Improvement of Takahashi's cloud model to simulate the CCN effects on convective cloud and precipitation development

The increase in Cloud Condensation Nuclei (CCN) concentration due to human activity may change cloud microphysical properties. First cloud droplet number concentration may increase, which is often accompanied by a reduction in droplet sizes, thereby reducing cloud precipitation efficiency. This may result in prolonged cloudiness, which gives rise to the so called “Albrecht” effect or the second indirect effect (Albrecht, 1989, Science). However this chain of reasoning may not always work. Especially when clouds include ice particles, the evolution of cloud and precipitation becomes very complicated. For example, for multi-cell cloud systems developing at high convective available potential energy (CAPE) conditions high CCN concentration might lead to more total precipitation and higher updraft velocities. Even for similar CAPE, the details of entire thermodynamic structure might lead to significantly different precipitation amount (Yang and Yum, 2007, Atmos. Res.).

This study aims first at improving the cloud model originally developed by Takahashi (Takahashi, 1976, J. Atmos. Sci.) and secondly at examining CCN effects on convective cloud and precipitation development in a more thorough manner than Yang and Yum (2007) did. Although bin microphysics is employed, this model assumes a prescribed initial droplet spectrum. One problem is that the prescribed initial droplet spectrum is too broad and fixed although supersaturation condition may change. New approach assumes that newly activated droplets have an arbitrarily chosen small size and the concentration of these droplets is determined by supersaturation following the Twomey's formulae. Collision efficiency data are also updated to more recent values. We find that these changes induce significantly different results from the previous calculations for some of the thermodynamic conditions examined (Yang and Yum, 2007). This reveals the importance of the model treatment of the physical processes occurring in the cloud. More detail will be discussed at the conference.