CCN and vertical velocity influences on droplet concentrations and supersaturations in clean and polluted stratus clouds

 
Poster PDF

Authors

James G. Hudson — Desert Research Institute
Stephen R Noble — Desert Research Institute

Category

Cloud Properties

Description

CCN and cloud microphysics measurements from two California marine stratus cloud projects, the Marine Stratus Experiment (MASE) and Physics of Stratocumulus Top (POST), are compared. Cloud droplet concentrations (Nc) were positively related to CCN concentrations (NCCN) over a wide NCCN range from clean to polluted during POST (Fig. 1). But for the high NCCN range of MASE (polluted), Nc decreased with NCCN (Fig. 1). As expected in both projects Nc was positively related to vertical velocity (W), which during POST was also positively correlated with NCCN. This unforeseen W-NCCN coupling thus tended to increase the NCCN-Nc and W-Nc relationships as each reinforced the other because both NCCN and W positively influence Nc. It is shown that without the coupling between W and NCCN during POST the NCCN-Nc and W-Nc correlation coefficients (R) would have been lower as they were in MASE where W and NCCN were independent (R = -0.09). The coupling or not of W with NCCN thus seems to be responsible for the difference between the W-Nc R of POST (0.61) from that of MASE (0.46); i.e., the 0.46 R of MASE is a pure relationship whereas the 0.61 R of POST is abetted by the positive NCCN-Nc relationship in POST. Subsets of the POST data with negative W-NCCN R have lower R for NCCN-Nc and W-Nc relationships. Likewise subsets of the MASE data with positive NCCN-W relationships have positive NCCN-Nc and W-Nc relationships. The latter is virtually unchanged.

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.