Understanding Clouds Across Scales: From Small-Scale Processes to Large-Scale Properties

• Applicant: Dr. Fabian Hoffmann
• Subject Area: Atmospheric Science
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 435878013

Clouds are an essential part of the Earth’s climate system with significant influence on the global radiation budget. However, our understanding of clouds in the climate system is highly uncertain, in large part because of the complex network of interactions and feedbacks originating from small-scale cloud processes, such as turbulent entrainment and mixing, radiation, and cloud microphysics, and the fact that these processes are usually neglected or only crudely parameterized, even in today’s high-resolution simulations. This is especially the case for low-level clouds, such as trade-wind shallow cumuli and Arctic mixed-phase clouds, which are the focus of this project. Using a novel modeling approach developed by the applicant, the project will bridge the gap between direct numerical simulation and large-eddy simulation, covering all relevant spatial scales from the entire cloud field down to the Kolmogorov lengthscale in one model. Resolving this wide range is essential to this project, enabling unprecedented insights on the effects of small-scale processes on entire cloud fields without the restrictions of a limited domain or insufficient resolution, as is the case for traditional modeling. Two upcoming field campaigns will provide complementary observations to this project.Entrainment and mixing have significant effects on the microphysical composition of a cloud and hence its radiative properties. These processes have been classically described by the interaction of turbulence and cloud microphysics. Recent work, however, has identified the saturation of mixed air as another crucial parameter, emphasizing the preconditioning of the mixed air by the interaction of turbulence, cloud microphysics, and radiation. Therefore, we aim to investigate these small-scale processes across the edge of individual shallow cumuli, their lifecycle, and within entire cloud fields, quantifying their large-scale impacts. Secondly, we will address entrainment and mixing in mixed-phase clouds, a largely uninvestigated topic so far. Our preliminary, idealized work already indicates significant impacts of these processes on the partitioning of water between the ice and the liquid phases, being responsible for creation and survival of pockets of supercooled liquid water. These small-scale features are suspected to decelerate the Wegener-Bergeron-Findeisen process and hence the dissipation of the cloud, with commensurate impacts on the global radiation budget. All in all, this project will further process-level understanding of small-scale cloud processes in trade-wind shallow cumuli and Arctic mixed-phase clouds. At the same time, the project will address the effects of these processes on the large-scale radiative properties of the investigated cloud types, and hence their role in the Earth’s climate system, as summarized in the project title “Understanding clouds across scales”.

• DFG Programme: Independent Junior Research Groups