A model intercomparison of CCN-limited tenuous clouds in the high Arctic

• Main Authors: Stevens, Robin, Loewe, Katharina, Dearden, Christopher, Dimitrelos, Antonios, Possner, Anna, Eirund, Gesa K., Raatikainen, Tomi, Hill, Adrian A., Shipway, Benjamin J., Wilkinson, Jonathan, Romakkaniemi, Sami, Tonttila, Juha, Laaksonen, Ari, Korhonen, Hannele, Connolly, Paul, Lohmann, Ulrike, Hoose, Corinna, Ekman, Annica M.L., Carslaw, Ken S., Field, Paul R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus 2018
• Subjects: Arctic
• Online Access: https://hdl.handle.net/20.500.11850/282796; https://doi.org/10.3929/ethz-b-000282796

We perform a model intercomparison of summertime high Arctic ( > 80°N) clouds observed during the 2008 Arctic Summer Cloud Ocean Study (ASCOS) campaign, when observed cloud condensation nuclei (CCN) concentrations fell below 1cm−3. Previous analyses have suggested that at these low CCN concentrations the liquid water content (LWC) and radiative properties of the clouds are determined primarily by the CCN concentrations, conditions that have previously been referred to as the tenuous cloud regime. The intercomparison includes results from three large eddy simulation models (UCLALES-SALSA, COSMO-LES, and MIMICA) and three numerical weather prediction models (COSMO-NWP, WRF, and UM-CASIM). We test the sensitivities of the model results to different treatments of cloud droplet activation, including prescribed cloud droplet number concentrations (CDNCs) and diagnostic CCN activation based on either fixed aerosol concentrations or prognostic aerosol with in-cloud processing. There remains considerable diversity even in experiments with prescribed CDNCs and prescribed ice crystal number concentrations (ICNC). The sensitivity of mixed-phase Arctic cloud properties to changes in CDNC depends on the representation of the cloud droplet size distribution within each model, which impacts autoconversion rates. Our results therefore suggest that properly estimating aerosol–cloud interactions requires an appropriate treatment of the cloud droplet size distribution within models, as well as in situ observations of hydrometeor size distributions to constrain them. The results strongly support the hypothesis that the liquid water content of these clouds is CCN limited. For the observed meteorological conditions, the cloud generally did not collapse when the CCN concentration was held constant at the relatively high CCN concentrations measured during the cloudy period, but the cloud thins or collapses as the CCN concentration is reduced. The CCN concentration at which collapse occurs varies substantially between models. Only one ...