Abstract
Abstract. The aerosol indirect effect on cloud microphysical and radiative properties is one of the largest uncertainties in climate simulations. In order to investigate the aerosol–cloud interactions, a total of 16 low-level stratus cloud cases under daytime coupled boundary-layer conditions are selected over the southern Great Plains (SGP) region of the United States. The physicochemical properties of aerosols and their impacts on cloud microphysical properties are examined using data collected from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the SGP site. The aerosol–cloud interaction index (ACIr) is used to quantify the aerosol impacts with respect to cloud-droplet effective radius. The mean value of ACIr calculated from all selected samples is 0.145±0.05 and ranges from 0.09 to 0.24 at a range of cloud liquid water paths (LWPs; LWP=20–300 g m−2). The magnitude of ACIr decreases with an increasing LWP, which suggests a diminished cloud microphysical response to aerosol loading, presumably due to enhanced condensational growth processes and enlarged particle sizes. The impact of aerosols with different light-absorbing abilities on the sensitivity of cloud microphysical responses is also investigated. In the presence of weak light-absorbing aerosols, the low-level clouds feature a higher number concentration of cloud condensation nuclei (NCCN) and smaller effective radii (re), while the opposite is true for strong light-absorbing aerosols. Furthermore, the mean activation ratio of aerosols to CCN (NCCN∕Na) for weakly (strongly) absorbing aerosols is 0.54 (0.45), owing to the aerosol microphysical effects, particularly the different aerosol compositions inferred by their absorptive properties. In terms of the sensitivity of cloud-droplet number concentration (Nd) to NCCN, the fraction of CCN that converted to cloud droplets (Nd∕NCCN) for the weakly (strongly) absorptive regime is 0.69 (0.54). The measured ACIr values in the weakly absorptive regime are relatively higher, indicating that clouds have greater microphysical responses to aerosols, owing to the favorable thermodynamic condition. The reduced ACIr values in the strongly absorptive regime are due to the cloud-layer heating effect induced by strong light-absorbing aerosols. Consequently, we expect larger shortwave radiative cooling effects from clouds in the weakly absorptive regime than those in the strongly absorptive regime.
Highlights
Clouds play a critical role in the Earth’s climate by acting as the dominant modulator of radiative transfer in the atmosphere and have substantial impacts on the global climate
Taking the uncertainties of aerosol–cloud interaction index (ACIr) into account, the ACIr in the two absorptive regimes cannot be separated well, owing to the enhanced condensational growth process accompanied by a higher liquid water paths (LWPs) and the diminished cloud response to aerosols associated with different ωabs values
No significant change in the weakly absorptive regime is evident, and the ACIr in the strongly absorptive regime decreased from 0.12 to 0.10, partly owing to the enhanced condensational growth process accompanied by a higher LWP, and inhibits the impact of lower-tropospheric stability (LTS)
Summary
Clouds play a critical role in the Earth’s climate by acting as the dominant modulator of radiative transfer in the atmosphere and have substantial impacts on the global climate. X. Zheng et al.: Aerosol–cloud interactions under different absorptive aerosol regimes ment of cloud-droplet number concentration (Nd) and the reduction of cloud-droplet effective radii (re), which results in an increase in cloud albedo. It is necessary to investigate the aerosol and cloud properties as well as the magnitude of the ACI index at the ARM SGP site in order to (a) enhance the understanding of the ACI and (b) reduce the uncertainty in quantifying the ACI and associated radiative effects when modeling aerosol influences on low-level continental clouds. 2. The development and analysis of the ACI for the 16 selected cases, the aerosol activation and cloud microphysical responses, and consequent cloud radiative effects under different aerosol absorptive regimes are investigated in Sect.
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