Abstract.A modal aerosol module (MAM) has been developed for the Community Atmosphere Model version 5 (CAM5), the atmospheric component of the Community Earth System Model version 1 (CESM1). MAM is capable of simulating the aerosol size distribution and both internal and external mixing between aerosol components, treating numerous complicated aerosol processes and aerosol physical, chemical and optical properties in a physically-based manner. Two MAM versions were developed: a more complete version with seven lognormal modes (MAM7), and a version with three lognormal modes (MAM3) for the purpose of long-term (decades to centuries) simulations. In this paper a description and evaluation of the aerosol module and its two representations are provided. Sensitivity of the aerosol lifecycle to simplifications in the representation of aerosol is discussed.Simulated sulfate and secondary organic aerosol (SOA) mass concentrations are remarkably similar between MAM3 and MAM7. Differences in primary organic matter (POM) and black carbon (BC) concentrations between MAM3 and MAM7 are also small (mostly within 10 %). The mineral dust global burden differs by 10 % and sea salt burden by 30-40 % between MAM3 and MAM7, mainly due to the different size ranges for dust and sea salt modes and different standard deviations of the log-normal size distribution for sea salt modes between MAM3 and MAM7. The model is able to qualitatively capture the observed geographical and temporal variations of aerosol mass and number concentrations, size distributions, and aerosol optical properties. However, there are noticeable biases; e.g., simulated BC concentrations are significantly lower than measurements in the Arctic. There is a low bias in modeled aerosol optical depth on the global scale, especially in the developing countries. These biases in aerosol simulations clearly indicate the need for improvements of aerosol processes (e.g., emission fluxes of anthropogenic aerosols and precursor gases in developing countries, boundary layer nucleation) and properties (e.g., primary aerosol emission size, POM hygroscopicity). In addition, the critical role of cloud properties (e.g., liquid water content, cloud fraction) responsible for the wet scavenging of aerosol is highlighted.

[1] A process-based treatment of ice supersaturation and ice nucleation is implemented in the National Center for Atmospheric Research Community Atmosphere Model (CAM). The new scheme is designed to allow (1) supersaturation with respect to ice, (2) ice nucleation by aerosol particles, and (3) ice cloud cover consistent with ice microphysics. The scheme is implemented with a two-moment microphysics code and is used to evaluate ice cloud nucleation mechanisms and supersaturation in CAM. The new model is able to reproduce field observations of ice mass and mixed phase cloud occurrence better than previous versions. The model is able to reproduce observed patterns and frequency of ice supersaturation. Simulations indicate homogeneous freezing of sulfate and heterogeneous freezing on dust are both important ice nucleation mechanisms, in different regions. Simulated cloud forcing and climate is sensitive to different formulations of the ice microphysics. Arctic surface radiative fluxes are sensitive to the parameterization of ice clouds. These results indicate that ice clouds are potentially an important part of understanding cloud forcing and potential cloud feedbacks, particularly in the Arctic.

Abstract. The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) examined the short-lived drivers of climate change in current climate models. Here we evaluate the 10 ACCMIP models that included aerosols, 8 of which also participated in the Coupled Model Intercomparison Project phase 5 (CMIP5). The models reproduce present-day total aerosol optical depth (AOD) relatively well, though many are biased low. Contributions from individual aerosol components are quite different, however, and most models underestimate east Asian AOD. The models capture most 1980–2000 AOD trends well, but underpredict increases over the Yellow/Eastern Sea. They strongly underestimate absorbing AOD in many regions. We examine both the direct radiative forcing (RF) and the forcing including rapid adjustments (effective radiative forcing; ERF, including direct and indirect effects). The models' all-sky 1850 to 2000 global mean annual average total aerosol RF is (mean; range) −0.26 W m−2; −0.06 to −0.49 W m−2. Screening based on model skill in capturing observed AOD yields a best estimate of −0.42 W m−2; −0.33 to −0.50 W m−2, including adjustment for missing aerosol components in some models. Many ACCMIP and CMIP5 models appear to produce substantially smaller aerosol RF than this best estimate. Climate feedbacks contribute substantially (35 to −58%) to modeled historical aerosol RF. The 1850 to 2000 aerosol ERF is −1.17 W m−2; −0.71 to −1.44 W m−2. Thus adjustments, including clouds, typically cause greater forcing than direct RF. Despite this, the multi-model spread relative to the mean is typically the same for ERF as it is for RF, or even smaller, over areas with substantial forcing. The largest 1850 to 2000 negative aerosol RF and ERF values are over and near Europe, south and east Asia and North America. ERF, however, is positive over the Sahara, the Karakoram, high Southern latitudes and especially the Arctic. Global aerosol RF peaks in most models around 1980, declining thereafter with only weak sensitivity to the Representative Concentration Pathway (RCP). One model, however, projects approximately stable RF levels, while two show increasingly negative RF due to nitrate (not included in most models). Aerosol ERF, in contrast, becomes more negative during 1980 to 2000. During this period, increased Asian emissions appear to have a larger impact on aerosol ERF than European and North American decreases due to their being upwind of the large, relatively pristine Pacific Ocean. There is no clear relationship between historical aerosol ERF and climate sensitivity in the CMIP5 subset of ACCMIP models. In the ACCMIP/CMIP5 models, historical aerosol ERF of about −0.8 to −1.5 W m−2 is most consistent with observed historical warming. Aerosol ERF masks a large portion of greenhouse forcing during the late 20th and early 21st century at the global scale. Regionally, aerosol ERF is so large that net forcing is negative over most industrialized and biomass burning regions through 1980, but remains strongly negative only over east and southeast Asia by 2000. Net forcing is strongly positive by 1980 over most deserts, the Arctic, Australia, and most tropical oceans. Both the magnitude of and area covered by positive forcing expand steadily thereafter.

In this paper, we discuss the results of 2000-2100 simulations following the emissions associated with the Representative Concentration Pathways (RCPs) with a chemistry-climate model, focusing on the changes in 1) atmospheric composition (troposphere and stratosphere) and 2) associated environmental parameters (such as nitrogen deposition). In particular, we find that tropospheric ozone is projected to decrease (RCP2.6, RCP4.5 and RCP6) or increase (RCP8.5) between 2000 and 2100, with variations in methane a strong contributor to this spread. The associated tropospheric ozone global radiative forcing is shown to be in agreement with the estimate used in the RCPs, except for RCP8.5. Surface ozone in 2100 is projected to change little compared from its 2000 distribution, a much-reduced impact from previous projections based on the A2 highemission scenario. In addition, globally-averaged stratospheric ozone is projected to recover at or beyond pre-1980 levels. Anthropogenic aerosols are projected to strongly decrease in the 21st century, a reflection of their projected decrease in emissions. Consequently, sulfate deposition is projected to strongly decrease. However, nitrogen deposition is projected to increase over certain regions because of the projected increase in NH 3 emissions.

Ozone (O₃) from 17 atmospheric chemistry models taking part in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) has been used to calculate tropospheric ozone radiative forcings (RFs). All models applied a common set of anthropogenic emissions, which are better constrained for the present-day than the past. Future anthropogenic emissions follow the four Representative Concentration Pathway (RCP) scenarios, which define a relatively narrow range of possible air pollution emissions. We calculate a value for the pre-industrial (1750) to present-day (2010) tropospheric ozone RF of 410 mW m⁻². The model range of pre-industrial to present-day changes in O₃ produces a spread (±1 standard deviation) in RFs of ±17%. Three different radiation schemes were used – we find differences in RFs between schemes (for the same ozone fields) of ±10%. Applying two different tropopause definitions gives differences in RFs of ±3%. Given additional (unquantified) uncertainties associated with emissions, climate-chemistry interactions and land-use change, we estimate an overall uncertainty of ±30% for the tropospheric ozone RF. Experiments carried out by a subset of six models attribute tropospheric ozone RF to increased emissions of methane (44±12%), nitrogen oxides (31 ± 9%), carbon monoxide (15 ± 3%) and non-methane volatile organic compounds (9 ± 2%); earlier studies attributed more of the tropospheric ozone RF to methane and less to nitrogen oxides. Normalising RFs to changes in tropospheric column ozone, we find a global mean normalised RF of 42 mW m⁻² DU⁻¹, a value similar to previous work. Using normalised RFs and future tropospheric column ozone projections we calculate future tropospheric ozone RFs (mW m⁻²; relative to 1750) for the four future scenarios (RCP2.6, RCP4.5, RCP6.0 and RCP8.5) of 350, 420, 370 and 460 (in 2030), and 200, 300, 280 and 600 (in 2100). Models show some coherent responses of ozone to climate change: decreases in the tropical lower troposphere, associated with increases in water vapour; and increases in the sub-tropical to mid-latitude upper troposphere, associated with increases in lightning and stratosphere-to-troposphere transport. Climate change has relatively small impacts on global mean tropospheric ozone RF