Modeling of cloud-aerosol interactions in warm boundary-layer clouds

The impact of soluble atmospheric aerosols acting as cloud condensation nuclei on microphysical processes within shallow convective clouds is well appreciated by the cloud physics community. More recently, such effects have attracted considerable attention of the climate community because of their potentially significant indirect effects on the Earth climate and geoengineering proposals. Impact of CCN on cloud microphysics and the development of drizzle/rain are relatively well understood at the process level and modeling techniques do exist to include such effects in large-eddy simulation (LES) models. The latter can be accomplished by applying, for instance, a double-moment warm-rain microphysics scheme or bin microphysics. There is also a mounting evidence that processing of aerosols by such clouds may feed back into the dynamics and result in profound changes of cloud structures. Perhaps the most striking and poorly understood example of such interactions are pockets of open cells (POCs) within otherwise solid subtropical stratiform decks. However, current approaches to simulate aerosol processing by precipitating clouds either involve significant simplifications (and thus offer incomplete picture of cloud-aerosol interactions) or apply approaches that are computationally inefficient (e.g., a two-dimensional bin microphysics, with the first dimension spanning aerosol radius and the second dimension spanning the drop radius).

This paper will present two contrasting approaches to simulate aerosol processing in bin-resolved microphysics. The first one is the Lagrangian Cloud Model, a mixed Eulerian/Lagrangian approach to atmospheric LES, with a two way coupling between Eulerian dynamics and thermodynamics and Lagrangian microphysics. Since Lagrangian representation of microphysics does not suffer from numerical diffusion in the radius space and solves full droplet growth equations, it may be considered an alternative for the bin approach. The Lagrangian Cloud Model has recently been extended to include collision/coalescence. The second approach in a novel aerosol-based bin microphysics, where the bins represent aerosol size (radius or mass), and not the drop size as in traditional bin schemes. Similarly to LCM, such an approach allows a faithful representation of aerosol processing by a drizzling/precipitating cloud. The paper will provide a brief overview of both techniques and examples of their applications to idealized simulations of warm boundary-layer clouds focusing on the aerosol processing.