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

Description

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 a 19-month set of comprehensive measurements of aerosols, clouds, and meteorology acquired during the Atmospheric Radiation Measurement Mobile Facility field campaign that took place over the northern coast of Graciosa Island in the Azores. Cloud droplet number concentrations (NC) and cloud optical depth (COD) significantly increased with increasing aerosol number concentration (Na). Cloud droplet effective radius (DER) significantly decreased with increasing Na. The significant correlations between cloud microphysical properties (NC, liquid water path (LWP), DER) and Na were found under more stable atmospheric conditions. The correlations between NC, LWP, DER, and Na were more significant under updraft conditions, while the correlation between COD and Na was more significant under downdraft conditions. The magnitude of the FIE under constant LWP conditions ranged from 0.023 to 0.101 depending on the different LWP values. Under more stable atmospheric 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 lower tropospheric stability varied according to the research approach used. The magnitude of the FIE had a larger variation with changing LWP under updraft conditions, and tended to be higher under updraft conditions for clouds with low LWP and under downdraft conditions for clouds with high LWP. These findings suggest that atmospheric dynamic and thermodynamic parameters contribute significantly to the various sensitivities of MBL clouds to aerosol perturbations.