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Author: Welti, André × Subject: cloud ×

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    Classical nucleation theory of immersion freezing: sensitivity of contact angle schemes to thermodynamic and kinetic parameters
    (Katlenburg-Lindau : EGU, 2017) Ickes, Luisa; Welti, André; Lohmann, Ulrike
    Heterogeneous ice formation by immersion freezing in mixed-phase clouds can be parameterized in general circulation models (GCMs) by classical nucleation theory (CNT). CNT parameterization schemes describe immersion freezing as a stochastic process, including the properties of insoluble aerosol particles in the droplets. There are different ways to parameterize the properties of aerosol particles (i.e., contact angle schemes), which are compiled and tested in this paper. The goal of this study is to find a parameterization scheme for GCMs to describe immersion freezing with the ability to shift and adjust the slope of the freezing curve compared to homogeneous freezing to match experimental data. We showed in a previous publication that the resulting freezing curves from CNT are very sensitive to unconstrained kinetic and thermodynamic parameters in the case of homogeneous freezing. Here we investigate how sensitive the outcome of a parameter estimation for contact angle schemes from experimental data is to unconstrained kinetic and thermodynamic parameters. We demonstrate that the parameters describing the contact angle schemes can mask the uncertainty in thermodynamic and kinetic parameters. Different CNT formulations are fitted to an extensive immersion freezing dataset consisting of size-selected measurements as a function of temperature and time for different mineral dust types, namely kaolinite, illite, montmorillonite, microcline (K-feldspar), and Arizona test dust. We investigated how accurate different CNT formulations (with estimated fit parameters for different contact angle schemes) reproduce the measured freezing data, especially the time and particle size dependence of the freezing process. The results are compared to a simplified deterministic freezing scheme. In this context, we evaluated which CNT-based parameterization scheme able to represent particle properties is the best choice to describe immersion freezing in a GCM.
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    The Arctic Cloud Puzzle: Using ACLOUD/PASCAL Multiplatform Observations to Unravel the Role of Clouds and Aerosol Particles in Arctic Amplification
    (Boston, Mass. : ASM, 2019) Wendisch, Manfred; Macke, Andreas; Ehrlich, André; Lüpkes, Christof; Mech, Mario; Chechin, Dmitry; Dethloff, Klaus; Velasco, Carola Barrientos; Bozem, Heiko; Brückner, Marlen; Clemen, Hans-Christian; Crewell, Susanne; Donth, Tobias; Dupuy, Regis; Ebell, Kerstin; Egerer, Ulrike; Engelmann, Ronny; Engler, Christa; Eppers, Oliver; Gehrmann, Martin; Gong, Xianda; Gottschalk, Matthias; Gourbeyre, Christophe; Griesche, Hannes; Hartmann, Jörg; Hartmann, Markus; Heinold, Bernd; Herber, Andreas; Herrmann, Hartmut; Heygster, Georg; Hoor, Peter; Jafariserajehlou, Soheila; Jäkel, Evelyn; Järvinen, Emma; Jourdan, Olivier; Kästner, Udo; Kecorius, Simonas; Knudsen, Erlend M.; Köllner, Franziska; Kretzschmar, Jan; Lelli, Luca; Leroy, Delphine; Maturilli, Marion; Mei, Linlu; Mertes, Stephan; Mioche, Guillaume; Neuber, Roland; Nicolaus, Marcel; Nomokonova, Tatiana; Notholt, Justus; Palm, Mathias; van Pinxteren, Manuela; Quaas, Johannes; Richter, Philipp; Ruiz-Donoso, Elena; Schäfer, Michael; Schmieder, Katja; Schnaiter, Martin; Schneider, Johannes; Schwarzenböck, Alfons; Seifert, Patric; Shupe, Matthew D.; Siebert, Holger; Spreen, Gunnar; Stapf, Johannes; Stratmann, Frank; Vogl, Teresa; Welti, André; Wex, Heike; Wiedensohler, Alfred; Zanatta, Marco; Zeppenfeld, Sebastian
    Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC) project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from below using similar but upward-looking remote sensing techniques as the Polar 5 aircraft. Several of the flights were carried out underneath collocated satellite tracks. The paper motivates the scientific objectives of the ACLOUD/PASCAL observations and describes the measured quantities, retrieved parameters, and the applied complementary instrumentation. Furthermore, it discusses selected measurement results and poses critical research questions to be answered in future papers analyzing the data from the two field campaigns.