Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions over the AMF-Azores Site

The response of marine boundary layer (MBL) cloud properties to aerosol loading, and the contribution of large-scale dynamic and thermodynamic conditions to the diversity in the sensitivity of MBL clouds properties to aerosol perturbations, as well as to the quantified first indirect effect (FIE), are investigated using a 19-month set of comprehensive measurements collected during the Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) campaign over the northern coast of Graciosa Island in the Azores. The cloud base height (CBH) and cloud top height (CTH) are not sensitive to the aerosol loading. Cloud droplet number concentrations (NC) and cloud optical depth (COD) significantly increase with increasing aerosol number concentration (Na). Cloud droplet effective radius (DER) significantly decreases with increasing Na. A positive feedback loop between aerosol loading and atmospheric stability, i.e., here more aerosols lead to weakened atmospheric vertical motion, more stability, and thus, greater aerosol loading, is observed. The correlation between CBH (CTH) and Na are not significantly sensitive to either dynamic or thermodynamic conditions, but significant correlations between cloud microphysical properties (NC, liquid water path (LWP), COD and DER) and Na are found under more stable atmospheric conditions. The LWP and COD show a significant correlation with Na for strong downdraft cases, while no significant correlations between NC (DER) and Na with vertical velocity are found. The FIE under constant LWP conditions is quantified and ranges from 0.034 to 0.082 depending on the different LWP values. The magnitude of the FIE increases by 31.4% when atmospheric stability changes from low to high and the magnitude of the FIE increases by 168.4% from updraft to downdraft conditions across all LWP bins. This suggests that atmospheric dynamic and thermodynamic parameters can contribute significantly to the various sensitivities of MBL clouds to aerosol perturbations.