Abstract
Abstract. 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 (ACIr) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during 2016–2018. The ACIr represents the relative change in cloud droplet effective radius re with respect to the relative change in cloud condensation nuclei (CCN) number concentration at 0.2 % supersaturation (NCCN,0.2 %) in the stratified water vapor environment. The ACIr values vary from −0.01 to 0.22 with increasing sub-cloud boundary layer precipitable water vapor (PWVBL) conditions, indicating that re 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 Nc 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 (Di), and PWVBL. The environmental effects on ACIr emerge after the data are stratified into different TKEw regimes. The ACIr values, under both lower and higher PWVBL conditions, more than double from the low-TKEw to high-TKEw 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 re. The enhanced activation of CCN and the cloud droplet condensational growth induced by the higher below-cloud CCN loading can effectively decrease re, which jointly presents as the increased ACIr. This study examines the importance of environmental effects on the ACIr assessments and provides observational constraints to future model evaluations of aerosol–cloud interactions.
Highlights
Clouds are one of the most important parts of the Earth’s climate system
Over the Atmospheric Radiation Measurement (ARM)-eastern North Atlantic (ENA) site, a total of 20 non-precipitating single-layered marine boundary layer (MBL) stratus and stratocumulus cloud cases have been selected in order to investigate the aerosol–cloud interaction (ACI)
The distributions of cloud condensation nuclei (CCN) and cloud properties for selected cases represent the typical characteristics of non-precipitating MBL clouds in a relatively clean environment over the remote oceanic area
Summary
Clouds are one of the most important parts of the Earth’s climate system. They can impact the global climate by modulating the radiative balance in the atmosphere. The changes in MBL cloud microphysical properties induced by aerosols have been investigated in previous studies using in situ measurements, groundand satellite-based observations, and model simulations in multiple oceanic areas such as the eastern Pacific and eastern Atlantic (Twohy et al, 2005; Lu et al, 2007; Hill et al, 2009; Costantino and Bréon, 2010; Mann et al, 2014; Dong et al, 2015; Diamond et al, 2018; Yang et al, 2019; Zhao et al, 2019; Wang et al, 2020)
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