What Controls Drizzle Initiation in Marine Stratocumulus?

Collisional growth in marine stratocumulus is the key process in the formation of drizzle, and thus in controlling the moisture and energy budgets, and radiative properties of such clouds. However, there exists substantial variability in microphysical properties of drizzle on length scales as small as tens of meters. The goal of this study is to better identify the processes that lead to collisional growth at cloud top, which is responsible for triggering drizzle and causing its variability. To achieve this goal, aircraft measurements at cloud top in marine stratocumulus from the VOCALS (VAMOS Ocean Cloud Aerosol Land Study) are analyzed to examine the relationship between collisionally-formed drops and dynamic (updraft velocity, turbulence), thermodynamic (temperature, moist static energy), and microphysical (liquid water content, properties of the drop size distribution) properties at cloud top. Our first finding is that collisionally-formed drops are found in air parcels that have resided at or near cloud top for longer times. Our second major finding is that the length scale of these air parcels is on the order of 1 m. This length scale is considerably smaller than that used in most large-eddy simulations of stratocumulus. These results form a picture where in these clouds, drizzle initiation is governed by a subset of small air parcels (order one cubic meter in volume) that randomly persist at or near cloud top for durations much longer than average such that drops within this volume can experience more collisions.