Lagrangian condensation microphysics with Twomey CCN activation

Since a cloud volume is a small fraction of the computational domain volume, the Twomey super-droplets provide significant computational advantage when compared to the original superdroplet methodology. Additional advantage comes from significantly longer time steps that can be used when modeling of CCN deliquescence is avoided. Moreover, other formulation of the droplet activation can be applied in case of low vertical resolution of the host model, for instance, linking the concentration of activated cloud droplets to the local updraft speed.

This paper discusses the development and testing of the Twomey super-droplet methodology, focusing on the activation and diffusional growth. Details of the activation implementation, transport of super-droplets in the physical space, and the coupling between super-droplets and the Eulerian temperature and water vapor field are discussed in detail. Some of these are relevant to the original superdroplet methodology as well and to the ice phase modeling using the Lagrangian approach.

As a computational example, the scheme is applied to an idealized moist thermal rising in a stratified environment, with the original superdroplet methodology providing a benchmark to which the new scheme is compared.