PNNL

Abstract

Much of large uncertainty in estimates of anthropogenic aerosol effects on climate arises from the multi-scale nature of the interactions between aerosol, cloud 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 aerosol and cloud 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 global scale in a computationally feasible way. Simulated model fields, including liquid water path (LWP), ice water path, cloud fraction, shortwave and longwave cloud forcing, precipitation, water vapor, and cloud droplet number concentration are in reasonable agreement with observations. The new model performs quantitatively similar to the previous version of the MMF model in terms of simulated cloud fraction and precipitation. 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 MMF, compared with 15.6% increase in LWP in large-scale clouds in CAM5). The much smaller increase in LWP in MMF is caused by a much smaller response in LWP to a given perturbation in CCN concentrations from the PI to PD in MMF, which is only about one-third of that in CAM5, and, to a lesser extent, by a smaller relative increase in CCN concentrations from the PI to PD in MMF, which is about 26% smaller than that in CAM5. The smaller relative increase in CCN concentrations in MMF is caused in part by a smaller increase in aerosol lifetime from the 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 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 ?) based on the examination of the Earth’s energy balance.