10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
A Model Intercomparison of CCN-Limited Tenuous Clouds in the High Arctic
Robin Stevens, HAMISH GORDON, Katharina Loewe, Christopher Dearden, Antonios Dimitrelos, Anna Possner, Gesa Eirund, Tomi Raatikainen, Adrian Hill, Benjamin Shipway, Jonathan Wilkinson, Sami Romakkaniemi, Juha Tonttila, Ari Laaksonen, Hannele Korhonen, Paul Connolly, Ulrike Lohmann, Corinna Hoose, Annica Ekman, Ken Carslaw, Paul Field, University of Leeds
Abstract Number: 30 Working Group: Clouds and Climate
Abstract A decrease in Arctic sea ice extent and thickness has been observed within recent decades. Further decreases in Arctic sea ice extent are expected to increase the fluxes of aerosol and aerosol precursor gases as well as latent heat and sensible heat from the open ocean surface within the Arctic. Additionally, Arctic aerosol concentrations could be significantly impacted by changes in non-local aerosol sources and changes in the long-range transport of this aerosol, and increases in shipping traffic as the Arctic becomes seasonally ice-free. This increase in shipping traffic would also be expected to yield an increased demand for accurate weather forecasts over the Arctic region. However, it remains unclear whether the net effect of these changes in aerosol concentrations and surface fluxes would result in an increase or a decrease in cloud cover or drizzle precipitation. The changes in cloud properties could strongly influence the radiation budget in the Arctic, resulting in feedbacks on the rate of sea-ice loss. Arctic clouds remain poorly understood, and the current representation of these processes in global climate models is most likely insufficient to realistically simulate long-term changes.
In order to better understand the processes controlling Arctic clouds and their uncertainties in current models, 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. We investigate the sensitivities of the model results to changes in these prescribed CDNC, fixed CCN, or prognostic CCN concentrations. Further, we examine the interaction of model sensitivity to CCN concentrations with changes in prescribed ice crystal number concentrations (ICNC). We also examine the interaction of model sensitivity to CCN concentrations with changes in surface heat and moisture fluxes due to a removal of sea-ice cover.
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. Our results also show that cooling of the sea-ice surface following cloud dissipation increases atmospheric stability near the surface, further suppressing cloud formation. This effect does not occur in simulations with a sea-ice-free surface.
There remains considerable diversity even in experiments with prescribed CDNCs and prescribed ICNCs. The sensitivity of mixed-phase Arctic cloud properties to changes in CDNC, and hence to changes in aerosol concentrations, 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.