Analysis of particle size distribution and scattering characteristics of water cloud in condensation and coalescence growth

Abstract

Study of scattering characteristics of water cloud is of great significance for weather forecasting, meteorological disaster warning, weather modification and research of radiation transmission in the lower atmosphere. In this paper, particle size distribution and scattering characteristics of water cloud in condensation and coalescence growth are studied by numerical simulation. The particle size distribution model of water cloud in the condensation growth and coalescence growth are established respectively. The dynamic process of the particle size distribution of water cloud in the condensation growth, the coalescence growth and the condensation-coalescence combination growth are analyzed. Then the scattering characteristics of water cloud in the droplet growth are studied with the Mie theory. The results show that with the condensation growing the full width at half maximum of particle size distribution, the effective radius and mode radius of water cloud increase continuously. The effective radius increases in the coalescence growth process and there are multiple peaks in the particle size distribution in the coalescence growth anaphase. The average growth rate of the effective radius of cloud droplets is 8 nm/s in the condensation-coalescence combination growth. The extinction coefficient and scattering coefficient of water cloud increase linearly with time increasing during the condensation growth or the coalescence growth. In the condensation-coalescence combination growth, the extinction coefficient and scattering coefficient increase exponentially with time increasing except at a wavelength of 3.2 mm; the asymmetric factors at the wavelengths of 1.064, 2.2, 3.7, 12 and 22 μm tend to be stable, while the asymmetric factor at the wavelength of 3.2 mm remains the same basically. Meanwhile, the lidar ratio at each of the wavelengths of 1.064 μm and 2.2 μm fluctuates near 20 sr, and that at the wavelength of 3.7 μm fluctuates greatly. In the growth process of cloud droplet, the single scattering albedo of water cloud decreases gradually at each of the wavelengths of 1.064, 2.2 and 3.7 μm, while it increases gradually at each of the wavelengths of 12, 22, 200 and 3.2 mm. The absolute value of Ångström exponent decreases gradually, which means that the wavelength-dependence of extinction coefficient decreases with cloud droplet growing. These research results reveal the change law of particle size distribution and the scattering characteristics of water cloud in condensation and coalescence growth. The results provide important reference for forecasting weather, studying earth-atmosphere radiation balance and correcting remote sensing data.