Open cellular structure in marine stratocumulus sheets

Geostationary and Sun-synchronous satellite data and in situ observations from ship cruises are used to investigate the formation of open cellular structure in marine stratocumulus clouds over the southeast Pacific (SEP). Open cellular convection either forms spontaneously as pockets of open cells (POCs) within overcast stratocumulus, or is advected into the region from midlatitude regions. POC formation occurs most frequently during the latter part of the night, demonstrating that this transition is not caused by solar absorption-driven decoupling. The transition preferentially occurs in clouds with low 11-3.9 mu m nighttime brightness temperature difference (BTD) which is found to be well correlated with both in situ measured accumulation mode aerosol concentration and cloud droplet concentration estimates derived from MODIS. Besides indicating that nighttime BTD is an excellent proxy for stratocumulus cloud droplet concentration N-d, this also suggests that low aerosol concentrations favor POC formation. Indeed, extremely low accumulation mode aerosol concentrations are found during the passage of open cell events over the ship. Free-tropospheric moisture is not found to be an important factor in POC formation. Significant subseasonal variability occurs in the fractional coverage of open cellular convection over the broader SEP. This coverage is well correlated with a MODIS-derived drizzle proxy (MDP) proportional to the ratio of liquid water path (LWP) to N-d for predominantly overcast regions. Both LWP and N-d variability influences the MDP. Periods of low MDP have significant positive large-scale N-d anomalies and are preceded by offshore winds at 850 hPa, which suggests a potential continental influence upon open cell formation over the SEP. Together, the results suggest important two-way interactions between aerosols and drizzle in marine stratocumulus and a role for drizzle in modulating the large-scale albedo of these cloud systems.