VOCALS: C-130 Observations and Numerical Modeling of Marine Boundary Layer Clouds during the VOCALS Regional Experiment (REx)

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The sparsely observed Southeast Pacific Ocean is covered by the world's largest subtropical sheet of stratocumulus clouds. Climate models poorly simulate these clouds with consequences for the global climate sensitivity and for their simulations of how atmospheric particles affect the climate. While model simulations are improving, better model treatments of clouds and particles are limited by gaps in the understanding of the interactions of clouds, aerosols, and drizzle. This project is a combination of modeling and observational syntheses that will use VOCALS-REx (the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment) to address these gaps. (VAMOS is an international science project, the Variability of the American Monsoon System).This study is a core project in VOCALS REx, which will take place over the Southeast Pacific Ocean during Oct-Nov 2008. Intensive observations will be made from aircraft, ships, and land sites. These data will be used to address hypotheses organized into two themes: aerosol-cloud-precipitation interactions in the marine boundary layer and chemical and physical couplings between the upper ocean, the land, and the atmosphere.The focus of this project is on data collected from the NSF C-130 aircraft to test hypotheses related to the interactions between aerosols, clouds and precipitation in stratocumulus clouds over the Southeast Pacific. Data from the C-130 and other platforms will be synthesized to understand the processes that determine the quantity of precipitation falling from low clouds, including the impact of aerosols, and mechanisms by which drizzle influences the dynamics and thermodynamics of the near-surface atmosphere, cloud structural and optical properties, and aerosol microphysics.The sampling strategy for the C-130 aircraft involves two distinct types of flights:. Cross-Section missions will involve flying cross-sections along 20 degrees South from the Northern Chilean coast out, across strong gradients in particle concentrations and cloud properties, to the climatological maximum in stratocumulus cloud cover at approximately 85 degrees West. These missions will build up a picture of the mean and variability in cloud and precipitation properties in the marine boundary layer, and aerosol physical and chemical properties both in and above that layer.. POC-Drift missions will study the microphysical and dynamical state and processes occurring in pockets of open cells (POCs) and contrast them with surrounding overcast stratocumulus. Flights will be coordinated with other participating aircraft and with the NOAA research vessel Ronald H Brown. The POC-Drift flight pattern conduct multi-flight (and possibly multi-aircraft) studies following individual POCs to observe aerosol and cloud evolution over a period longer than a single flight.Large eddy simulations (LES) will be conducted to build upon the observational analysis. Aerosol-cloud-drizzle interactions will be incorporated into in the LES framework using a simplified aerosol source-sink budget equation, and this will be used, together with the C-130 observations, to test hypotheses concerning the feedbacks between aerosols, clouds and precipitation over the SEP.Broader impacts of this project are in forming the foundation of two doctoral theses. This project is a central component of the VOCALS Regional Experiment, for which the PI is the lead scientist. The observational datasets generated as part of the VOCALS-REx will play an important role in model intercomparisons carried out by the GEWEX (Global Energy and Water Cycle Experiment) Cloud System Study boundary-layer cloud working group. In addition, the PIs will produce materials for Windows to the Universe, a bilingual educational outreach program.