Simulation Study on Influencing Factors of Cloud Optical ThicknessSimulation Study on Influencing Factors of Cloud Optical Thickness

Cloud optical thickness impacts the radiation balance of the earth–atmosphere system and is a key factor in climate change prediction. Furthermore, variation in cloud supersaturation predominantly relies on the physicochemical and activation characteristics of aerosols and the vertical velocity of updraft. These factors affect the activation process and condensation growth process within the cloud, ultimately changing the cloud optical thickness. Based on the simulation results of the adiabatic bubble model, the influences of vertical velocity, aerosol number concentration, and aerosol chemical composition on the cloud optical thickness were studied. The simulation results can reproduce the first indirect radiation effect of aerosols and show a positive correlation between cloud optical thickness and vertical velocity. When the liquid water content in the cloud remained constant, increasing the vertical velocity and aerosol number concentration increased the cloud droplet number concentration while reducing the cloud droplet effective radius. This phenomenon increased the total surface area of cloud droplets, ultimately enhancing the cloud albedo. However, with the rapid increase in vertical velocity and aerosol number concentration, the growth rate of cloud droplet number concentration and the cloud droplet effective radius slowed down simultaneously, consequently leading to a reduction in the growth rate of the cloud droplet total surface area, cloud albedo, and cloud optical thickness. Moreover, when the aerosol chemical composition comprises organic carbon, ammonium sulfate, and sea salt at the same aerosol number concentration, the cloud droplet effective radius decreases, leading to an increase in the total surface area and a consequent increase in the optical thickness of the cloud. The findings of this work have clarified the influence mechanism of the aforementioned factors on cloud optical thickness and deepened the theoretical understanding of the first indirect effects of aerosols.