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Elucidating the Role of Clouds-Circulation Coupling in Climate

Periodic Reporting for period 4 - EUREC4A (Elucidating the Role of Clouds-Circulation Coupling in Climate)

Reporting period: 2021-02-01 to 2022-07-31

The EUREC4A (Elucidating the role of cloud-circulation coupling in climate) project focused on two fundamental questions of climate and atmospheric science: How sensitive is the Earth’s surface temperature to an increase in atmospheric greenhouse gases? and What role does the organization of the atmosphere into rain bands, cloud clusters or storms play in climate? Answering these seemingly different questions is important to improve estimates of the rate and magnitude of global warming over the next decades, and to better understand and predict the evolution of weather and climate at the regional scale.

One of the main sources of uncertainty in the Earth’s climate sensitivity to greenhouse gases is the response of clouds to changes in their environment, particularly the response of low-level clouds that occur in the trade wind regions. To reduce this uncertainty, we designed and led (with B. Stevens, in collaboration with European, US and Caribbean teams) the first airborne field campaign specifically designed to test hypothesized mechanisms whereby changes in trade wind clouds with global warming act to amplify this warming, a positive feedback which would increase Earth's climate sensitivity. Beyond this specific objective, the field campaign aimed to better understand the physical processes through which trade wind clouds interact with their environment. The EUREC4A field campaign took place over the tropical Atlantic near Barbados in January and February 2020.

In parallel, we have investigated how clouds and convection organize themselves in space, forming clumps or patterns, and explored the role this spatial organization might play in climate. In doing so, we sought to address one of the main questions of the World Climate Research Programme’s Grand Challenge on Clouds, Circulation and Climate Sensitivity. In particular, we examined the role of cloud organization in the Earth's radiation budget, large-scale atmospheric circulation, and extreme precipitation. We also studied how anvil clouds, which form in the upper atmosphere and can cover large areas, respond to climate perturbations or natural or anthropogenic origin.

The project addressed these challenges with a team of PhD students, post-docs and engineers, in collaboration with a number of collaborators, and by exploiting the synergy between models and observations on a wide range of scales.
The EUREC4A field campaign marked a turning point in our ability to observationally study the factors influencing clouds and their coupling to atmospheric circulations. By developing novel methodologies and experimental techniques, such as the measurement of area-averaged vertical motions using dropsondes, the estimate of the convective mass flux from the mass budget of the atmospheric boundary layer or the measurement of the cloud cover around the cloud base level using horizontal lidar and radar remote sensing, we have demonstrated that trade cumulus clouds are more dynamically controlled by convective and mesoscale vertical motions than thermodynamically controlled by humidity variations, and that the coupling between clouds and lower-tropospheric mixing does not desiccate clouds. This finding refutes the mechanism by which some climate models predict a strong amplification of global warming by trade cumulus clouds. Furthermore, by revising our physical interpretation of the vertical structure of the trade-wind atmosphere, by revealing the ubiquity of shallow circulations and cloud patterns at the mesoscale (20-200 km) and by demonstrating their importance in controlling moisture and radiation in trade wind regions, we have advanced our fundamental and conceptual understanding of the tropical atmosphere.

In parallel, our project tested the physical hypothesis that the anvil cloud coverage in the tropical atmosphere is largely controlled by the stability and radiative cooling of the clear-sky areas surrounding the clouds. We showed that this hypothesis was supported by satellite observations and meteorological analyses at the interannual time scale and that the same mechanism explained the response of anvil clouds to a range of anthropogenic or natural perturbations, including explosive volcanic eruptions. These findings thus advanced our understanding of how a major cloud type responds to a range of environmental changes.

Finally, our project showed that the spatial arrangement of clouds in the tropics influenced climate in several important ways. It has provided the first observational evidence that the degree of aggregation (or clumping) of deep convective clouds is a primary modulator of the tropical Earth’s radiation budget, impacts the intensity of extreme precipitation events, and the structure and width of tropical rain belts. Based on idealized simulations in which the atmosphere interacts with the ocean, we also suggested that the interplay between convective aggregation, sea surface temperature and radiation could generate some internal climate variability and influence the sensitivity of the Earth’s surface temperature to an increase in greenhouse gases in the atmosphere.

Overall, the project has resulted in 50 peer-reviewed publications, and the EUREC4A field campaign in dozens of observational data sets. The project has also produced a generation of Early Career Scientists versed in exploiting the synergy between observational and modeling approaches, and in linking cloud processes to the atmospheric circulations in which they are embedded while elucidating their role in climate.
Due to its innovative design and experimental strategies, the EUREC4A field campaign has attracted great interest across the international community. With the participation of dozens of institutions from Europe, the United States and the Caribbean, four research aircraft, four research vessels, advanced ground based remote sensing and a wide variety of tethered and new remotely piloted platforms observing the atmosphere, the upper ocean and the air-sea interface, and its coordination with modeling initiatives, the EUREC4A campaign has become a capstone field project of the World Climate Research Programme.

Beside its scientific results published in leading and high-profile journals, the ERC project has developed new experimental techniques that radically change our ability to observe clouds, circulation and their coupling. In particular, our ability to measure area-averaged vertical motions of the atmosphere on the scale of 20-200 km has filled a long-standing observational gap in atmospheric science. Our ability to perform horizontal lidar and radar remote sensing from an aircraft has also opened new avenues to characterize cloud fields and their spatial organization. These measurements have already led (directly or indirectly) to important discoveries, and they will likely be exploited in future field campaigns.

Our project has provided evidence of the importance, for humidity, clouds and radiation, of atmospheric circulations and convective organizations occurring at the mesoscale (20-200 km). This finding questions our understanding of the climate system and the models currently used to predict climate and climate change at the global scale. It has contributed to develop a new field of research on the underlying physics, and the implications for climate, of the mesoscale organization of convection.