Aerosols-cloud microphysics-thermodynamics-turbulence: evaluating supersaturation in a marine stratocumulus cloud 1Leibniz Institute for Tropospheric Research (IfT), Permoserstr. 15, 04318 Leipzig, Germany
05 Mar 2012
2Department of Physics, Michigan Technological University, Houghton, Michigan, USA
Received: 11 October 2011 – Published in Atmos. Chem. Phys. Discuss.: 04 November 2011 Abstract. 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.
Revised: 02 February 2012 – Accepted: 20 February 2012 – Published: 05 March 2012
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., 12, 2459-2468, doi:10.5194/acp-12-2459-2012, 2012.