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Elucidating the Role of Clouds-Circulation Coupling in Climate

Periodic Reporting for period 2 - EUREC4A (Elucidating the Role of Clouds-Circulation Coupling in Climate)

Reporting period: 2018-02-01 to 2019-07-31

The EUREC4A project focuses on two fundamental questions of climate and atmospheric science: How sensitive is Earth’s surface temperature to an increase of greenhouse gases in the atmosphere? and What governs the organization of the atmosphere into rain bands, cloud clusters and storms? 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.

With a team of PhD students, post-docs and engineers, and in collaboration with a number of collaborators, we are tackling these challenges through a combination of observational and modelling approaches.

One of the main sources of uncertainty in the Earth’s climate sensitivity to greenhouse gases is the response of low-level clouds to global warming. To reduce this uncertainty, I will lead (jointly with B. Stevens, and in collaboration with many European and US teams) the first airborne field campaign specifically designed to better understand the physical processes through which low clouds interact with their environment. The EUREC4A field campaign will take place in the trade wind region of the tropical Atlantic on Jan-Feb 2020 (Bony et al., Surveys in Geophysics, 2017,

In parallel, we are exploring the physical mechanisms through which the atmosphere can organize in moist and dry areas, and how the spatial organization of clouds can influence climate. We are investigating in particular the role of the cloud organization in the Earth's radiation balance, in the large-scale atmospheric circulation, and in atmospheric hydrology and extremes.
The results so far have shown that the clustering of deep clouds (such as those involved in thunderstorms) interacts strongly with the ocean surface temperature, that the coupling between clustering, surface temperature and radiation can generate internal variability in the climate system, and that it can influence the Earth’s radiation budget and the characteristics of the tropical rain belts.

In parallel, we have been preparing the EUREC4A field campaign by developing and testing novel experimental strategies to better understand how marine low-clouds interact with their environment. This includes the development of methodologies that will allow us to measure the large-scale vertical motions in the atmosphere, the mass flux at the base of shallow clouds, and the area covered by clouds at their base.
One particular achievement has been the demonstration that it was possible to measure with dropsondes the vertical profile of large-scale atmospheric vertical motions, a measurement which had been considered so far as a long-standing observational challenge of atmospheric science.

Through its design and novel experimental strategies, the EUREC4A field campaign has raised a lot of interest across the international community, and it is now growing into a capstone field project of the World Climate Research Programme. More and more teams from Europe and from the US are planning to join to develop complementary investigations, and observations from several additional aircraft and research vessels are currently in discussion. Fifty years after the BOMEX (Barbados Oceanographic and Meteorological Experiment), the EUREC4A field campaign promises to shed a new light on our understanding of the coupling between clouds and their environment in the trade-wind regions.
Shallow and deep clouds observed over the tropical Atlantic during NARVAL2