Final Report Summary - ATMOFLEX (Turbulent Transport in the Atmosphere: Fluctuations and Extreme Events)
A major part of the physical an chemical processes occurring in the atmosphere involves the turbulent transport of tiny particles. Current studies and models use a formulation in terms of mean fields, where strong variations are oversimplified. Devising an accurate understanding of the influence of air turbulence and of the extreme fluctuations that it generates remains a challenging issue. The ERC project AtmoFlex aimed at coordinating and integrating theoretical, numerical, and experimental efforts to develop a new statistical understanding of the role of fluctuations in atmospheric transport processes. Our work covered individual as well as collective behaviors and aimed at providing a systematic and unified description of targeted specific processes involving suspended drops or particles: the dispersion of pollutants from a source, the growth by condensation and coagulation of droplets in clouds, the scavenging, settling and re-suspension of aerosols.
During this project, significant advances have been made in quantifying concentration fluctuations due to the turbulent motion of the carrier fluid. This is for instance crucial to quantify the likeliness of finding a local pollutant concentration exceeding a high threshold. We have studied two mechanisms leading to large fluctuations and whose better understanding is key for improving risk models. The first is intrinsically related to the nature of the turbulent transport itself and can be quantified in terms of random walks in random environments and of relative dispersion between tracers of a turbulent flow. The second source of fluctuations comes from the fact that most particulate matter that is suspended in the atmosphere is constituted of finite-size particles that are heavier than the air. Their mass causes their ejection from the turbulent eddies and favors their concentration in high-strain downwelling regions. We have performed a systematic quantification of the correlations between such concentrations and the airflow structures.
Other topics on which significant advances have been made include the growth by condensation and coalescence of water droplets in turbulent clouds. We have shown that fluctuations are again central since they lead to the formation of a few droplets with sizes much larger than the bulk of the distribution. Once such “lucky droplets” appeared, strong gravity effects become dominants. They undergo a snowball effect and grow at a very fast rate. These events finally dominate the evolution of the whole size distribution. This was the first time such a phenomenon has been observed and quantified as a function of the parameters of the system. It is very likely that the accomplished work will lead to new paradigms in the study of cloud formation and to new models for explaining the observed fast time scales of rain initiation.
Let us also mention some advances on the study of large neutrally-buoyant particles in turbulence. We have characterized and quantified various dynamical properties related to their slip and their modification of the surrounding flow. Also, we have been able to provide the first experimental evidence that their global effects is a net attenuation of a developed turbulent flow. Such results can have important applications in industry for reducing the drag of various flows transporting impurities.
To conclude, let us stress that our approach is based on the simultaneous use of tools borrowed from statistical physics, the theory of large deviations and random dynamical systems, of massive direct-numerical simulations on the largest European computing centers, and of advanced experimental techniques using particle tracking. One of the expected outcomes is to provide a new framework for improving and refining the methods used in meteorology and atmospheric sciences and to answer the long-standing question of the effects of suspended particles onto climate.
During this project, significant advances have been made in quantifying concentration fluctuations due to the turbulent motion of the carrier fluid. This is for instance crucial to quantify the likeliness of finding a local pollutant concentration exceeding a high threshold. We have studied two mechanisms leading to large fluctuations and whose better understanding is key for improving risk models. The first is intrinsically related to the nature of the turbulent transport itself and can be quantified in terms of random walks in random environments and of relative dispersion between tracers of a turbulent flow. The second source of fluctuations comes from the fact that most particulate matter that is suspended in the atmosphere is constituted of finite-size particles that are heavier than the air. Their mass causes their ejection from the turbulent eddies and favors their concentration in high-strain downwelling regions. We have performed a systematic quantification of the correlations between such concentrations and the airflow structures.
Other topics on which significant advances have been made include the growth by condensation and coalescence of water droplets in turbulent clouds. We have shown that fluctuations are again central since they lead to the formation of a few droplets with sizes much larger than the bulk of the distribution. Once such “lucky droplets” appeared, strong gravity effects become dominants. They undergo a snowball effect and grow at a very fast rate. These events finally dominate the evolution of the whole size distribution. This was the first time such a phenomenon has been observed and quantified as a function of the parameters of the system. It is very likely that the accomplished work will lead to new paradigms in the study of cloud formation and to new models for explaining the observed fast time scales of rain initiation.
Let us also mention some advances on the study of large neutrally-buoyant particles in turbulence. We have characterized and quantified various dynamical properties related to their slip and their modification of the surrounding flow. Also, we have been able to provide the first experimental evidence that their global effects is a net attenuation of a developed turbulent flow. Such results can have important applications in industry for reducing the drag of various flows transporting impurities.
To conclude, let us stress that our approach is based on the simultaneous use of tools borrowed from statistical physics, the theory of large deviations and random dynamical systems, of massive direct-numerical simulations on the largest European computing centers, and of advanced experimental techniques using particle tracking. One of the expected outcomes is to provide a new framework for improving and refining the methods used in meteorology and atmospheric sciences and to answer the long-standing question of the effects of suspended particles onto climate.