Microphysical and kinematic structures within drizzling stratocumulus in the southeast Pacific

Vast regions of subtropical stratocumulus (Sc) clouds are a significant source of cooling in the earth's radiation budget. Southeast Pacific Sc also play a supporting role in the seasonal cycle of the East Pacific Ocean and the El Nino-Southern Oscillation. Yet these clouds have proven difficult for climate models to simulate. The role drizzle plays in the dynamics of the Sc-topped boundary layer is also uncertain and its parameterization difficult. Observations obtained during the 2001 East Pacific Investigation of Climate stratocumulus study (EPIC Sc) were designed to provide insight into the Sc physical processes with an eventual goal of improving model parameterizations. The unique EPIC data set combines comprehensive surface and ship-based remote sensing measurements and includes high temporal and spatial resolution data from both a vertically-pointing millimeter cloud radar (MMCR) and a scanning C-band radar.

Recent findings based on ship and radar data from EPIC Sc show the importance of mesoscale (10-100 km) variability in both cloud and drizzle properties. To further these results, we use the C-band radar's RHI scans to characterize the detailed vertical structure of drizzle cells, including cells in all stages of their life cycle. Our findings are compared with information derived from the MMCR time height cross sections.

Preliminary results indicate that strong drizzle cells (reflectivity cores greater than 15 dBZ) from both open-cell and closed-cell cloud regimes have quite similar microphysical and kinematic properties. Within drizzle cells, precipitation growth is evident from cloud top to just below cloud base. Patterns of convergence near cloud base and divergence near cloud top are accompanied by a shear layer in the upper 200-300 m of cloud. In contrast, echo regions outside of clearly defined drizzle cells show more pronounced shear through a deeper layer. Because drizzle potentially plays an important role in modulating the mesoscale circulations in Sc-topped boundary layers, these statistical drizzle-cell features are important for cloud-resolving and other models to reproduce.