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Volume 14, issue 22
Atmos. Chem. Phys., 14, 12573-12592, 2014
https://doi.org/10.5194/acp-14-12573-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Arctic Summer Cloud Ocean Study (ASCOS) (ACP/AMT/OS inter-journal...

Atmos. Chem. Phys., 14, 12573-12592, 2014
https://doi.org/10.5194/acp-14-12573-2014
© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 28 Nov 2014

Research article | 28 Nov 2014

The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface

G. Sotiropoulou1,2, J. Sedlar1,2, M. Tjernström1,2, M. D. Shupe3,4, I. M. Brooks5, and P. O. G. Persson3,4 G. Sotiropoulou et al.
  • 1Department of Meteorology, Stockholm University, Stockholm, Sweden
  • 2Bert Bolin Center for Climate Research, Stockholm University, Stockholm, Sweden
  • 3Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado, USA
  • 4NOAA Earth System Research Laboratory, Boulder, Colorado, USA
  • 5Institute for Climate & Atmospheric Science, School of Earth & Environment, University of Leeds, Leeds, UK

Abstract. The vertical structure of Arctic low-level clouds and Arctic boundary layer is studied, using observations from ASCOS (Arctic Summer Cloud Ocean Study), in the central Arctic, in late summer 2008. Two general types of cloud structures are examined: the "neutrally stratified" and "stably stratified" clouds. Neutrally stratified are mixed-phase clouds where radiative-cooling near cloud top produces turbulence that generates a cloud-driven mixed layer. When this layer mixes with the surface-generated turbulence, the cloud layer is coupled to the surface, whereas when such an interaction does not occur, it remains decoupled; the latter state is most frequently observed. The decoupled clouds are usually higher compared to the coupled; differences in thickness or cloud water properties between the two cases are however not found. The surface fluxes are also very similar for both states. The decoupled clouds exhibit a bimodal thermodynamic structure, depending on the depth of the sub-cloud mixed layer (SCML): clouds with shallower SCMLs are disconnected from the surface by weak inversions, whereas those that lay over a deeper SCML are associated with stronger inversions at the decoupling height. Neutrally stratified clouds generally precipitate; the evaporation/sublimation of precipitation often enhances the decoupling state. Finally, stably stratified clouds are usually lower, geometrically and optically thinner, non-precipitating liquid-water clouds, not containing enough liquid to drive efficient mixing through cloud-top cooling.

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During ASCOS, clouds are more frequently decoupled from the surface than coupled to it; when coupling occurs it is primary driven by the cloud. Decoupled clouds have a bimodal structure; they are either weakly or strongly decoupled from the surface; the enhancement of the decoupling is possibly due to sublimation of precipitation. Stable clouds (no cloud-driven mixing) are also observed; those are optically thin, often single-phase liquid, with no or negligible precipitation (e.g. fog).
During ASCOS, clouds are more frequently decoupled from the surface than coupled to it; when...
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