Anthropogenic Aerosol Influences on Mixed-Phase Clouds

Ulrike Lohmann

doi:10.1007/s40641-017-0059-9

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-of-the-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.

Highlights

Emissions of anthropogenic aerosols and their precursor gases have substantially increased since pre-industrial times
In simulations with the ECHAM6 global climate models (GCMs) coupled to the HAM2 aerosol scheme that include aerosol effects on stratiform warm and mixed-phase clouds (MPCs), the sensitivity of ERFaci+ari to aerosol processing, i.e., how important it is to account for the number of aerosol particles that are incorporated into cloud droplets and ice crystals during activation, nucleation, scavenging, and collisions, was investigated [73]
Mixed-phase clouds remain one of the least studied cloud types because they are not as easy to access as warm clouds and require instrumentation that distinguishes cloud droplets from ice crystals

Summary

Introduction

Emissions of anthropogenic aerosols and their precursor gases have substantially increased since pre-industrial times. In simulations with the ECHAM6 GCM coupled to the HAM2 aerosol scheme that include aerosol effects on stratiform warm and MPCs, the sensitivity of ERFaci+ari to aerosol processing, i.e., how important it is to account for the number of aerosol particles that are incorporated into cloud droplets and ice crystals during activation, nucleation, scavenging, and collisions, was investigated [73]. In a multi-scale modeling framework, where the relevant cloud-scale circulations are resolved, ERFaci+ari is reduced by 40% from −1.8 W m−2 down to −1.05 W m−2 [98] yielding one of the lowest ERFaci+ari estimates in that category (Fig. 5) In this framework, the liquid water path increases much less in response to anthropogenic aerosols because more buffering mechanisms, such as enhanced evaporation of the smaller polluted cloud droplets, can take place. One promising approach is the spectral convective parameterization representing the statistical effects of a heterogeneous ensemble of cumulus clouds [3] extended with an explicit cloud model based on a one-dimensional steady-state entraining plume [43]

Conclusions

Findings

10. Cambridge