Improvement of the parameterization of the initial cloud droplet size distribution in a convective cloud model with detailed microphysics

In a previous study, Yang and Yum (2007) investigated the CCN effects on the cloud and precipitation developments under different thermodynamic conditions, using the Takahashi's 2D convective cloud model with detailed microphysics. Two different maximum CCN concentrations, maritime (300 cm-3) and continental (1000 cm-3), were used as input CCN data and the model was run on eight thermodynamic conditions obtained from observational soundings. Generally, a decrease in CCN number concentration enhanced cloud and precipitation developments but the CCN effects also critically depended on the given thermodynamic conditions. However, the Takahashi model has two major problems - one is that the initial droplet distribution is prescribed and the other is the excessive latent heat release to accommodate the generation of initial cloud droplets, especially when CCN concentrations are extremely high. Here we implement an improved method to parameterize the nucleation process, where the newly activated droplet distribution is determined by the supersaturation: as the supersaturation becomes higher, activated droplet concentration becomes higher but the size distribution is shifted toward the smaller sizes. Unlike the model runs with the original setting that showed more intense cloud development for the extreme continental CCN (5000 cm-3) than for the continental CCN for some thermodynamic conditions, the model runs with the improved parameterization of the nucleation process produced weaker convective clouds and slower onset of precipitation for the extreme continental clouds than for the continental as well as maritime clouds. As before, the CCN effects are more prominent in warm clouds than in cold clouds. The improved method will be further highlighted and the contrasts of the cloud and precipitation developments for the three CCN concentrations will be closely examined at the conference.

Yang, Hee-Jung, and Seong Soo Yum, 2007, Effects of prescribed initial cloud droplet spectra on cloud and precipitation developments under different thermodynamic conditions: a modeling and observational study. Atmos. Res., doi:10.1016/j.atmosres.2007.04.004