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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2017. This work is distributed under
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Research article
11 Dec 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).
A Model Intercomparison of CCN-Limited Tenuous Clouds in the High Arctic
Robin G. Stevens1, Katharina Loewe2, Christopher Dearden3, Antonios Dimitrelos4, Anna Possner5,6, Gesa K. Eirund5, Tomi Raatikainen7, Adrian A. Hill8, Benjamin J. Shipway8, Jonathan Wilkinson8, Sami Romakkaniemi9, Juha Tonttila9, Ari Laaksonen7, Hannele Korhonen7, Paul Connolly3, Ulrike Lohmann5, Corinna Hoose2, Annica M. L. Ekman4, Ken S. Carslaw1, and Paul R. Field1,8 1Institute of Climate and Atmospherice Science, School of Earth and Environment, University of Leeds, Leeds, UK
2Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
3Centre for Atmospheric Science, School of Earth and Environmental Sciences, University of Manchester, Manchester, UK
4Department of Meteorology, Stockholm University, Stockholm, Sweden
5Institute for Atmospheric and Climate Science, Eidgenössische Technische Hochschule, Zürich, Switzerland
6Department of Global Ecology, Carnegie Institution for Science, Stanford, CA, USA
7Finnish Meteorological Institute, Helsinki, Finland
8Met Office, Exeter, UK
9Finnish Meteorological Institute, Kuopio, Finland
Abstract. 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 1 cm−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 (CDNC) 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 on 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 model predicts complete dissipation of the cloud due to glaciation, and this occurs only for the largest prescribed ICNC tested in this study. Global and regional models with either prescribed CDNCs or prescribed aerosol concentrations would not reproduce these dissipation events. Additionally, future increases in Arctic aerosol concentrations would be expected to decrease the frequency of occurrence of such cloud dissipation events, with implications for the radiative balance at the surface. Our results also show that cooling of the sea-ice surface following cloud dissipation increases atmospheric stability near the surface, further suppressing cloud formation. Therefore, this suggests that linkages between aerosol and clouds, as well as linkages between clouds, surface temperatures and atmospheric stability need to be considered for weather and climate predictions in this region.

Citation: Stevens, R. G., Loewe, K., Dearden, C., Dimitrelos, A., Possner, A., Eirund, G. K., Raatikainen, T., Hill, A. A., Shipway, B. J., Wilkinson, J., Romakkaniemi, S., Tonttila, J., Laaksonen, A., Korhonen, H., Connolly, P., Lohmann, U., Hoose, C., Ekman, A. M. L., Carslaw, K. S., and Field, P. R.: A Model Intercomparison of CCN-Limited Tenuous Clouds in the High Arctic, Atmos. Chem. Phys. Discuss.,, in review, 2017.
Robin G. Stevens et al.
Robin G. Stevens et al.
Robin G. Stevens et al.


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Short summary
We perform a model intercomparison of summertime high Arctic clouds. Observed concentrations of aerosol particles necessary for cloud formation fell to extremely low values, coincident with a transition from cloudy to nearly cloud-free conditions. Previous analyses have suggested that at these low concentrations, the radiative properties of the clouds are determined primarily by these particle concentrations. The model results strongly support this hypothesis.
We perform a model intercomparison of summertime high Arctic clouds. Observed concentrations of...