Abstract
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.
Highlights
The atmosphere’s radiation budget and aerosol particles are linked via (1) the radiative properties of the aerosol particles themselves and (2) via influencing cloud microphysics and, cloud radiative properties
The analysis is based on observations with the helicopter-borne measurement platform Airborne Cloud Turbulence Observation System (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 N d = 464 cm−3
Within the framework of this paper, we focus on three approaches for characterizing supersaturation fluctuations: (1) aerosol number size distribution inside and outside the stratocumulus cloud and the resulting activation properties as well as a comparison with observed cloud droplet number concentration, (2) water vapor supersaturation at the cloud base derived from highly resolved thermodynamic data, and (3) a sensitivity analysis of the influence of measured vertical velocity fluctuations on the supersaturation field determined with a cloud microphysical parcel model
Summary
The atmosphere’s radiation budget and aerosol particles are linked via (1) the radiative properties of the aerosol particles themselves (direct aerosol effect) and (2) via influencing cloud microphysics (indirect aerosol effect) and, cloud radiative properties. We evaluate the magnitude of supersaturation fluctuations (S ) in a turbulent marine stratocumulus layer over the Baltic Sea. The measurements were obtained with the helicopter-borne platform ACTOS (Airborne Cloud Turbulence Observation System, Siebert et al, 2006); its true air speed of only 15 to 20 m s−1 allows us to compare highly resolved and spatially collocated thermodynamic and cloud microphysical properties with microphysical properties of the interstitial aerosol nearby the turbulent cloud layer. Within the framework of this paper, we focus on three approaches for characterizing supersaturation fluctuations: (1) aerosol number size distribution inside and outside the stratocumulus cloud and the resulting activation properties as well as a comparison with observed cloud droplet number concentration, (2) water vapor supersaturation at the cloud base derived from highly resolved thermodynamic data, and (3) a sensitivity analysis of the influence of measured vertical velocity fluctuations on the supersaturation field determined with a cloud microphysical parcel model
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