Global numerical simulations of atmospheric ice crystals
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Abstract
A comprehensive ice nucleation parameterization (Barahona and Nenes, 2009b, hereafter BN09) has been implemented in the global chemistry-climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry) in order to improve the representation of ice crystal number concentration (ICNC). This parameterization takes into account the competition for water vapour between homogeneous and heterogeneous nucleation in cirrus clouds and the influence of different aerosol components on heterogeneous nucleation. Furthermore, the effect of pre-existing ice crystals, which deplete water vapour through their diffusional growth, has been included in the new algorithm. Thus, the implementation of BN09 allows one to simulate processes which are neglected by the standard configuration of EMAC used so far.
The modelled ICNCs obtained by using BN09 in the cirrus regime agree with the observations, as BN09 strongly reduces the ICNCs in the upper troposphere with respect to the standard model configuration. We found that the effect of pre-existing ice crystals is the main cause of such reduction. On the other hand, the ICNC reduction due to the water vapour competition between the ice nucleation mechanisms is very weak, thus, homogeneous ice nucleation is the dominant nucleation mechanism in cirrus clouds.
Focusing on the contributions of different aerosol components to immersion/condensation and deposition nucleation simulated via a parameterization included in BN09 (Phillips et al. 2013), we found that most of the new ice crystals in the cirrus regime derive from soluble organic compounds and in the mixed-phase regime from black carbon. Dust is on average less important than black carbon, and bioaerosols contribute by ~20% to form new ice crystals in the lower troposphere.
Finally, the analysis of the relative importance of the physical processes which produce and remove ice crystals shows that ice nucleation is the most important source in the upper troposphere, while convective detrainment and instantaneous freezing are more important at lower altitudes, at temperatures lower than -35°C. Sedimentation, which is the most important sink of ice crystals in the upper troposphere, is the main source in the mixed-phase regime.