Indirect effects in Arctic liquid-phase clouds during ISDAC
Poster PDF
Authors
Steven J. Ghan — Pacific Northwest National Laboratory
Mikhail Ovchinnikov — Pacific Northwest National Laboratory
J. Walter Strapp — Environment Canada
Peter Liu — Environment Canada
Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Warren Richard Leaitch — Imre Consulting - Climate Chem. Meas. and Research Section
Michael Earle — Environment Canada
Nicole Shantz — Environment Canada
Dan Imre — Imre Consulting
Category
Aerosol-Cloud-Radiation Interactions
Description
Aircraft measurements during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) in April 2008 are used to investigate aerosol indirect effects in Arctic clouds. Two aerosol-cloud regimes are considered in this analysis: single-layer stratocumulus cloud with below-cloud aerosol concentrations (Na) below 300 cm-3 on April 8 and April 26–27 (clean cases); and inhomogeneous layered cloud with Na > 500 cm-3 below cloud base on April 19–20, concurrent with a biomass burning episode (polluted cases). Vertical profiles through cloud in each regime are used to determine cloud microphysical and optical properties, while horizontal flight legs below cloud are used to characterize the physicochemical properties of precursor aerosol. The former are used to assess relationships between droplet effective radius (Re) and the cloud optical depth (τ) or albedo (A), which have been applied previously to investigations of indirect effects based on aircraft and satellite observations (vertical profile component); the latter are used in an aerosol-cloud droplet closure study, which examines the roles of aerosol number concentration, physicochemical properties, and vertical (updraft) velocity in droplet activation (droplet closure component).
The analysis in the vertical profile component showed positive correlations between Re and τ for both clean and polluted cases, which are characteristic of optically thin clouds such as those in the present analysis. The average Re for polluted cases was slightly larger than that for clean cases, despite significantly higher Nd, contrary to the typical trend for indirect effects, in which droplet size decreases with the number of droplets formed. This discrepancy was attributed to the higher liquid water path (LWP) for polluted cases relative to clean cases.
In the droplet closure component, analysis using an adiabatic cloud parcel model indicated that most of the below-cloud aerosol in clean cases was activated to form droplets, and that activation was relatively insensitive to the updraft velocity. For the polluted case, a smaller number fraction of the below-cloud aerosol was activated, owing to the competition for vapour amongst the more numerous particles, which limited activation to larger and/or more hygroscopic particles; this case was therefore more sensitive to the updraft velocity. The results also indicated that for these cases, particle size was more important for activation than chemical composition and mixing state.
