A study of the microphysical mechanism for correlation patterns between droplet radius and optical thickness of warm clouds off the coast of California as simulated by a downscaling spectral bin microphysical model

Three-dimensional downscaling simulations using a Spectral Bin Microphysics model (SBM) were conducted to investigate effects of aerosols and dynamics of atmosphere on the microphysical properties of marine stratocumulus (Sc) off the coast of California. We implemented the regeneration process of aerosols into the bin type weather forecasting model named JMA-NHM+HUCM, and conducted sensitivity experiments to change how to treat the regeneration process of aerosols. Simulated results showed that the regeneration process is essential in SBM to reproduce the observed optical properties of Scfrom the satellite imager. We also conducted sensitivity experiments to examine how the correlation patterns between droplet radius and optical thickness of cloud (figure 1) are influenced by aerosol and dynamics of the atmosphere. These experiments showed that the amount of aerosol modify the correlation patterns through changing the cloud particle number concentrations, while dynamics of the atmosphere modifies correlation patterns through changing the liquid water path. The present study suggests that the successful reproduction of the characteristics of correlation pattern by previous two-dimensional idealized studies (Suzuki et al., 2006, 2010a, 2010b) is from two reasons. One is number of cloud in simulated domain. Previous study analyzed only a cloud and cloud property is sensitive to the environmental condition, whereas this study analyzed various cloud and cloud microphysical characteristics cannot be sensitive because each cloud buffer the effect. The other is simplicity of aerosol and dynamical condition in previous study. Previous study simulated uniform aerosol and dynamical condition, whereas this study simulated various dynamical and aerosol condition. Figure1: Scatter plot between effective radius and optical thickness of cloud