The GASS Microphysics Intercomparison Project: How Different Are Aerosol-Cloud-Precipitation Interactions from a Variety of Microphysics Schemes?

Differences between aerosol characteristics in areas of closed and open cells within stratocumulus over the subtropical eastern Pacific have been observed. This is important because open and closed cells have dramatically different cloud characteristics, with almost 100% cloud cover for the closed cells and significantly lower (below 20%) cloud cover for the open cells. Aerosol processing at the boundary between open and closed cells has been argued to play an important role, perhaps even control, the transition from open- to closed-cell circulations. Moreover, the effects of aerosol processing are thought to lead to the formation of ultra clean layers in the marine boundary layer.

In this work we present a report on the GASS microphysics project and the International Cloud Modelling Workshop projects, i.e. the Kinematic Driver – Aerosol (KiD-A) project. This project was motivated by the aforementioned differences between closed and open cell within stratocumulus and the specific aim of this project is to better understand in-cloud aerosol processing as well as investigate how aerosol processing depends on the numerical techniques. In order to achieve this aim, KiD-A uses a publicly available kinematic framework to compare langrangian microphysics schemes, spectral bin microphsyics schemes and bulk microphysics schemes. This is the first time such an intercomparison has taken place. The preliminary results indicated generally good agreement between all models in the absence of sedimentation and collision-coalescence. However, when all processes occur, the models vary substantially, with the more detailed models exhibiting a much larger range in cloud characteristics (e.g., liquid water path) than that of the simpler models. Subsequent work has been performed to elucidate the cause of the large model spread by closely examining the drop size distributions predicted by the different models as well as the representation of collection and sedimentation in the models. In this presentation these new results will be discussed.