Characterizing cold pools underneath marine stratocumulus using data from VOCALS REx

Previous modeling and observational studies have reported the formation of cold pools from the evaporation of drizzle under stratocumuli. Their importance to the marine boundary layer dynamics and to cloud and precipitation structures remains uncertain. During the VOCALS Regional Experiment in 2008, the NCAR C-130 flew fourteen research flights to characterize the stratocumulus-topped marine boundary layer in the southeast Pacific. We use data from straight and level legs in the subcloud layer (150 m altitude) to identify cold pool features. We then construct composites of changes in thermodynamic, microphysical, and cloud properties for these cold pools to gain a better understanding of their formation, dynamics, and dissipation, and their potential impact on cloud and drizzle organization.

The 25th and 75th percentile values of observed cold pool size range from 1.5 to 18.5 kilometers. The composites show that the cold pools are universally moister than the surrounding air, and the pools tend to be associated with precipitating regions of the marine boundary layer. The moistening inside the cold pool results in elevated values of equivalent potential temperature inside the cold pools, consistent with previous studies. The composites also show horizontal wind convergence and positive vertical winds at the edges of cold pool. Satellite data are used to examine the cloud structural properties associated with cold pools. In most cases, cold pools are found where the stratocumulus deck becomes broken.