Anthropogenic Aerosol Influences on Mixed-Phase Clouds

Abstract

Aerosol effects on mixed-phase clouds (MPCs) are more complex than in warm clouds because aerosol particles can act both as cloud condensation nuclei and as ice nucleating particles and more microphysical pathways exist. Stratiform MPCs are most prevalent in the Arctic where cloud top cooling enables heterogeneous ice formation and in orographic terrain where large updrafts prevail. Recently, aerosol effects on stratiform MPCs have also been considered in global climate models. The estimated effective aerosol radiative forcing due to aerosol-cloud and aerosol-radiation interactions (ERFaci+ari) at the top-ofthe-atmosphere (TOA) in stratiform warm and MPCs, which is an update of the estimate given in the fifth assessment report (AR5) of the Intergovernmental Panel of Climate Change, is −1.2 W m −2 with a 5-95 % range between −0.8 and −2.0 W m −2 . Since AR5, only one new estimate of ERFaci+ari including aerosol effects on both stratiform and convective clouds of −1.4 W m −2 has been published. In all cases, ERFaci+ari is dominated by changes in the shortwave TOA radiation with changes in the longwave TOA radiation amounting to 0.15 W m −2 .

Aerosol indirect effects on the nighttime Arctic Ocean surface from thin, predominantly liquid clouds

“…The deactivation effect, whereby sulfates reduce icenucleating particle efficiency (Du et al, 2011;Girard et al, 2005Girard et al, , 2013Lohmann, 2017), could also be consistent with our observations. Some limited in situ data support the occurrence of this mechanism (Jouan et al, 2012), but remote sensing data are contradictory (Grenier et al, 2009;Grenier and Blanchet, 2010), perhaps in part because of high uncertainties in below-cloud aerosols and a focus on ice-phase clouds, where it is more difficult for CALIPSO to accurately separate aerosols from ice particles.…”

Initiation of secondary ice production in clouds

“…The low or nonexistent values of N (lim) INP calculated in this study indicate that perturbations in cloud condensation nuclei concentrations are more influential on mixed-phase partitioning than those in INP concentrations, with the caveat that thermodynamic conditions are appropriate for secondary production. If the mixed-phase cloud is polluted by more cloud condensation nuclei, the higher droplet number will mean that fewer droplets reach a sufficient size to shatter or rime efficiently (This last factor has been called the riming indirect effect (Borys et al, 2003;Lance et al, 2011;Lohmann, 2017)). And in these cases, the supercooled liquid fraction remains higher, and the cloud reflects more shortwave radiation.…”

The Radiative Forcing of Aerosol–Cloud Interactions in Liquid Clouds: Wrestling and Embracing Uncertainty

“…In this section, we summarize studies from these promising categories. We do not attempt to be exhaustive in summarizing GCM-based estimates; [78] and [79] provide an excellent compilation of such studies since AR5.…”

“…We will organize our discussion around estimates of the climate forcing of the anthropogenic aerosol, conventionally expressed as an effective radiative forcing (ERF) relative to the preindustrial climate state. We will restrict our discussion to aerosolcloud interactions in warm (liquid-phase) clouds, given their tight connection to other unresolved climate change questions such as cloud feedbacks and climate sensitivity; we refer the reader to [78] for a comprehensive discussion of mixed-and ice-phase aerosol-cloud interactions (ACI). We will not cover questions associated with the abundance, size distribution, composition, and anthropogenic fraction of the atmospheric aerosol, since many other excellent papers have already done so (e.g., [106,108]).…”