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Volume 18, issue 15 | Copyright

Special issue: BACCHUS – Impact of Biogenic versus Anthropogenic emissions...

Atmos. Chem. Phys., 18, 11041-11071, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Aug 2018

Research article | 08 Aug 2018

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

Robin G. Stevens1,a, Katharina Loewe2, Christopher Dearden3,b, 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 Robin G. Stevens et al.
  • 1Institute of Climate and Atmospheric 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
  • anow at: Air Quality Research Division, Environment and Climate Change Canada, Dorval, Canada
  • bnow at: the Centre of Excellence for Modelling the Atmosphere and Climate, School of Earth and Environment, University of Leeds, Leeds, UK

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 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 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.

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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...