|
Atmos. Chem. Phys. Discuss., 11, 29777-29805, 2011
www.atmos-chem-phys-discuss.net/11/29777/2011/ doi:10.5194/acpd-11-29777-2011 © Author(s) 2011. This work is distributed under the Creative Commons Attribution 3.0 License. |
Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud
1Leibniz Institute for Tropospheric Research (IfT), Permoserstr. 15, 04318 Leipzig, Germany
2Department of Physics, Michigan Technological University, Houghton, Michigan, USA
Abstract. This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence parameters 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 spectral 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 to 435±87 cm−3 and compares well with the observed median droplet number concentration of Nd=456 cm−3. Furthermore, a 50% activation diameter of Dp50 ≈ 115 nm was derived, which was linked to a critical supersaturation Scrit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σS' =0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S') are calculated based on highly-resolved thermodynamic data showing a standard deviation of S' ranging within 0.1% ≤ σS' ≤ .3%. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a spectral cloud microphysical model. These results show highest fluctuations of S' with σS' =0.1% at cloud base and a decreasing σS' 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.
Citation: Ditas, F., Shaw, R. A., Siebert, H., Simmel, M., Wehner, B., and Wiedensohler, A.: Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud, Atmos. Chem. Phys. Discuss., 11, 29777-29805, doi:10.5194/acpd-11-29777-2011, 2011.