Abstract
Abstract. A new cloud microphysical scheme including a detailed parameterization for aerosol-driven ice formation in cirrus clouds is implemented in the global ECHAM/MESSy Atmospheric Chemistry (EMAC) chemistry–climate model and coupled to the third generation of the Modal Aerosol Dynamics model for Europe adapted for global applications (MADE3) aerosol submodel. The new scheme is able to consistently simulate three regimes of stratiform clouds – liquid, mixed-, and ice-phase (cirrus) clouds – considering the activation of aerosol particles to form cloud droplets and the nucleation of ice crystals. In the cirrus regime, it allows for the competition between homogeneous and heterogeneous freezing for the available supersaturated water vapor, taking into account different types of ice-nucleating particles, whose specific ice-nucleating properties can be flexibly varied in the model setup. The new model configuration is tuned to find the optimal set of parameters that minimizes the model deviations with respect to observations. A detailed evaluation is also performed comparing the model results for standard cloud and radiation variables with a comprehensive set of observations from satellite retrievals and in situ measurements. The performance of EMAC-MADE3 in this new coupled configuration is in line with similar global coupled models and with other global aerosol models featuring ice cloud parameterizations. Some remaining discrepancies, namely a high positive bias in liquid water path in the Northern Hemisphere and overestimated (underestimated) cloud droplet number concentrations over the tropical oceans (in the extratropical regions), which are both a common problem in these kinds of models, need to be taken into account in future applications of the model. To further demonstrate the readiness of the new model system for application studies, an estimate of the anthropogenic aerosol effective radiative forcing (ERF) is provided, showing that EMAC-MADE3 simulates a relatively strong aerosol-induced cooling but within the range reported in the Intergovernmental Panel on Climate Change (IPCC) assessments.
Highlights
The impact of aerosol on atmospheric composition and climate still represents one of the largest uncertainties in the quantification of anthropogenic climate change (Boucher et al, 2013)
We describe and evaluate the implementation of a new two-moment cloud microphysical scheme (Kuebbeler et al, 2014) into the EMAC global model (ECHAM/MESSy Atmospheric Chemistry; Jöckel et al, 2010) and its coupling to the aerosol microphysics submodel MADE3 (Modal Aerosol Dynamics model for Europe adapted for global applications, third generation; Kaiser et al, 2019)
We focus on cloud and radiation variables and analyze the performance of EMAC-MADE3 in reproducing essential quantities compared to satellite and in situ observations
Summary
The impact of aerosol on atmospheric composition and climate still represents one of the largest uncertainties in the quantification of anthropogenic climate change (Boucher et al, 2013). Penner et al (2018) included an ice-formation parameterization for cirrus clouds in the NCAR-CAM5.3 model coupled to the IMPACT aerosol model, distinguishing three aerosol mixing states in three size modes Their resulting estimates of the radiative forcing from various emission sources show a large negative climate impact due to aerosol-induced cirrus modifications. The compelling need for additional insights into this issue motivated the extension of MADE3 towards a better resolved representation of INP properties (Kaiser et al, 2019) and the coupling to a new cloud scheme with a detailed parameterization for aerosol-induced ice formation in cirrus clouds, which is described in the present paper.
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