11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Wednesday, 5 June 2002: 11:31 AM
Sensitivity of stratocumulus optical depths to droplet concentrations: Satellite observations and large-eddy simulations (Formerly Paper Number P4.8)
Andrew S. Ackerman, NASA/Ames Research Center, Moffett Field, CA; and D. E. Stevens, O. B. Toon, and J. A. Coakley Jr.
A number of observations and simulations have shown that increased droplet concentrations in ship tracks increase their total cross-sectional area, thereby enhancing cloud albedo and providing a negative (cooling) radiative forcing at the surface and the top of the atmosphere. In some cases cloud water has been found to be enhanced in ship tracks, which has been attributed to suppression of drizzle and implies an enhanced susceptibility of cloud albedo to droplet concentrations. However, observations from aircraft and satellite indicate that on average cloud water is instead reduced in daytime ship tracks. Such a reduction in liquid water may be attributable to cloud-burning caused by solar heating by soot within the ship exhaust (e.g., Ackerman et al., 2000), or by increased precipitation resulting from giant nuclei in the ship exhaust (e.g., Feingold et al., 1999).

We will summarize the observational evidence and present results from large-eddy simulations that evaluate these mechanisms. Along the way we will present our insights into the interpretation of satellite retrievals of cloud microphysical properties.

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