AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds
JAMES HUDSON, Stephen Noble, Desert Research Institute
Abstract Number: 31 Working Group: Aerosols, Clouds, and Climate
Abstract Cloud microphysics and CCN measurements in two marine stratus cloud projects are presented and analyzed. Results show that the increase of cloud droplet concentrations (N$_c) with higher CCN concentrations (N$_(CCN)) rolls off for N$_(CCN) at 1% supersaturation (S) (N$_(1%)) above 400 cm$^(-3). At such high concentrations N$_c was no longer correlated with N$_(CCN), but is related to vertical velocity (W) or variations of W (sigma$_w), which are more relevant to stratus clouds. This changeover from predominate N$_c dependence on N$_(CCN) to N$_c dependence on W or sigma$_w is due to the higher slope (k) of CCN spectra that is typically found at lower S. The higher k at lower S becomes more relevant for determining N$_c because of the reduction of cloud S due to greater competition among the higher droplet concentrations that are induced by high N$_(CCN). Higher k of input CCN spectra tends to increase the apparent influence of W or sigma$_w variations relative to the influence of N$_(CCN) variations on N$_c. This changeover at high N$_(CCN) thus seems to put a limit on the indirect aerosol effect (IAE). Although these results appear to reduce the influence of the aerosol (CCN) on cloud microphysics this is really not the case because it is both the concentration and especially the shape (k) of the CCN spectrum that produces the conditions where the dynamics becomes more important for determining N$_c and cloud microphysics.
On the other hand, in clean maritime air (N$_(1%) <~ 200 cm$^(-3)) stratus cloud N$_c often exceeded N$_(1%), which means cloud S > 1%, which considerably exceeds conventional wisdom that stratus cloud S is less than 0.3%. This means that smaller (i.e., 20 nm) more numerous CCN can produce stratus cloud droplets in the more susceptible clouds that are so important for IAE.