Environmental Science Division (EVS) a Division of Argonne National Laboratory
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VOCALS Case Study

To improve prediction of ocean cloud cover, EVS teamed with other institutes in a study of stratocumulus clouds over the South Pacific Ocean during the VOCALS field experiment.

The oceans cover about 70% of Earth’s surface, and clouds are present, on average, over 70% of the ocean surface. Vast sheets of low-lying stratocumulus clouds are present over the eastern Pacific Ocean, off the coast of California to the north and off the coast of Chile and Peru to the south.

When viewed from space, a stark contrast can be seen between the ocean surface and extensive horizontal layers of stratocumulus clouds. The ocean surface looks dark, as it reflects only about 2% of the solar radiation back to space, whereas the stratocumulus clouds reflect about 60% of radiation back to space and look like bright cotton balls.

Hence, when they are present, oceanic stratocumulus clouds cool the Earth-atmosphere system we call climate, by reflecting a greater amount of radiation back to space. Past climate model simulations have shown that an increase in the coverage of stratocumulus clouds by about 2% can cool Earth enough to compensate for the warming due to the doubling of carbon dioxide in the atmosphere.

Stratocumulus clouds form when water vapor resulting from evaporation is transported upward from the ocean surface far enough to cool the vapor and form cloud droplets. Hence, to predict the stratocumulus cloud cover, it is crucial to understand both the factors determining the transport of water vapor upward from the surface and the processes responsible for the transport.

A multi-agency, multi-institute observational field campaign titled “Variability of the American Monsoon System (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx)” was conducted off the coast of Chile and Peru in the fall of 2008. Three research aircraft, two research ships, and several scientists from the United States and other countries participated in this field campaign. EVS researchers specifically used data collected by the instruments onboard the research ship Ronald H. Brown, which included Doppler radar, Doppler lidar, radiometers, and weather balloons.

The research ship <cite>Ronald H. Brown</cite> used during the VOCALS field experiment.
The research ship Ronald H. Brown used during the VOCALS field experiment. [Source: VOCALS-REx team/UCAR]

We found that the principal mechanism controlling the transport of water vapor to clouds is radiative cooling near the cloud tops, together with the difference between the sea surface temperature and the air temperature. By taking into account both of these mechanisms and the change in the wind speed with height, we can predict most of the transport of air (and water vapor) from the ocean surface to the clouds.

The findings of the study have been published in a Journal of Applied Meteorology and Climatology article titled “Turbulence and Radiation in Stratocumulus-Topped Marine Boundary Layers: A Case Study from VOCALS-REx.”

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photo of Virendra Ghate
Assistant Atmospheric Scientist
Capabilities: Process-level studies using data collected by atmospheric in situ and remote sensing instruments; characterization of atmospheric dynamics.