The Impact of Cloud Dynamical Processes on Cloud Droplets in Parameterized Shallow Cumulus

Cumulus clouds are strongly diluted by entrainment mixing. Interactions between turbulent and microphysical processes are relevant to indirect effects of aerosols on climate. For instance, cloud radiative effects depend on cloud droplet number concentrations and sizes. Entrainment of air dilutes cloud droplet number concentrations and liquid water contents and may also affect the activation of aerosol particles. Despite their great potential significance, interactions between turbulent and microphysical processes are either omitted or highly simplified in climate models. Here, a parameterization for non-homogeneous transient shallow convection (von Salzen and McFarlane, 2002) is extended to study the sensitivity of cloud droplets to homogeneous and extremely inhomogeneous mixing processes. The new parameterization is based on a combination of a highly efficient prognostic treatment for aerosol activation and a diagnostic representation of the effects of mixing on cloud droplets. Observations of thermodynamic and microphysical properties of cumulus clouds as well as microphysical and chemical properties of aerosol particles are used for tests of the parameterization in a single column model. The observations are taken from the Canadian airborne cloud measurement program that was part of the ICARTT intensive in the summer of 2004.