RAL SEMINAR: Impact of Aerosols and Turbulence on Clouds: Process Level Investigations Using Experiments, Theory and Modeling

Clouds are critical for both short-term weather patterns and long-term climate change since they affect radiative forcing and the hydrological cycle. Of particular importance, clouds and associated microphysical processes significantly impact climate sensitivity. Macro-scale cloud properties intimately depend on small-scale properties such as cloud particle size and concentration, which are directly coupled to the turbulent flow field and aerosol concentration. This talk will cover aerosol-cloud-turbulence interactions with a focus on microphysical details. This problem is investigated using three different approaches -- theory, laboratory experiments, and numerical modeling. Experiments using the Michigan Tech Pi Cloud Chamber show a turbulence-induced broadening of droplet size distributions (DSDs), and the aerosol concentration modulates this broadening process. The theoretical DSD shapes in turbulent clouds (obtained from solutions of the Fokker-Planck equation) will be discussed. A theoretical DSD expression for fluctuating supersaturation with a size-dependent droplet removal rate gives the most favorable comparison to the measurements. However, even this optimal distribution breaks down for broad aerosol size distributions. A numerical modeling study of the influence of turbulence, entrainment, and secondary activation (activation above the cloud base) on cumulus clouds will then be presented. This problem was investigated using a futuristic Lagrangian cloud microphysics scheme (“super-droplet method’’) in a large-eddy simulation (LES) framework using the CM1 cloud model. The Lagrangian microphysics was coupled to subgrid-scale schemes for turbulent transport and supersaturation fluctuations impacting droplet activation and condensation growth. Subgrid-scale turbulent fluctuations cause broadening of cloud DSDs and increased activation of cloud droplets. DSD width also increases with increasing entrainment-induced dilution, except in the most diluted cloud regions. Secondary activation affects DSDs in two contrasting ways: narrowing with adiabatic fraction in highly diluted regions and broadening in relatively less diluted. Impacts of cloud DSD evolution on precipitation generation will also be discussed; comparison with simulations using a bin microphysics scheme suggests that a larger DSD width in bin simulations (likely due to numerical diffusion) initiates rain earlier and enhances rain development in a positive feedback loop. The talk will end with a broader discussion of how these process-level investigations of aerosol-cloud-turbulence interactions could improve the bulk representation of clouds in weather and climate models.