Cold pools and aerosols under stratocumuli

Modeling studies have investigated aerosol-cloud precipitation feedbacks associated with drizzle cells in stratocumuli, but the precise mechanisms remain uncertain. The formation of drizzle drops is expected to be a major sink for accumulation mode particles, but evaporation of these drizzle drops in the subcloud layer might increase the number of coarse and even giant aerosol particles. Cold pools are observed to form under strongly drizzling stratocumuli from the evaporative cooling of precipitation that occurs below the clouds. Understanding the microphysical contrasts between these cold pools and the surrounding air can help increase understanding of possible microphysical feedbacks of drizzle on the stratocumulus.

During the VOCALS Regional Experiment, 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 investigate the microphysical changes in the context of cloud and thermodynamic properties inside and outside of the cold pools.

Composites of size-resolved aerosol properties suggest that there is an increase of approximately 50% in the number concentration of coarse mode particles (diameter > 1 micrometer) inside the cold pools. While the concentration of particles with diameters of 0.1-1 micrometer also show some increase, the relative changes are much smaller (of order 10%). There are a number of possible explanations for the observed increase of larger particles: increased stability associated with stratification in the lower subcloud layer, increased relative humidity inside cold pools, and increased surface flux of aerosols from an increase in wind speed. We investigate whether these explanations are quantitatively consistent with the observed thermodynamic and microphysical characteristics inside and outside of the cold pools.