Comment les cumulus s’organisent-ils ? Lessons de la campagne EUREC4A / What controls the organization of shallow clouds ? Lessons from the EUREC4A campaign.

Séminaire organisé par Sandrine Bony et Bjorn Stevens du 21 au 26 Mars 2022 

 

Participants 

Franziska Aemisegger (ETH, Zurich), Anna Lea Albright (LMD, Paris), Sandrine Bony (LMD, CNRS, Paris), Pierre-Etienne Brilouet (CNRM, Toulouse), Fleur Couvreux (Météo-France, Toulouse), Julien Delanoë (LATMOS, Guyancourt), Kerry Emanuel (MIT, USA), Geet George (MPI, Hamburg), Silke Gross (DLR, Munich), Tobias Koelling (MPI, Hamburg), Marie Lothon (LAERO, Toulouse), Bernhard Mayer (Munich University), Ann Kristin Naumann (Hamburg University), Nicolas Rochetin (ENS, Paris), Hauke Schulz (MPI, Hamburg), Pier Siebesma (Delft University), Bjorn Stevens (MPI for Meteorology, Hamburg), Raphaela Vogel (LMD, Paris).

 

Résumé 

Flower pattern / motif fleuri

Les nuages de type cumulus qui se forment dans les premiers kilomètres de l’atmosphère constituent le type de nuage le plus fréquent sur Terre. Ces nuages peuvent apparaître isolément ou en groupes, formant des motifs caractéristiques. La disposition spatiale de ces nuages, appelée organisation, est associée à une grande diversité de formes et d’aspects tels que cellules, arcs, fleurs, sucre, gravier ou arêtes de poisson. Des travaux recherche ont montré que l’organisation des nuages influençait la météorologie et le climat de différentes façons, mais les mécanismes physiques expliquant cette organisation restent largement mystérieux. La campagne de terrain internationale EUREC4A (Elucidating the role of cloud-circulation coupling in climate) qui s’est déroulée en janv-fév 2020 au large de la Barbade pour étudier l’interaction des nuages de convection avec leur environnement est une opportunité pour mieux comprendre l’organisation des nuages. Après deux ans de crise sanitaire, ce séminaire était la première réunion en personne depuis la campagne. L’objectif du Séminaire était de discuter i) de ce que nous avons déjà appris de la campagne EUREC4A concernant l’organisation mésoéchelle de la convection et des nuages, ii) des questions qui restent à élucider et  iii) des travaux de recherche et des collaborations qui aideraient à résoudre ces questions plus efficacement. Le séminaire a réuni 18 participants (dont 45 % de femmes, 9 scientifiques en début de carrière et 9 scientifiques confirmés) venus de France, d’Allemagne, de Suisse, des Pays-Bas et des États-Unis, avec un mélange d’expertise allant de l’observation des nuages, à la modélisation ou la théorie. Les deux premiers jours, chaque participant a présenté ses résultats et ses réflexions sur l’organisation des cumulus, à partir de quoi nous avons identifié six sujets de discussion que nous avons abordés en petits groupes ou en séance plénière. Grâce à ce Séminaire, nous avons pu définir des axes de collaboration autour de l’analyse des observations EUREC4A et des simulations associées, nous avons défini l’ébauche d’un article résumant les aspects clés des observations EUREC4A que les modèles numériques devraient être en mesure de représenter, et nous avons discuté des observations qui, dans le futur, pourraient nous aider à résoudre les grandes questions liées à l’organisation mésoéchelle des nuages. Enfin, dans la lignée des rencontres entre Art et Science chères à la Fondation des Treilles, une jeune participante (Anna Lea Albright) passionnée de nuages et de peinture a donné un soir un séminaire sur : Comment les peintures de Turner et Monet montrent les tendances de la pollution atmosphérique au cours du 19th siècle. Cet exposé fut l’occasion de discussions et de collaborations avec les historiennes de l’Art en résidence à la Fondation. Le séminaire restera dans les mémoires comme une rencontre extrêmement productive et stimulante dans un cadre merveilleux. Tous les participants sont très reconnaissants à la Fondation des Treilles et à son personnel pour avoir permis cette rencontre dans d’aussi bonnes conditions.

 

Summary 

The shallow cumuli that form within the first few kilometers of the atmosphere constitute the most frequent  cloud type on Earth. These clouds can appear in isolation or in groups, forming horizontal patterns. The spatial arrangement of these clouds, called organisation, can take a spectacular variety of forms and shapes such as cells, arcs, flowers, sugar, gravel or fish bones. Past research has shown that the organisation of clouds can influence weather and climate in many important ways, but the physical mechanisms explaining this organisation remain largely mysterious. The international field campaign EUREC4A (Elucidating the role of cloud-circulation coupling in climate) that took place in Jan-Feb 2020 over the tropical Atlantic ocean east of Barbados to study the interaction of shallow cumulus clouds with their environment is an opportunity to better understand the cloud organisation. After two years of sanitary crisis, this Seminar was the first in-person meeting since the field work. Its objective was i) to discuss what has already been learned from the EUREC4A campaign about the mesoscale organisation of convection and clouds, ii) what we would like to understand further, and iii) what could be done to facilitate this understanding. The seminar brought together 18 participants (including 45 % female scientists, 9 early career scientists and 9 senior scientists) from France, Germany, Switzerland, the Netherlands and the USA, with a mixture of observational, modelling or theoretical backgrounds. During the first two days, each participant presented his/her recent results and thoughts about the organisation of cumulus clouds, and then we identified six main topics to be discussed further either in small groups or in plenary. The outcome of the Seminar was the definition of new collaborative work around the analysis of EUREC4A observations and simulations, the discussion of new observations that could help fill remaining gaps of our knowledge, and the outline of an article summarizing the key aspects of EUREC4A observations that numerical models should be able to represent. Finally, in the spirit of the meetings between Art and Science dear to the Fondation des Treilles, a young participant (Anna Lea Albright) passionate about clouds and painting gave an evening seminar on the theme: Paintings by Turner and Monet Depict Trends in 19th Century Air Pollution. Her talk was an opportunity for discussion and collaboration with the Foundation’s resident art historians. The seminar will be remembered as an extremely productive and stimulating event in a beautiful setting. All participants are very grateful to the Fondation des Treilles and its staff for having made this Seminar possible in such exceptional conditions.

 

Report 

EUREC4A observations shed a new light on the clouds and the vertical structure, spatial organisation, and circulations of the atmosphere in the trade-wind regions. It was discussed by Anna Lea Albright, who showed how EUREC4A transformed our appreciation and interpretation of transition layers between the well-mixed boundary layer and the cloud layer, by Geet George, who showed the ubiquity of shallow mesoscale overturning circulations and their importance of these circulations for the moisture variance in the lower troposphere, and Raphaela Vogel, who showed the importance of the dynamical control of the trade-wind cloudiness relative to its thermodynamical control, and the implications for this finding for the response of trade-cumuli to global warming. Pierre-Etienne Brilouet and Marie Lothon showed that during EUREC4A the characteristic lengths scales of variability of the boundary layer were not only the classical ones inferred from turbulence theory, but also depicted a strong variability at the mesoscale. The possible role in this mesoscale organisation of cold pools driven by the reevaporation of precipitation was discussed by Nicolas Rochetin. In turn, Ann Kristin Naumann showed how the organisation of convection was influencing the intensity or precipitation in particular conditions. Franziska Aemisegger presented the water stable isotopic measurements made during EUREC4A and showed how the isotopic composition of water could provide insights about the local vs remote (e.g. extratropical) processes controlling the relative occurrence of the different mesoscale patterns of cloudiness. Bernhard Mayer showed how new hyperspectral observations of clouds could, through a detailed analysis of the backscatter glory, help retrieve microphysical properties of clouds, and the question arose as to whether the different cloud patterns were associated with different cloud microphysical properties.

To interpret the ubiquity of mesoscale circulations and variability in the atmosphere of the trade-wind regions, Kerry Emanuel proposed a theory derived from boundary-layer quasi-equilibrium and Pier Siebesma proposed another consistent theoretical framework aiming to explain the development of mesoscale instabilities and the growth of humidity flucturations in atmospheric convection.

Then several presentations discussed the ability of numerical models to reproduce the mesoscale variability and cloud patterns that were observed during EUREC4A. Fleur Couvreux showed that the large-eddy simulation model Meso-NH was able to reproduce the ‘Flowers’ patterns of cloudiness observed during EUREC4A, and that the AROME convection-resolving model was able to reproduce part of the observed mesoscale variability of cloudiness and humidity. Hauke Schulz presented large-eddy simulations of the whole EUREC4A field campaign by the ICON model and showed that the model reproduced the large variability of cloud patterns, with however more difficulty in simulating ‘Flowers’. Finally, several participants noted that models tend to strongly underestimate the mesoscale variance of moisture within the subcloud-layer compared to observations, a feature that remains to be interpreted.

Finally, several presentations discussed the new questions raised by the recent EUREC4A findings, and discussed their implications for our understanding of the more general issue of the mesoscale organisation of shallow and deep convection in the tropical atmosphere. Bjorn Stevens questioned the physical processes that control the structure of the Inter-Tropical Convergence Zone over the tropical Atlantic, and presented ideas to investigate these processes further through a mix of airborne and satellite observations over the tropical Atlantic. Sandrine Bony showed how the different mesoscale patterns of cloudiness observed during EUREC4A were related to different mixtures of cloud populations scaled by subcloud-layer properties, and discussed how the universality of these results in regimes of shallow and deep convection could be tested with new observations. Then, Silke Gross and Julien Delanoë discussed how these different ideas could be tested in future campaigns using current and new remote sensing techniques and instrumentation, and Tobias Kölling discussed novel approaches to data sharing and collaboration.

Finally, during an evening seminar, Anna Lea Albright presented her recent work that suggests that the language of the sky has imprinted its historical evolution, and hence past climate changes, on art and culture. She showed how trends in air pollution can be identified through the paintings of J.M.W. Turner and Claude Monet, who painted scenes of London and Paris in the 19th century during early industrialization. She  discussed how art and science are both products of human observation and imagination and give complementary perspectives about the natural world and our role as actors and observers of environmental change.


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