organisation of CLoUdS, and implications for Tropical cyclones and for the Energetics of the tropics, in current and in a waRming climate

Project Information

CLUSTER

Grant agreement ID: 805041

DOI

10.3030/805041

Project closed

EC signature date 24 September 2018

Start date 1 June 2019

End date 31 May 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 078 021,00

EU contribution

€ 1 078 021,00

Coordinated by

INSTITUTE OF SCIENCE AND TECHNOLOGY AUSTRIA
Austria

Periodic Reporting for period 5 - CLUSTER (organisation of CLoUdS, and implications for Tropical cyclones and for the Energetics of the tropics, in current and in a waRming climate)

Reporting period: 2024-09-01 to 2025-05-31

Summary of the context and overall objectives of the project

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 was 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.

The first phase of the CLUSTER project focused on identifying the key physical processes that drive convective organisation. These processes were examined through idealised simulations and observations of the tropical atmosphere.

The second phase explored how convective organisation influences major atmospheric phenomena such as tropical cyclones, the hydrological cycle, and extreme precipitation.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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 investigated in detail the role of the two feedbacks recently identified as being key for aggregation, the radiative feedback and the moisture-memory (Hwong & Muller 2024, GRL). This led to several publications including a review article on theoretical advances in our understanding of cloud clustering (Muller et al 2022, ARFM) and a publication in physics today which was highlighted on their cover (Muller & Abramian 2023, Phys. Today).

Beyond idealized models and theory, more complex settings were investigated, as well as data collected during the observational campaign EUREC4A, which led to several publications (e.g. Albright et al 2021, ESCD; Fildier et al 2023, AGU Advances). We also contributed to the growing literature on the importance and implications of this phenomenon for the tropical atmosphere, notably for precipitation extremes (Bao et al, 2024 Sciences Advances) and tropical cyclones (Polesello et al 2025, JAMES). These results received media coverage, invited talks (including a keynote talk at a recent Hackathon (May 2025 MPI-Hamburg) and were presented in outreach events (such as Pint of Science, Think and Drink or high school climate events).

Overall, by combining theoretical analysis, high-resolution modelling, and both in-situ and satellite observations, CLUSTER has advanced our understanding of the physical mechanisms underpinning convective aggregation. It has also helped quantify the impact of convective organisation on tropical cyclones, precipitation extremes, and the overall energy balance of the tropics. Notably, as the climate warms, changes in convective organisation are expected to amplify precipitation extremes beyond what thermodynamics alone would predict. Therefore, incorporating convective organisation into climate models is essential for improving rainfall projections in a warming world.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

CLUSTER has advanced our fundamental understanding of convective organisation. For instance, the detailed structure of radiative cooling in clear-sky conditions remains poorly understood, despite its crucial role in radiative feedback and its ability to drive convective aggregation. Using data from the EUREC4A field campaign, we analysed radiative cooling profiles and introduced a new theoretical scaling. This work advances our understanding of the radiative feedback and its sensitivity to thermodynamic profiles.

We also clarified the moisture-memory feedback, particularly how rain evaporation helps organise convection into squall lines. A new, simple theoretical model was developed to predict squall line orientation, which was validated through high-resolution simulations.

A key feature in squall line organisation is the formation of cold pools, which are regions of negatively buoyant air below precipitating clouds, caused by partial rain evaporation and latent cooling. Our idealised simulations revealed how rain evaporation influences convective organisation.

The theoretical foundations built in the first phase of CLUSTER enabled us to explore convective organisation in more complex contexts, including realistic simulations and observations of the tropical atmosphere. The second phase focused on implications for key geophysical phenomena, such as tropical cyclones, precipitation, and tropical energetics. CLUSTER helped clarify the role of radiation in modulating cyclone intensity, and revealed a novel result: interactions between tropical cyclones and the stratosphere also influence cyclone strength.

We also investigated the physical processes that lead to the largest cloud systems in realistic global high-resolution simulations. Cloud cover plays a key role in cloud radiative feedback, with implications for climate sensitivity. CLUSTER identified the internal feedbacks that lead to extensive cloud cover.

Finally, CLUSTER showed that tropical daily precipitation extremes are projected to intensify with warming. These extremes are stronger in more organised convective states, where storms are larger but fewer. In a warmer climate with increased organisation, oceanic precipitation extremes grow faster than thermodynamic expectations. Given that precipitation extremes are among the most destructive natural hazards, these findings highlight the societal and climatic significance of convective organisation in the tropics.

Idealised numerical simulation of convective organisation: In the absence of feedback mechanisms