Improved Parameterization of Water Vapor Transport in Stratocumulus Clouds

Summary

Vast areas of low-level stratocumulus clouds are observed over the southeast Pacific west of Chile and Peru. These low-level clouds are significantly brighter and reflect more solar radiation than the ocean and thus have a large impact on Earth’s radiation budget. Accurate representation of processes controlling the formation and lifecycle of these clouds in global climate model (GCM) simulations is important for future climate predictions. Since many of the processes associated with these clouds occur at spatial scales poorly resolved by GCMs, these cloud formation processes and their associated effects on Earth’s radiation budget must be parameterized.

Stratocumulus clouds are formed and maintained by turbulent processes in the marine boundary layer that transport water vapor upward from the ocean surface. To predict stratocumulus cloud cover, it is important to understand the factors controlling this water vapor transport. A research team funded by the U.S. Department of Energy’s Atmospheric System Research program used observations from a multi-agency field campaign to examine the processes controlling water vapor transport in these clouds. In a unique analysis, data from a Doppler radar and lidar were combined to observe the turbulence structure of the entire stratocumulus-topped marine boundary layer from cloud top to cloud base. These data were complemented by measurements of the cloud liquid water and atmospheric water vapor from a microwave radiometer and surface flux measurements.

The researchers found that the principal mechanism controlling transport of water vapor to clouds is radiative cooling near the tops of the clouds, together with the difference between the sea surface temperature and the air temperature. By taking into account this new information and the change in the wind speed with height, they were able to predict most of the upward transport of air and water vapor from the ocean surface to the clouds. This new formulation of the convective velocity scale could improve GCM parameterizations of stratocumulus cloud formation and evolution.

References

Ghate, V. P., B. A. Albrecht, M. A. Miller, A. Brewer, and C. W. Fairall. 2014. “Turbulence and Radiation in Stratocumulus-Topped Marine Boundary Layers: A Case Study from VOCALS-Rex,” Journal of Applied Meteorology and Climatology 53, 117-35. DOI: http://dx.doi.org/10.1175/JAMC-D-12-0225.1.