Image credits:
Dilute suspension of flexiblefibers in turbulent channel flow
(D. Di Giusto, C. Marchioli)
Flyer:
Click here to download.Confirmed Keynote Speakers
Professor Gholamhossein (Mohsen) Bagheri
Max Planck Institute for Dynamics and Self-Organization (GER)
The Mystery of the Lucky Droplet: Can Turbulence Trigger Rain in Warm Clouds?
Abstract:
While major advances have been made in understanding clouds and their role in weather and climate, some everyday atmospheric phenomena remain unexplained. A notable example is the rapid initiation of rain in warm clouds (i.e., clouds composed entirely of liquid droplets), which produces up to 70% of tropical rainfall. Turbulence-induced processes are among the leading mechanisms proposed to explain this long-standing problem. Such processes are largely hypothesized to arise from the fact that cloud droplets, although small, do not perfectly follow the flow and instead behave as inertial particles, depending on their size and the local turbulence intensity. Studying these interactions, however, remains extremely challenging. Realistic cloud conditions are difficult to reproduce in laboratory experiments or numerical simulations, while airborne measurements often lack the spatial and temporal resolution required to simultaneously resolve cloud microphysics and turbulence. In this talk, I will present recent findings from novel airborne measurements using an instrumented tethered aerostat equipped with the fastest holographic imaging system and the first airborne particle image velocimetry measurements inside clouds. These techniques enabled collocated, instantaneous measurements of cloud microphysics and turbulence within shallow cumulus clouds in the tropical trade-wind region near Barbados. Our observations reveal that precipitating clouds exhibit highly localized clustering of droplets within regions spanning roughly one meter, where the probability of finding droplet pairs at sub-millimeter separations is several times greater than expected for a random distribution. Furthermore, these clustering events are strongly correlated with intense turbulent fluctuations. Our analysis suggests that these structures are consistent with the centrifugal ejection of inertial particles from vortical regions, rather than being primarily associated with entrainment and mixing processes. These observations provide some of the first direct evidence supporting the ``lucky droplet'' hypothesis, in which rare and localized microphysical events contribute disproportionately to the initiation of warm rain.
Brief Bio:
Dr. Gholamhossein (Mohsen) Bagheri studied mechanical engineering with a specialization in fluid mechanics at Persian Gulf University and Shahid Bahonar University of Kerman, where he earned his BA and MSc degrees, respectively. He subsequently completed his PhD at University of Geneva. During his doctoral research, he carried out experimental and numerical investigations into the dynamics of irregular particles in laminar and turbulent flows, advancing the understanding and predictive modeling of the transport dynamics of non-spherical particles. Following his PhD, Dr. Bagheri was awarded a fellowship from the Swiss National Science Foundation to join the Max Planck Institute for Dynamics and Self-Organization as a visiting scientist. Since 2019, he has led a research group at the institute. His current research focuses on cloud microphysics and atmospheric turbulence using the Max Planck CloudKites platform (i.e. an instrumented tethered balloon platform), the characterization of respiratory particle emissions and airborne pathogen transmission, and the experimental and computational study of non-spherical particle dynamics, including ice crystals, volcanic ash, and microplastics. Through this interdisciplinary work, Dr. Bagheri contributes to advancing both fundamental fluid mechanics and its applications in atmospheric science, environmental science, and public health.
Max Planck Institute for Dynamics and Self-Organization (GER)
The Mystery of the Lucky Droplet: Can Turbulence Trigger Rain in Warm Clouds?
Abstract:
While major advances have been made in understanding clouds and their role in weather and climate, some everyday atmospheric phenomena remain unexplained. A notable example is the rapid initiation of rain in warm clouds (i.e., clouds composed entirely of liquid droplets), which produces up to 70% of tropical rainfall. Turbulence-induced processes are among the leading mechanisms proposed to explain this long-standing problem. Such processes are largely hypothesized to arise from the fact that cloud droplets, although small, do not perfectly follow the flow and instead behave as inertial particles, depending on their size and the local turbulence intensity. Studying these interactions, however, remains extremely challenging. Realistic cloud conditions are difficult to reproduce in laboratory experiments or numerical simulations, while airborne measurements often lack the spatial and temporal resolution required to simultaneously resolve cloud microphysics and turbulence. In this talk, I will present recent findings from novel airborne measurements using an instrumented tethered aerostat equipped with the fastest holographic imaging system and the first airborne particle image velocimetry measurements inside clouds. These techniques enabled collocated, instantaneous measurements of cloud microphysics and turbulence within shallow cumulus clouds in the tropical trade-wind region near Barbados. Our observations reveal that precipitating clouds exhibit highly localized clustering of droplets within regions spanning roughly one meter, where the probability of finding droplet pairs at sub-millimeter separations is several times greater than expected for a random distribution. Furthermore, these clustering events are strongly correlated with intense turbulent fluctuations. Our analysis suggests that these structures are consistent with the centrifugal ejection of inertial particles from vortical regions, rather than being primarily associated with entrainment and mixing processes. These observations provide some of the first direct evidence supporting the ``lucky droplet'' hypothesis, in which rare and localized microphysical events contribute disproportionately to the initiation of warm rain.
Brief Bio:
Dr. Gholamhossein (Mohsen) Bagheri studied mechanical engineering with a specialization in fluid mechanics at Persian Gulf University and Shahid Bahonar University of Kerman, where he earned his BA and MSc degrees, respectively. He subsequently completed his PhD at University of Geneva. During his doctoral research, he carried out experimental and numerical investigations into the dynamics of irregular particles in laminar and turbulent flows, advancing the understanding and predictive modeling of the transport dynamics of non-spherical particles. Following his PhD, Dr. Bagheri was awarded a fellowship from the Swiss National Science Foundation to join the Max Planck Institute for Dynamics and Self-Organization as a visiting scientist. Since 2019, he has led a research group at the institute. His current research focuses on cloud microphysics and atmospheric turbulence using the Max Planck CloudKites platform (i.e. an instrumented tethered balloon platform), the characterization of respiratory particle emissions and airborne pathogen transmission, and the experimental and computational study of non-spherical particle dynamics, including ice crystals, volcanic ash, and microplastics. Through this interdisciplinary work, Dr. Bagheri contributes to advancing both fundamental fluid mechanics and its applications in atmospheric science, environmental science, and public health.
