atmospheric ice formation processes when mineral dust is present. The observed long-lasting cirrus event could be fully explained by the presence of dust, i.e., without the need for homogeneous ice nucleation processes.
Highlights
Heterogeneous ice formation is an important pathway of aerosol–cloud interaction
In strong contrast to these Arctic and mid-latitude aerosol– cloud closure studies at clean-air and low aerosol concentration levels, our remote-sensing-based ice-nucleating particle concentration (INPC)–ice crystal number concentration (ICNC) closure approach deals with stratiform altocumulus and cirrus layers which developed in pronounced Saharan dust layers in the middle and upper troposphere over Cyprus
An in-depth comparison of aerosol mass profiles, INPC, and ice-nucleating particle concentration (INP)-relevant aerosol parameters obtained from UAV flights and derived from the lidar observations was performed during the Cyprus-2016 campaign (Schrod et al, 2017; Mamali et al, 2018; Marinou et al, 2019)
Summary
Heterogeneous ice formation is an important pathway of aerosol–cloud interaction. Via heterogeneous freezing and nucleation mechanisms, solid aerosol particles trigger the nucleation of ice crystals at relatively high temperatures from 0 to about −35 ◦C. In strong contrast to these Arctic and mid-latitude aerosol– cloud closure studies at clean-air and low aerosol concentration levels, our remote-sensing-based INPC–ICNC closure approach deals with stratiform altocumulus and cirrus layers which developed in pronounced Saharan dust layers in the middle and upper troposphere over Cyprus (at Middle Eastern meteorological conditions). Such clouds were frequently observed during our field campaigns, conducted in each of the spring seasons from 2015 to 2018, and are obviously very common in the eastern Mediterranean. The Hallett–Mossop SIP process is associated with a splinter ejection during the riming of ice crystals at temperatures between −3 and −8 ◦C (Hallett and Mossop, 1974)
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