Abstract
Abstract. Many global aerosol and climate models, including the widely used Community Atmosphere Model version 5 (CAM5), have large biases in predicting aerosols in remote regions such as the upper troposphere and high latitudes. In this study, we conduct CAM5 sensitivity simulations to understand the role of key processes associated with aerosol transformation and wet removal affecting the vertical and horizontal long-range transport of aerosols to the remote regions. Improvements are made to processes that are currently not well represented in CAM5, which are guided by surface and aircraft measurements together with results from a multi-scale aerosol–climate model that explicitly represents convection and aerosol–cloud interactions at cloud-resolving scales. We pay particular attention to black carbon (BC) due to its importance in the Earth system and the availability of measurements. We introduce into CAM5 a new unified scheme for convective transport and aerosol wet removal with explicit aerosol activation above convective cloud base. This new implementation reduces the excessive BC aloft to better simulate observed BC profiles that show decreasing mixing ratios in the mid- to upper-troposphere. After implementing this new unified convective scheme, we examine wet removal of submicron aerosols that occurs primarily through cloud processes. The wet removal depends strongly on the subgrid-scale liquid cloud fraction and the rate of conversion of liquid water to precipitation. These processes lead to very strong wet removal of BC and other aerosols over mid- to high latitudes during winter months. With our improvements, the Arctic BC burden has a 10-fold (5-fold) increase in the winter (summer) months, resulting in a much-better simulation of the BC seasonal cycle as well. Arctic sulphate and other aerosol species also increase but to a lesser extent. An explicit treatment of BC aging with slower aging assumptions produces an additional 30-fold (5-fold) increase in the Arctic winter (summer) BC burden. This BC aging treatment, however, has minimal effect on other underpredicted species. Interestingly, our modifications to CAM5 that aim at improving prediction of high-latitude and upper-tropospheric aerosols also produce much-better aerosol optical depth (AOD) over various other regions globally when compared to multi-year AERONET retrievals. The improved aerosol distributions have impacts on other aspects of CAM5, improving the simulation of global mean liquid water path and cloud forcing.
Highlights
As one of the most uncertain forcing agents in the Earth’s climate system, aerosols and their representation in climate models continue to be a challenge for climate research
The interesting behavior at high latitudes may be important to the local black carbon (BC) removal and burden in the Arctic, but the results indicate that wet scavenging at mid-latitudes largely controls how much BC gets to the Arctic
In this study we have evaluated process representations associated with aerosol–cloud interactions, cloud microphysics and macrophysics, aerosol transformation, convective transport and aerosol wet removal in Community Atmosphere Model version 5 (CAM5) that are key to determining the amount of aerosols reaching remote regions
Summary
As one of the most uncertain forcing agents in the Earth’s climate system, aerosols and their representation in climate models continue to be a challenge for climate research. The atmospheric component of the CESM, the Community Atmosphere Model version 5 (CAM5; Neale et al, 2010), includes relatively comprehensive representations of aerosols and mechanisms for interactions with clouds and climate (Gettelman et al, 2008; Liu et al, 2012; Ghan et al, 2012). Like many other global aerosol and climate models (Kinne et al, 2006; Koch et al, 2009b; Qian et al, 2012), CAM5 produces a relatively poor simulation of aerosols and clouds in remote regions (upper troposphere and high latitudes) compared to those in other regions. H. Wang et al.: Improving remote aerosol distributions in CAM5 concentrations, during the Arctic haze season (winter to early spring), and overpredicts upper-tropospheric aerosols in lower latitudes (Wang et al, 2011b; Liu et al, 2012). We describe changes designed to reduce the biases, interpret those changes in the context of the basic physics of aerosol–cloud interactions, and demonstrate the improvements to the CAM5 simulation of aerosols in remote regions and changes to global aerosol distributions
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