Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign

摘要

This study investigates the response of marine boundary layer (MBL) cloud properties to aerosol loading by accounting for the contributions of large-scale dynamic and thermodynamic conditions and quantifies the first indirect effect (FIE). It makes use of 19-month measurements of aerosols, clouds, and meteorology acquired during the Atmospheric Radiation Measurement Mobile Facility field campaign over the Azores. Cloud droplet number concentrations N-c and cloud optical depth (COD) significantly increased with increasing aerosol number concentration N-a. Cloud droplet effective radius (DER) significantly decreased with increasing N-a. The correlations between cloud microphysical properties [N-c, liquid water path (LWP), and DER] and N-a were stronger under more stable conditions. The correlations between N-c, LWP, DER, and Na were stronger under ascending motion conditions, while the correlation between COD and N-a was stronger under descending-motion conditions. The magnitude and corresponding uncertainty of the FIE {=[-partial derivative ln(DER)/partial derivative ln(N-a) at constant LWP} ranged from 0.060 +/- 0.022 to 0.101 +/- 0.006 depending on the different LWP values. Under more stable conditions, cloud-base heights were generally lower than those under less stable conditions. This enabled a more effective interaction with aerosols, resulting in a larger value for the FIE. However, the dependence of the response of cloud properties to aerosol perturbations on stability varied according to whether ground- or satellite-based DER retrievals were used. The magnitude of the FIE had a larger variation with changing LWP under ascending-motion conditions and tended to be higher under ascending-motion conditions for clouds with low LWP and under descending-motion conditions for clouds with high LWP. A contrasting dependence of FIE on atmospheric stability estimated from the surface and satellite cloud properties retrievals reported in this study underscores the importance of assessing all-level properties of clouds in aerosol cloud interaction studies.