PNNL

Abstract

Marine stratocumulus clouds are the “global reflectors,” sharply contrasting with the underlying dark ocean surface and exerting a net cooling on Earth’s climate. The magnitude of this cooling remains uncertain, in part due to the averaged representation of microphysical processes such as the dropletto-drizzle transition in global climate models (GCMs). Current GCMs parameterize cloud droplet size distributions as broad, cloud-averaged gammas. Using digital holographic measurements of discrete stratocumulus cloud volumes, we find that cloud drop size distributions are narrow at the cm scale and never resemble the cloud average. These local distributions tend to form pockets of similar-looking cloud regions each characterized by a size distribution shape that is diluted to varying degrees. These observations open the way for novel modeling representations of microphysical processes.