2CLOUDPHYS Design of a Convection-Cloud Chamber for Exploring Condensational and Collisional Growth Processes Relevant to Precipitation Formation

The formation of drizzle depends on microphysical interactions between clouds droplets and aerosol particles, including activation, growth by condensation, and growth by coalescence. The latter is often parameterized as autoconversion. Understanding these interactions is challenging because the atmosphere is ever changing and the initial and boundary conditions are poorly constrained. Turning to the laboratory is one way to make progress. In a convection-cloud chamber, a cloud is generated by feeding aerosols into a water-supersaturated environment created by moist convection driven by a temperature gradient. All microphysical processes that are usually considered sub-grid-scale even in detailed models, are fully represented. The cloud microphysical and optical properties produced in a convection-cloud chamber are similar to those observed in stratocumulus. A consortium of universities, national laboratories, and industry/international partners, with support from DOE and NSF, has developed a design for a large convection-cloud chamber able to study the physics of processes involving long residence times such as drizzle initiation and aerosol/cloud chemistry. The proposed height of approximately 10 m provides sufficiently large droplet residence times that aqueous-phase processing of aerosol and growth by collision-coalescence can be investigated. These capabilities will allow interactions within a fully-coupled aerosol-cloud-drizzle system to be explored and directly compared to detailed models. Effort has been focused on modeling and simulation related to the design of the chamber, development of measurement methods suitable for resolving chemical and physical processes of interest in such a large chamber, and technical engineering aspects of the chamber design. Specific examples to be shown include modeling of aerosol processing and collisional growth in the large chamber, new measurement methods for detecting aerosol activation and the existence of rare, large drizzle drops, and technologies for high-rate aerosol production and for maintaining appropriate chamber boundary conditions. This effort also informs the use of a large chamber for analogous ice and mixed-phase processes that depend on collisional growth.