AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Aerosol-Vertical Velocity Coupling Effects on Stratus Cloud Microphycs
STEPHEN NOBLE, James Hudson, Desert Research Institute
Abstract Number: 138 Working Group: Aerosols, Clouds, and Climate
Abstract It is well known that increasing atmospheric aerosol leads to greater cloud albedo and longer cloud lifetimes, the indirect aerosol effect (IAE), which increases cloud droplet concentration (N$_c) and decreases cloud droplet mean diameters (MD). In large areas of marine stratocumuli, IAE produces cooling that offsets greenhouse warming. However, changes in N$_c and MD can lead to feedbacks that further change cloud microphysics and even IAE. Observations of cloud microphysics, dynamics, and below cloud CCN from two marine stratocumulus field campaigns are analyzed. Results show limited evidence of feedbacks involving vertical velocity (W) and possibly latent heat exchanges.
Cloud supersaturation (S) can be measured two ways. Because aerosol below stratocumuli often have bimodal particle size distributions, which is said to be from cloud processing, the size between the two modes of the distributions can be related to cloud S through assumptions of aerosol composition. A comparison between N$_c and the CCN spectra produces another cloud S estimate that is usually higher. Droplets with small MD are not as likely to be involved with the cloud processing because when dry air is mixed into the cloud from above they more readily evaporate, which makes latent heat exchange. This evaporative cooling then increases W (sigma$_W) perturbations.
Furthermore, smaller MD due to increased CCN suppresses drizzle. Less drizzle decreases evaporative cooling below cloud, which reduces the stability of the boundary layer (Ackerman et al. 2004) and thus increases buoyancy and turbulence i.e., greater sigma$_W. Also, sigma$_W can be increased with latent heat release from condensation on smaller more numerous droplets with more surface area that produce more rapid condensation. These feedbacks can reduce IAE.