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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: Zheng, Xiaojian, Xi, Baike, Dong, Xiquan, Wu, Peng, Logan, Timothy, Wang, Yuan
• Format: Text
• Language: English
• Published: 2022
• Subjects: North Atlantic
• Online Access: https://doi.org/10.5194/acp-22-335-2022; https://acp.copernicus.org/articles/22/335/2022/

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,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 ( PWV BL ) 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 PWV BL . 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 ( TKE w ), the decoupling index ( D i ), and PWV BL . The environmental effects on ACI r emerge after the data are stratified into different TKE w regimes. The ACI r values, under both lower and higher PWV BL 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.