Speaker is Prof. Raymond Shaw (Michigan Technological University).

Abstract:
Macroscale dynamics and thermodynamics dictates the amount of condensed mass in a cloud, but in order to know cloud optical and precipitation properties, we have to know how that mass is distributed amongst droplets and crystals. There is no escape: microphysics matters for macroscopic cloud properties of relevance to climate. Over the last decade we have explored microphysical aspects of aerosol-cloud interactions using the “Pi convection-cloud chamber” – a pi-cubic-meter turbulent cloud generated by feeding aerosols into a water-supersaturated environment created by moist Rayleigh-Benard convection. All microphysical processes that are usually considered “sub-grid-scale” even in detailed models, are fully represented, so we can think of this as a single “cloud pixel.” The resulting cloud microphysical properties are similar to those observed in stratocumulus, and the Pi chamber has provided insight into the role of turbulence on cloud droplet activation and growth, the formation of persistent mixed-phase clouds, and the cloud dispersion indirect effect. Much of this research supports the conceptual picture of limiting regimes with “fast” versus “slow” microphysics compared to turbulent mixing time scales. A minimalist model describes scaling laws for cloud droplet radius, number concentration, and liquid water content for these regimes. This opens a path to adapting the scaling laws to stratocumulus clouds in the atmosphere.