Overlaying the cool southeast Pacific Ocean is the most persistent subtropical stratocumulus deck in the world. These clouds are a source of significant negative radiative forcing, and they play a supporting role in the seasonal cycle of the East Pacific Ocean and El Nino-Southern Oscillation, but they have proven difficult for climate models to simulate. The EPIC 2001 stratocumulus study conducted the first major in-situ investigation of cloud and boundary layer processes in this region. Its main goals were (1) to characterize the cloud and boundary layer structure, compared to the better studied northeast Pacific stratocumulus region, (2) to investigate the importance of drizzle in regulating cloud thickness and albedo, and (3) to provide a context for understanding long-term measurements from a flux-reference buoy at 20 S, 85 W, and for testing and improving climate model parameterizations in stratocumulus regions. Such parameterizations play key roles in determining the sensitivity of climate to anthropogenic perturbations because they determine cloud feedbacks on climate change.

Comprehensive ship-based remote sensing and surface measurements were taken during a two week cruise in October 2001. These included three-hourly soundings, a scanning 5 cm and a vertically-pointing 8 mm radar, a lidar and radiometer for examining clouds and precipitation, and measurements of aerosols and surface turbulent and radiative fluxes. A well-mixed boundary layer with a regular, pronounced diurnal cycle of cloud thickness and boundary layer depth was observed, modulated by an unexpectedly strong remotely-forced diurnal cycle of subsidence. Mesoscale organization of cloud and drizzle was ubiquitous, and nighttime drizzle was substantial except in the more polluted near-coastal region. This integrated dataset is attractive for comparisons with both climate and detailed process models.