A Large-Droplet Mode and Prognostic Number Concentration of Cloud Droplets in the Colorado State University Regional Atmospheric Modeling System (RAMS). Part I: Module Descriptions and Supercell Test Simulations

The microphysics module of the version of the Regional Atmospheric Modeling System (RAMS) maintained at Colorado State University has undergone a series of improvements, including the addition of a large-clouddroplet mode from 40 to 80 mm in diameter and the prognostic number concentration of cloud droplets through activation of cloud condensation nuclei (CCN) and giant CCN (GCCN). The large-droplet mode was included to represent the dual modes of cloud droplets that often appear in nature. The activation of CCN is parameterized through the use of a Lagrangian parcel model that considers ambient cloud conditions for the nucleation of cloud droplets from aerosol. These new additions were tested in simulations of a supercell thunderstorm initiated from a warm, moist bubble. Model response was explored in regard to the microphysics sensitivity to the largedroplet mode, number concentrations of CCN and GCCN, size distributions of these nuclei, and the presence of nuclei sources and sinks.

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