Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud

• Published in: Atmospheric chemistry and physics Vol. 12; no. 5; pp. 2459 - 2468
• Main Authors: Ditas, F, Shaw, R. A, Siebert, H, Simmel, M, Wehner, B, Wiedensohler, A
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 05.03.2012; Copernicus Publications
• Subjects: Analysis; Atmospheric physics; Stratocumulus clouds; Thermodynamics; Turbulence (Fluid dynamics)
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-12-2459-2012

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±87 cm−3 and compares well with the observed median droplet number concentration of Nd = 464 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'≤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 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.