Periodic Reporting for period 3 - CLUSTER (organisation of CLoUdS, and implications for Tropical cyclones and for the Energetics of the tropics, in current and in a waRming climate)
Periodo di rendicontazione: 2021-09-01 al 2023-02-28
Few geophysical phenomena are as spectacular as tropical cyclones, with their eye surrounded by sharp cloudy eyewalls. There are other types of spatially organised convection (convection refers to overturning of air within which clouds are embedded), in fact organised convection is ubiquitous in the tropics. But it is still poorly understood and poorly represented in convective parameterisations of global climate models, despite its strong societal and climatic impact. It is associated with extreme weather, and with dramatic changes of the large scales, including drying of the atmosphere and increased outgoing longwave radiation to space. The latter can have dramatic consequences on tropical energetics, and hence on global climate. Thus, convective organisation could be a key missing ingredient in current estimates of climate sensitivity from climate models.
The goal of CLUSTER is to lead to improved fundamental understanding of convective organisation. It is closely related to the World Climate Research Programme (WCRP) grand challenge: Clouds, circulation and climate sensitivity. Grand challenges identify areas of emphasis in the coming decade, targeting specific barriers preventing progress in critical areas of climate science.
Until recently, progress on this topic was hindered by high numerical cost and lack of fundamental understanding. Advances in computer power combined with new discoveries based on idealised frameworks, theory and observational findings, make this the ideal time to determine the fundamental processes governing convective organisation in nature. Using a synergy of theory, high-resolution cloud-resolving simulations, and in-situ and satellite observations, CLUSTER will specifically target two feedbacks recently identified as being essential to convective aggregation, and assess their impact on tropical cyclones, large-scale properties including precipitation extremes, and energetics of the tropics.
The goal of CLUSTER is to lead to improved fundamental understanding of convective organisation. It is closely related to the World Climate Research Programme (WCRP) grand challenge: Clouds, circulation and climate sensitivity. Grand challenges identify areas of emphasis in the coming decade, targeting specific barriers preventing progress in critical areas of climate science.
Until recently, progress on this topic was hindered by high numerical cost and lack of fundamental understanding. Advances in computer power combined with new discoveries based on idealised frameworks, theory and observational findings, make this the ideal time to determine the fundamental processes governing convective organisation in nature. Using a synergy of theory, high-resolution cloud-resolving simulations, and in-situ and satellite observations, CLUSTER will specifically target two feedbacks recently identified as being essential to convective aggregation, and assess their impact on tropical cyclones, large-scale properties including precipitation extremes, and energetics of the tropics.
Idealized simulations of the tropical atmosphere have predicted that clouds can spontaneously clump together in space, despite perfectly homogeneous settings. This phenomenon has been called self-aggregation, and it results in a state where a moist cloudy region with intense deep convective storms is surrounded by extremely dry subsiding air devoid of deep clouds.
Since the beginning of the project CLUSTER, our work helped clarify the physics of this phenomenon, in theoretical simple models and in numerical models in idealized settings, highlighting the physical processes believed to play a key role in convective self-aggregation. We notably investigated in detail the role of the two feedbacks recently identified as being key for aggregation, the radiative feedback and the moisture-memory. This led to several publications including a recent review article on theoretical advances in our understanding of cloud clustering.
Beyond idealized models and theory, more complex settings and surface interactions were investigated, as well as the data collected during the observational campaign EUREC4A. Our results show the important role of the ocean surface and ocean eddies in the organization of tropical storms. We also contributed to the understanding and the growing literature, on the importance and implications of this phenomenon for the tropical atmosphere, notably for precipitation extremes and tropical cyclone intensification. These important implications of cloud clustering will be further explored in the remaining time of the project.
Since the beginning of the project CLUSTER, our work helped clarify the physics of this phenomenon, in theoretical simple models and in numerical models in idealized settings, highlighting the physical processes believed to play a key role in convective self-aggregation. We notably investigated in detail the role of the two feedbacks recently identified as being key for aggregation, the radiative feedback and the moisture-memory. This led to several publications including a recent review article on theoretical advances in our understanding of cloud clustering.
Beyond idealized models and theory, more complex settings and surface interactions were investigated, as well as the data collected during the observational campaign EUREC4A. Our results show the important role of the ocean surface and ocean eddies in the organization of tropical storms. We also contributed to the understanding and the growing literature, on the importance and implications of this phenomenon for the tropical atmosphere, notably for precipitation extremes and tropical cyclone intensification. These important implications of cloud clustering will be further explored in the remaining time of the project.
The detailed structure of radiative cooling in the cloud-free boundary layer remains poorly understood, despite its importance for the radiative feedback and its ability to drive convective aggregation. We analyzed radiative cooling profiles from the EUREC4A observational field campaign, and proposed a novel theoretical scaling for these profiles. This work contributes to the theoretical understanding of the radiative feedback, and its sensitivity to thermodynamic profiles and to global warming.
The moisture-memory feedback, notably the role of rain evaporation in organizing convection into squall lines, was clarified using a novel simple theoretical model. This new theory allowed to predict the orientation of squall lines, as verified in high-resolution numerical simulations.
The theoretical knowledge gained in the first half of CLUSTER allows us now to assess implications for important geophysical phenomena, notably tropical cyclones and tropical energetics. Implications in our warming climate is also investigating in light of the physical processes highlighted in our recent work. Our overall goal is to gain fundamental understanding of the physical processes involved in current and in a warming climate, as well as implications for tropical cyclogenesis and intensification, for the energetics of the tropics.
The moisture-memory feedback, notably the role of rain evaporation in organizing convection into squall lines, was clarified using a novel simple theoretical model. This new theory allowed to predict the orientation of squall lines, as verified in high-resolution numerical simulations.
The theoretical knowledge gained in the first half of CLUSTER allows us now to assess implications for important geophysical phenomena, notably tropical cyclones and tropical energetics. Implications in our warming climate is also investigating in light of the physical processes highlighted in our recent work. Our overall goal is to gain fundamental understanding of the physical processes involved in current and in a warming climate, as well as implications for tropical cyclogenesis and intensification, for the energetics of the tropics.