PNNL

Abstract

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 average cloud microphysical and optical properties. Positive correlations between the cloud droplet effective radius (Re) and cloud optical depth (t) are observed for both clean and polluted cases, which are characteristic of optically-thin, non-precipitating clouds. Average Re values for each case are ~ 6.2 µm, despite significantly higher droplet number concentrations (Nd) in the polluted cases. The apparent independence of Re and Nd simplifies the description of indirect effects, such that t and the cloud albedo (a) can be described by relatively simple functions of the cloud liquid water path. Adiabatic cloud parcel model simulations show that the marked differences in Na between the regimes account largely for differences in droplet activation, but that the properties of precursor aerosol also play a role, particularly for polluted cases where competition for vapour amongst the more numerous particles limits activation to larger and/or more hygroscopic particles. The similarity of Re for clean and polluted cases is attributed to compensating droplet growth processes for different initial droplet size distributions.