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
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.