Sensitivity of remote aerosol distributions to representation of cloud–aerosol interactions in a global climate model

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 muchbetter 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 uppertropospheric 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.

Assessing the CAM5 physics suite in the WRF-Chem model: implementation, resolution sensitivity, and a first evaluation for a regional case study

“…These interactions occur at subgrid scales in most atmospheric models except for large eddy simulations. Deficiencies in the description of these interactions are believed to lead to high estimates of aerosol indirect forcing and the underestimation of aerosol concentrations in remote regions such as the Arctic Lee et al, 2013;Rasch et al, 2000;Shindell et al, 2008;Textor et al, 2006;Wang et al, 2013). The aerosol indirect forcing can also be amplified due to data aggregation from grid-box averages (McComiskey and Feingold, 2012).…”

“…We first focus on the high-latitudes because it is a vulnerable region for climate change (e.g., Screen and Simmonds, 2010;Serreze et al, 2009), and there are still large uncertainties regarding the role of aerosols in the Arctic (north of 66.5 • N) with documented deficiencies in aerosol transport into the Arctic simulated by GCMs Textor et al, 2006;Wang et al, 2013).…”

An AeroCom assessment of black carbon in Arctic snow and sea ice

“…Koch et al (2009) evaluated Arctic atmospheric BC in AeroCom phase I models and found that increasing BC lifetime, which is accomplished by decreasing the aging rate or by reducing removal by ice clouds, has a large impact on BC surface concentrations in remote regions. Analysis of surface measurements at Barrow, Alaska, indicates that the seasonal cycle of "Arctic haze" is dominated by wet scavenging rather than efficiency of transport pathways from source regions (Garrett et al, 2010;Browse et al, 2012;Lund and Berntsen, 2012;Wang et al, 2013). Liu et al (2011) concluded that the simulation of BC in the Arctic is significantly improved by using a parameterization of BC aging rate that is proportional to the OH radical concentration, reducing dry deposition velocities over ice and snow, and decreasing ice cloud wet removal efficiency.…”

Climatic Responses to Future Trans‐Arctic Shipping

“…However, we emphasize that the magnitude of this cooling effect (~1 °C) is roughly an order of magnitude smaller than the overall regional warming in RCP 8.5 (~10 °C). Although the CESM model that we use is more sophisticated than those used in prior studies, it is possible that the cooling we simulate may reflect enhanced liquid water path caused by a large decrease in the rate of conversion of cloud to rainwater when cloud droplet number increases (Wang et al, 2013; Zhou & Penner, 2017). The current atmospheric component of CESM (CAM5) has a more realistic representation of cloud properties than its predecessor (CAM4), with a substantial improvement in Arctic cloud seasonal cycle (Kay et al, 2012).…”

“…Lastly, an increase in atmospheric particulates that serve as CCN will tend to increase cloud albedo by encouraging formation of smaller and more numerous cloud droplets (Twomey, 1977), particularly in liquid clouds (Christensen et al, 2014; Morrison et al, 2012). Although the influence of particulate emissions in the Arctic has historically been small and localized relative to more populous lower latitudes (Gong et al, 2018; Peters et al, 2011), increases in shipping emissions could substantially raise CCN, enhance cloud formation, and induce a climatic response (Mueller, 2018; Wang et al, 2013; Yang et al, 2018). …”