Entrainment of aerosols and their activation in shallow cumulus clouds - Large-Eddy Simulations with an Embedded Lagrangian Particle Model
Final Report Abstract
The project investigated the interaction of aerosols and clouds, a major uncertainty in our understanding of the climate system, using a novel cloud modeling approach: a so-called Lagrangian cloud model, from which only five implementations exists (to our best knowledge). Within this approach, cloud microphysics are represented by individually simulated particles. As a result, the use of parameterizations can be restricted to a minimum enabling more reliable results than other models. Moreover, the tracking of the individually simulated particles also allows new ways for investigating aerosol-cloud interactions, which have been applied throughout the project. The project primarily investigated the activation of aerosols, i.e., the transformation of aerosols into cloud droplets, and how this process depends on the associated dynamics of entrainment, i.e., the mixing of environmental air into the cloud. We quantified how much the different ways of aerosol entrainment contributed to the number of aerosols inside a shallow cumulus cloud, and revealed that the activation characteristics of laterally entrained aerosols are different from those aerosols, which are entrained through the cloud base. Implications for the parameterization of these processes have been discussed. Furthermore, the theory usually applied for describing the activation of aerosols, the Köhler theory, has been validated, and their limits of applicability have been identified. Moreover, so-called spurious supersaturations have been investigated, which are numerical artifacts that are able to falsify the microphysical behavior of clouds by the spurious activation of aerosols. Using theoretical arguments, the general dependence of spurious supersaturations on numerical and cloud-microphysical parameters has been revealed. These results might enable a reduction of spurious supersaturations in both traditional Eulerian and novel Lagrangian cloud models in the future. The effects of aerosols on clouds and precipitation have been further investigated by a study on the initiation of rain, revealing that effects of turbulence on the collection of cloud droplets are more dominant in aerosol-laden than in pristine conditions. Finally, the project also contributed to the general improvement of Lagrangian cloud models by intercomparing all available collection algorithms, uncovering their individual shortcomings and advantages. All in all, the project deepened our process-level understanding of the interaction of aerosols and clouds and improved our ability to model these processes. Moreover, it significantly contributed to the advancement of Lagrangian cloud models in general, enabling the maturing of this novel and promising modeling approach.
Publications
- 2015: Entrainment of aerosols and their activation in a shallow cumulus cloud studied with a coupled LCM-LES approach, Atmos. Res., 156, 43-57
Hoffmann, F., S. Raasch, and Y. Noh
(See online at https://doi.org/10.1016/j.atmosres.2014.12.008) - 2015: The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives, Geosci. Model Dev., 8, 2515-2551
Maronga, B., M. Gryschka, R. Heinze, F. Hoffmann, F. Kanani-Sühring, M. Keck, K. Ketelsen, M. O. Oliver Letzel, M. Sühring, and S. Raasch
(See online at https://doi.org/10.5194/gmd-8-2515-2015) - 2016: The effect of spurious cloud edge supersaturations in Lagrangian cloud models: An analytical and numerical study, Mon. Wea. Rev., 144, 107-118
Hoffmann, F.
(See online at https://doi.org/10.1175/MWR-D-15-0234.1) - 2017: Collection/aggregation algorithms in Lagrangian cloud microphysical models: Rigorous evaluation in box model simulations, Geosci. Model Dev., 10, 1521- 1548
Unterstrasser, S., F. Hoffmann, and M. Lerch
(See online at https://doi.org/10.5194/gmd-10-1521-2017) - 2017: On the Validity of Köhler Activation Theory: How do Collision and Coalescence Affect the Activation of Aerosols?, Atmos. Chem. Phys.
Hoffmann, F.
(See online at https://dx.doi.org/10.5194/acp-2017-134) - 2017: The route to raindrop formation in a shallow cumulus cloud simulated by a Lagrangian cloud model, J. Atmos. Sci., 74, 2125-2142
Hoffmann, F., Y. Noh, and S. Raasch
(See online at https://doi.org/10.1175/JAS-D-16-0220.1)
