CCN and vertical velocity influences on droplet concentrations and supersaturations in clean and polluted stratus clouds
Authors
James G. Hudson — Desert Research Institute
Stephen R Noble — Desert Research Institute
Category
Cloud Properties
Description
The unexpected positive relationship between W and NCCN has been theoretically predicted by Xue and Feingold (2006) because of the easier evaporation of the smaller cloud droplets of polluted clouds. This causes greater latent heat exchange, which produces TKE and buoyancy gradients that enhance mixing and entrainment (Blyth et al. 1988) that can cause further droplet evaporation and thus positive feedback to the W NCCN positive relationship.
The supersaturation (S) of stratus clouds is important because this determines the particles that form cloud droplets in these most climatically important clouds. Conventional wisdom has held that S of stratus is < 0.3%, which means that only particles larger than 60 nm are capable of nucleating stratus cloud droplets. Results of this study confirm Hudson et al. (2010) that S in clean stratus clouds exceeds 1%, but S in polluted stratus clouds usually exceeds 0.1% and are not as low as 0.03% as indicated by Hudson et al. (2010)(Fig. 2). Figure 2 also demonstrates the decrease of cloud S with NCCN, which is due to competition among droplets for condensate at higher NCCN.