Assessing Nonlinear Responses of Low-Level Warm Clouds Under the Impacts of Absorbing Aerosols Using the Cloud-Aerosol Mixing Ratio

Abstract. Air masses carrying absorbing aerosols play a dual role by altering ambient supersaturation and acting as cloud condensation nuclei (CCN), thus contributing to both the semi-direct and indirect aerosol effects. However, in real cloud development processes, aerosol influences can simultaneously act to suppress cloud formation or enhance droplet production, resulting in microphysical characteristics that remain difficult to describe. In the study, an aerosol and cloud microphysics experiment, focusing on warm cloud events strongly coupled with biomass-burning aerosols transported from the Southeast Asia Peninsula, was conducted in the spring of 2024 at the Lulin Atmospheric Background Station (LABS, 2862 m a.s.l) in Taiwan. One-minute resolution of in-situ data was used in the data analysis, allowing exploration of the microphysical responses under varying mixing states of absorbing aerosol concentration and cloud liquid water content. This study applies the cloud-aerosol mixing ratio (e.g., mass concentration mixing ratio, MCr) in conjunction with the ACI index to describe the behavior of aerosol-cloud interactions better. Results identify two distinct responses: (1) under high MCr conditions (MCr > 4.5), clouds exhibit features of the Twomey effect (ACI ≈ 0.08); (2) under low MCr conditions (MCr = 3–4.5), high aerosol loading coincides with elevated temperatures, lower relative humidity, and a reduction in small droplets (<10 μm), consistent with a semi-direct effect. This study offers a comprehensive explanation of how aerosols affect cloud systems in East Asia. It also underscores the crucial role of cloud-aerosol mixing ratios in characterizing nonlinear cloud microphysical responses.