Environmental effects on aerosol-cloud interaction in non-precipitating marine boundary layer (MBL) clouds over the eastern North Atlantic

Over the eastern North Atlantic (ENA) ocean, a total of 20 non-precipitating single-layer marine boundary layer (MBL) stratus and stratocumulus cloud cases are selected to investigate the impacts of the environmental variables on the aerosol-cloud interaction (ACI(r)) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during 2016-2018. The ACI(r) represents the relative change in cloud droplet effective radius r(e) with respect to the relative change in cloud condensation nuclei (CCN) number concentration at 0.2 % supersaturation (N-CCN,N- 0.2 %) in the stratified water vapor environment. The ACI(r) values vary from -0.01 to 0.22 with increasing sub-cloud boundary layer precipitable water vapor (PWVBL) conditions, indicating that r(e) is more sensitive to the CCN loading under sufficient water vapor supply, owing to the combined effect of enhanced condensational growth and coalescence processes associated with higher N-c and PWVBL. The principal component analysis shows that the most pronounced pattern during the selected cases is the co-variations in the MBL conditions characterized by the vertical component of turbulence kinetic energy (TKEW), the decoupling index (D-i), and PWVBL. The environmental effects on ACI(r) emerge after the data are stratified into different TKEW regimes. The ACI(r) values, under both lower and higher PWVBL conditions, more than double from the low-TKE w to high-TKE w regime. This can be explained by the fact that stronger boundary layer turbulence maintains a well-mixed MBL, strengthening the connection between cloud microphysical properties and the below-cloud CCN and moisture sources. With sufficient water vapor and low CCN loading, the active coalescence process broadens the cloud droplet size spectra and consequently results in an enlargement of r(e). The enhanced activation of CCN and the cloud droplet condensational growth induced by the higher below-cloud CCN loading can effectively decrease r(e), which jointly presents as the increased ACI(r). This study examines the importance of environmental effects on the ACI(r) assessments and provides observational constraints to future model evaluations of aerosol-cloud interactions.