Applying super-droplets as a compact representation of warm-rain microphysics for aerosol-cloud-aerosol interactions

Aerosols provide nuclei on which cloud droplets are formed (cloud condensation nuclei - CCN). Hence, aerosol size spectrum and its chemical composition influence the microphysical properties of clouds. Clouds may, in turn, influence aerosol characteristics of their environment. The relevant processes include wet deposition (rainout or washout) and CCN recycling through evaporation of cloud droplets and drizzle drops. Recycled CCN may have altered physicochemical properties if the evaporated droplets go through collisional growth or irreversible chemical reactions (e.g. SO2 oxidation). The main challenge of representing these processes in a numerical cloud model stems from the need to track the properties of activated CCN throughout the cloud lifecycle. Lack of such "memory" characterizes the so-called bulk, multi-moment as well as bin representations of cloud microphysics.

In this study we apply the particle-based scheme of Shima et al. 2009. Modeled particles (aka super-droplets) are a numerical proxy for a multiplicity of real-world CCN, cloud, drizzle or rain particles of the same size, nucleus type, and position. Tracking cloud nucleus properties is an inherent feature of the particle-based frameworks, making them suitable for studying aerosol-cloud-aerosol interactions. Moreover, the super-droplet approach is characterized by linear scalability in the number of computational particles, and no numerical diffusion in its condensational and collisional (Monte-Carlo) growth schemes.

We will present simulations using a kinematic model of a stratocumulus cloud. The simulations are carried out using a setup inspired by the VOCALS campaign (Grabowski& Lebo, see http://www.rap.ucar.edu/~gthompsn/workshop2012/case1/). The initial conditions are defined by the dry aerosol size spectrum and the vertical profiles of the temperature and humidity. The prescribed flow mimics a single eddy spanning from the sea surface up to the inversion. Thus, within the simulated domain both updraft and downdraft regions are present. Employment of a kinematic framework allows (and limits) to focus solely on the microphysical aspects of aerosol-cloud-aerosol interactions, excluding any coupling with cloud dynamics, and simplifying the underlying numerics.

The super-droplet approach features particle-level formulation of condensational growth (including CCN activation and evaporation), and collisional growth. This presentation will be focused on representation of aqueous-phase chemistry within the super-droplet framework. This extension is being implemented with the aim to cover processes leading e.g. to presence of the so-called "Hoppel gap" in the size spectrum of the cloud-processed aerosol.