Precipitation Susceptibility and Effects of Giant Cloud Condensation Nuclei Artificially Introduced into Sratocumulus Clouds

To study the effect of giant cloud condensation nuclei (GCCN) on precipitation processes in stratocumulus clouds, 1–10 μm diameter salt particles (salt powder) were released from an aircraft while flying near the cloud top on 3 August 2011 off the central coast of California. The seeded area was subsequently sampled from the aircraft that was equipped with aerosol, cloud, and precipitation probes and an upward-facing cloud radar.

During post-seeding sampling, made 30–60 min after seeding, the mean cloud droplet size increased, the droplet number concentration decreased, and large drop (e.g., diameter larger than 10 μm) concentration increased. Average drizzle rates increased from about 0.05 to 0.20 mmh^-1, and the liquid water path decreased from about 52 to 43 gm^-2. Strong radar returns associated with drizzle were observed on the post-seeding flights and were accompanied by a substantial depletion of the cloud liquid water content. The changes were large enough to suggest that the salt particles resulted in a four-fold increase in the cloud-base rainfall rate and depletion of the cloud water due to rainout. Further, the sensitivity of precipitation response to aerosol loading (precipitation susceptibility) reduced to nearly zero during the post-seeding flights. In contrast, a case is shown where the cloud was already precipitating (on 10 August) and the effect of adding GCCN to the cloud was insignificant.