CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds

Abstract

Cloud microphysics and cloud condensation nuclei (CCN) measurements from two marine stratus cloud projects are presented and analyzed. Results show that the increase of cloud droplet concentrations N c with CCN concentrations N CCN rolls off for N CCN at 1% supersaturation (S)N 1% above 400 cm 23 . Moreover, at such high concentrations N c was not so well correlated with N CCN but tended to be more closely related to vertical velocity W or variations of W (s w ). This changeover from predominate N c dependence on N CCN to N c dependence on W or s w is due to the higher slope k of CCN spectra at lower S, which is made more relevant by the lower cloud S that is forced by higher N CCN . Higher k makes greater influence of W or s w variations than N CCN variations on N c . This changeover at high N CCN thus seems to limit the indirect aerosol effect (IAE).On the other hand, in clean-air stratus cloud S often exceeded 1% and decreased to slightly less than 0.1% in polluted conditions. This means that smaller CCN [those with higher critical S (S c )], which are generally more numerous than larger CCN (lower S c ), are capable of producing stratus cloud droplets, especially when they are advected into clean marine air masses where they can induce IAE. Positive correlations between turbulence s w and N CCN are attributed to greater differential latent heat exchange of smaller more numerous cloud droplets that evaporate more readily. Such apparent CCN influences on cloud dynamics tend to support trends that oppose conventional IAE, that is, less rather than greater cloudiness in polluted environments.

Response of Marine Boundary Layer Cloud Properties to Aerosol Perturbations Associated with Meteorological Conditions from the 19-Month AMF-Azores Campaign

“…More water vapor is supplied to the atmosphere, which decreases the competition for water vapor between CCN activation of aerosols, resulting in more CCN and cloud droplets when there is an inversion at the top of the boundary layer (Su et al 2010;Dong et al 2015). As v increases, N c decreases, which is consistent with results reported by others (e.g., Kim et al 2008;Hudson and Noble 2014;West et al 2014). This is because the stronger ascending motion enables more of the smaller aerosols to activate as a result of the increase in maximum supersaturation (West et al 2014).…”

Atmospheric new particle formation as a source of CCN in the eastern Mediterranean marine boundary layer

“…The maximum supersaturation usually remains below 0.3 % in polluted boundary-layer clouds, while higher supersaturations close to or even larger than 1 % have been reported under clean conditions and in convective clouds (Ditas et al, 2012;Hammer et al, 2014;Hudson and Noble, 2014). As discussed in Sect.…”

A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network

“…The two measurement locations discussed here are interesting with regard to the ratio of presumed cloud droplet number concentration (CDNC) to the total aerosol particle number concentration. It has been reported that, although under the clean and convective conditions ambient S c may reach as high as 1.0 %, in the polluted boundary layer S c usually remains below 0.3 % (Ditas et al, 2012;Hammer et al, 2014;Hudson and Noble, 2014). If one assumes this value, a comparatively small fraction of aerosol in northern Finland and central Netherlands would potentially activate into cloud droplets if exposed to this S c .…”