Aerosol indirect effects in the PNNL-MMF multi-scale aerosol-climate model

 

Authors

Evgueni Kassianov — Pacific Northwest National Laboratory
Steven J. Ghan — Pacific Northwest National Laboratory
Mikhail Ovchinnikov — Pacific Northwest National Laboratory
Hugh Clifton Morrison — University Corporation for Atmospheric Research
Xiaohong Liu — Texas A&M University
Yun Qian — Pacific Northwest National Laboratory
Dick C Easter — Pacific Northwest National Laboratory
Minghuai Wang — Nanjing University

Category

Aerosol-Cloud-Radiation Interactions

Description

Scatter plots and regressions of (a) the relative changes [(PD-PI)/PI] in annual-mean CCN concentrations (at 0.1% supersaturation) in CAM5 versus the MMF , (b) the relative changes in annual-mean liquid water path (LWP) versus the relative changes in annual-mean CCN concentrations in the MMF model, and (c) like (b) but in the CAM5 model. LWP in CAM5 only includes the contribution from large-scale clouds. Annual mean data are sampled on each GCM grid column from 60°S to 60°N. CCN concentrations are averaged over the lowest eight model levels (surface to about 800 hPa). Red lines and equations are from the linear regression. (a) shows that the relative increase in CCN concentrations from PI to PD in the MMF is about 26% smaller than that in CAM5, while (b) and (c) show that the response in LWP to a given perturbation in cloud condensation nuclei (CCN) concentrations from PI to PD in the MMF is about one-third of that in CAM5.
Much of the large uncertainty in estimates of anthropogenic aerosol effects on climate arises from the multi-scale nature of the interactions between aerosols, clouds, and large-scale dynamics, which are difficult to represent in conventional global climate models (GCMs). In this study, we use a multi-scale aerosol-climate model that treats aerosols and clouds across multiple scales to study aerosol indirect effects. This multi-scale aerosol-climate model is an extension of a multi-scale modeling framework (MMF) model that embeds a cloud-resolving model (CRM) within each grid cell of a GCM. The extension allows the explicit simulation of aerosol/cloud interactions in both stratiform and convective clouds on the global scale in a computationally feasible way. The simulated change in shortwave cloud forcing from anthropogenic aerosols is -0.77 W m-2, which is less than half of that in the host GCM (NCAR CAM5) (-1.79 W m-2) and is also at the low end of the estimates of most other conventional global aerosol-climate models. The smaller forcing in the MMF model is attributed to its smaller increase in LWP from preindustrial conditions (PI) to present day (PD): 3.9% in the MMF, compared with 15.6% increase in LWP in large-scale clouds in CAM5. The much smaller increase in LWP in the MMF is caused by a much smaller response in LWP to a given perturbation in cloud condensation nuclei (CCN) concentrations from PI to PD in the MMF (about one-third of that in CAM5), and, to a lesser extent, by a smaller relative increase in CCN concentrations from PI to PD in the MMF (about 26% smaller than that in CAM5). The smaller relative increase in CCN concentrations in the MMF is caused in part by a smaller increase in aerosol lifetime from PI to PD in the MMF, a positive feedback in aerosol indirect effects induced by cloud lifetime effects. The smaller response in LWP to anthropogenic aerosols in the MMF model is consistent with observations and with high-resolution model studies, which may indicate that aerosol indirect effects simulated in conventional global climate models are overestimated and point to the need to use global high-resolution models, such as MMF models or global CRMs, to study aerosol indirect effects. The simulated total anthropogenic aerosol effect in the MMF is -1.05 W m-2, which is close to the Murphy et al. (2009) inverse estimate of -1.1±0.4 W m-2 (1 σ) based on the examination of the Earth’s energy balance.