Turbulence and clouds in the atmosphere: From the last unresolved classical physics problem to the largest uncertainty in climate projections

Clouds play a key role in the Earth's energy balance. Small changes in low-cloud area cover have a comparable magnitude on the global energy budget as that of the anthropogenic emissions of greenhouse gases. Clouds are interconnected with atmospheric turbulence because turbulent transport causes the variations of temperature and humidity that lead to cloud formation. In recent years, the growth of computing power combined with advances in atmospheric physics modeling have yielded high-fidelity simulations establishing numerical simulation as a major tool for cloud physics discovery. Focusing on high-resolution simulation of atmospheric boundary-layer clouds, we discuss how classical turbulence techniques are used to advance our understanding of clouds. The development of a physics-based turbulence closure based on asymptotic exact solutions of the equations of motion is presented. The newly developed turbulence model is used in fine-scale modeling of cloudy atmospheric boundary layers. Fine-scale cloud modeling is used to investigate the development of cloud horizontal organization, an important mechanism in the interaction of the boundary layer with the large-scale planetary circulation. Finally, we discuss how findings from high-resolution modeling of clouds are used to inform the development of convection parameterizations in climate models.