ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-2459-2012 Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud Ditas F. 1 Shaw R. A. 1 2 Siebert H. 1 Simmel M. 1 Wehner B. 1 Wiedensohler A. 1 Leibniz Institute for Tropospheric Research (IfT), Permoserstr. 15, 04318 Leipzig, Germany Department of Physics, Michigan Technological University, Houghton, Michigan, USA 05 03 2012 12 5 2459 2468 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/2459/2012/acp-12-2459-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/2459/2012/acp-12-2459-2012.pdf

This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435&plusmn;87 cm<sup>−3</sup> and compares well with the observed median droplet number concentration of <span style="border-top: 1px solid #000; color: #000;"><i>N</i></span><sub>d</sub> = 464 cm<sup>−3</sup>. Furthermore, a 50% activation diameter of <i>D</i><sub>p50</sub>≈115 nm was derived, which was linked to a critical supersaturation <i>S</i><sub>crit</sub> of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of &sigma;<sub><i>S'</i></sub> = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (<i>S'</i>) are calculated based on highly-resolved thermodynamic data showing a standard deviation of <i>S'</i> ranging within 0.1%≤&sigma;<sub><i>S'</i></sub>≤0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of <i>S'</i> with &sigma;<sub><i>S'</i></sub>=0.1% at cloud base and a decreasing &sigma;<sub><i>S'</i></sub> with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.

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