Southeast Pacific Stratocumulus Clouds, Precipitation and Boundary Layer Structure Sampled Along 20 S During VOCALS-REx

The VOCALS-REx field campaign in Oct.-Nov. 2008 provided a wealth of airborne in-situ and remote sensing observations of the SE Pacific stratocumulus-topped boundary layer. One goal was to document its mean structure and variability along 20S, from the Chilean coast at 70W to a WHOI stratus climate reference buoy 1600 km offshore at 85W, across a large climatological gradient in aerosol concentrations and a significant gradient in boundary layer structure. Two aircraft, the NSF C-130 and the UK BAe146 flew dedicated flight patterns alternating between legs sampling near the surface, within the cloud, and above the capping inversion. The C-130 carried the University of Wyoming cloud radar and upward pointing lidar, and the BAe146 dropped sondes from 6 km. 4 C-130 flights sampled the full 1600 km, and 6 other C-130 flights and 5 BAe flights sampled 1000 km offshore. An ONR Twin Otter also frequently sampled 150 km offshore, and radiosondes at Iquique documented the coastal structure. These data complements Ron Brown ship observations along this transect. The airborne observations were from a few specific times of day but provide broad spatial coverage, while the ship observations document the prominent diurnal cycle. The resulting dataset provides a detailed transect along 20S of the vertical structure of the boundary layer and lower troposphere, turbulence statistics, cloud and aerosol properties, and drizzle. These are excellent comparisons for large scale models and satellite retrievals, and also demonstrate relationships of intrinsic scientific value.

Some key findings follow. The boundary layer was universally capped by a strong, sharp inversion, typically 10-12 K. The inversion was around 1000 m near the coast and rose offshore to typically 1600 m at the buoy. The near-surface LCL was typically about 800-900 m at all longitudes. This was associated with a thin Sc layer near the coast, well mixed at night but less so during the day, and Cu rising into a decoupled Sc layer more than 500 km offshore. The aerosol gradients also maximized 500 km offshore, with nearly pristine conditions common at the buoy. Radar observations showed that in the offshore regime, drizzling mesoscale cells of varying strengths were nearly ubiquitous, even when cloud cover was 100%. Since peak cloud liquid water path was much larger offshore than near to the coast, the offshore drizzle cannot be entirely attributed to the lack of aerosol. The persistence of stratocumulus in this region is partly due to extreme cloudtop longwave cooling of over 100 W m-2 under the extraordinarily dry air aloft, which promoted strong turbulence in the stratocumulus layer. Relative humidities of 1% or less were common above the inversion offshore. Near the coast, the free-tropospheric air was often moister, with an origin over S America, resulting in less cloud-top cooling. More than 200 km offshore, the air above the boundary layer typically contained very few accumulation-mode aerosol particles. REx observations suggest that the nearshore increase in cloud droplet concentration stems from anthropogenic aerosols injected into the boundary layer in and near the coast, rather than being a signal entrained from above.