A novel, open-source library of bulk and particle-based warm-rain cloud microphysics schemes

This paper will introduce libcloudph++, a library of algorithms for representing cloud microphysics in numerical models. In the current release, libcloudph++ features implementations of three distinct warm-rain cloud microphysics schemes. The first two schemes are the so-called single-moment bulk and double-moment bulk schemes that belong to the Eulerian class of methods. However, the main highlight of the library is the novel Lagrangian particle-based scheme, where the so-called computational particles are tracked through the model domain. Information associated with those particles travels along their trajectories and the local properties of a given volume can be diagnosed taking into account the properties of particles contained within it. The coalescence in the particle-based scheme is represented by a Monte-Carlo scheme (so-called Super-droplet method) and the aerosol composition is described by the kappa-Koehler parameterization. In the library, Lagrangian scheme is used to represent the dispersed phase (atmospheric aerosol, cloud droplets, rain drops), while the continuous phase (moisture, heat) is represented with the Eulerian approach.

The library is designed with the aim of creating a collection of algorithms to be used within models of different dimensionality, different dynamical cores, different parallelization strategies, and in principle models written in different programming languages. The library code and documentation are released as free (meaning both gratis & libre) and open-source software. Releasing the software as a library-type package enables its users to easily benefit from developments of other researchers (by gaining access to enhancements, corrections, or entirely new schemes). Library approach facilitates collaboration and reduces development time and maintenance effort by imposing separation of cloud microphysics logic from other source code components such as model dynamical core or parallelization logic. Such strict separation is also a prerequisite for genuine software testing.

A two-dimensional prescribed-flow single-eddy framework will be used as a testbed to present microphysical schemes implemented in the library. Discussion will focus on the particle-based scheme and on comparisons of its capabilities and limitations with respect to the bulk schemes.