The Impact of Giant Cloud Condensation Nuclei on Drizzle Formation in Stratocumulus: Implications for Cloud Radiative Properties

Abstract

The impact of giant and ultragiant cloud condensation nuclei (>5-μm radius) on drizzle formation in stratocumuli is investigated within a number of modeling frameworks. These include a simple box model of collection, a trajectory ensemble model (comprising an ensemble of Lagrangian parcel models), a 2D eddy-resolving model, and a 3D large-eddy simulation model. Observed concentrations of giant cloud condensation nuclei (GCCN) over the ocean at ambient conditions indicate that 20-μm radius haze particles exist in concentrations of between 10⁻⁴ and 10⁻² cm⁻³, depending on ambient wind speed and seastate. It is shown that these concentrations are sufficient to move a nonprecipitating stratocumulus into a precipitating state at typical cloud condensation nucleus (CCN) concentrations of 50 to 250 cm⁻³, with higher concentrations of GCCN being required at higher CCN concentrations. However, at lower CCN concentrations, drizzle is often active anyway and the addition of GCCN has little impact. At high CCN concentrations, drizzle development is slow and GCCN have the greatest potential for enhancing the collection process. Thus, although drizzle production decreases with increasing CCN concentration, the relative impact of GCCN increases with increasing CCN concentration. It is also shown that in the absence of GCCN, a shift in the modal radius of the CCN distribution to larger sizes suppresses drizzle because larger modal radii enable the activation of larger droplet number concentrations. Finally, calculations of the impact of GCCN on cloud optical properties are performed over a range of parameter space. Results indicate that the presence of GCCN moderates the effect of CCN on optical properties quite significantly. In the absence of GCCN, an increase in CCN from 50 to 150 cm⁻³ results in a threefold increase in albedo; when GCCN exist at a concentration of 10⁻³ cm⁻³, the increase in albedo is only twofold. Thus the variable presence of GCCN represents yet another uncertainty in estimating the influence of anthropogenic activity on climate.