EUREC4A-OA will implement ad-hoc innovative observations and a hierarchy of numerical simulations focusing on mesoscale and submesoscale ocean dynamics and the atmospheric boundary layer at scales ranging from 20 m to 1000 km over the northwest tropical North Atlantic. The aim is to advance our knowledge of the phenomenology and representation of air-sea interactions, physical and biogeochemical ocean small-scale non-linear processes in ESMs but also in NWPs, S2Ss and decadal forecasts operational systems. EUREC4A-OA will bring together international specialists of ocean, atmosphere physical and biogeochemical observations and numerical modelling as well as scientists working on numerical parameterization, operational systems and future projections to address four objectives: 1) Assessing the impact of the diurnal cycle on energy, water and CO2 ocean-atmosphere exchanges and quantifying the modification of diurnal cycle and the related exchanges by meso-scale and submeso-scale features and other extreme conditions; 2) The identification and quantification of the processes ruling the ocean-atmosphere exchanges and uptake of heat, momentum and CO2 at the ocean nonlinear small scales (from a few tens of meters to 500 km); 3) The role of various processes (diurnal cycle, ocean nonlinear small scales, boundary layer aerosols) on the atmosphere shallow convection and cloud formation; 4) To provide improved models metrics and parameterizations for the above processes to be integrated in operational prediction systems and ESMs. EUREC4A-OA associated partners (12 international institutions contributing with more than 35 scientists) will cooperate in integrating new knowledge into improved model metrics and parameterizations. EUREC4A-OA results will enhance capability to deliver novel information that will have a significant impact on science and society.

Groundwater (GW) constitutes 30% of the fresh water resources, which are subjected to increasing withdrawals. When shallow enough, it can also sustain soil moisture, thus increase evapotranspiration, with potential impact on the climate system (in particular temperatures and precipitation). Its large residence time can also increase the Earth system’s memory, with consequences on the persistence of extreme events, hydro-climatic predictability, and anthropogenic climate change, particularly the magnitude of regional warming. Our main goal is to explore the impacts of GW on regional and global climate, and its links to water resources availability, through model analyses. To this end, our Franco-Taiwanese consortium offers a unique opportunity to compare the sensitivity of simulated climate to different GW parametrizations within 3 different climate models: the French IPSL and CNRM-GAME climate models, and the American NCAR climate model (CESM), modified and used here by the Taiwanese team. All teams have experience in international intercomparison projects, and they have all recently emerged as important actors of the research on groundwater in climate models: the IPSL team and Min-Hui Lo have pioneered the analysis of the sensitivity of global simulated climate to GW, while the CNRM-GAME team achieved significant advances regarding the global-scale parameterization of GW and its coupling with rivers and land surfaces. The project includes two transversal tasks: T0. Coordination; T5. International workshops; and the research program is organized into 4 successive scientific tasks: T1. Sensitivity to fixed water table depths (WTD), to identify the patterns of “active WTD”, below which GW do not impact regional climate T2. Dynamic WTD over the recent period, to assess the potential of realistic GW parametrizations to improve the simulated climate, with a focus on land/atmosphere feedback and persistence/ memory in the Earth system T3. Dynamic WTD and climate change, with two complementary questions: (1) What is the influence of GW on the climate change trajectory? (2) What is the impact of climate change on water resources (including GW)? T4. Dynamic WTD with withdrawals, which artificially increase soil moisture via irrigation, with potential impacts on climate until water resources get exhausted. I-GEM is also intended to consolidate the potential of France and Taiwan in the interdisciplinary research field of the global water cycle, by tightening the links between these two countries, and by federating the French community (IPSL and CNRM-GAME). We also aim at enhancing the visibility of French and Taiwanese teams, by developing closer links with European and North-American leaders in large-scale modeling of GW. To this end, we want to organize two international workshops on the role of GW in climate models, one in Taiwan and one in France, with a broad audience (T5).

Latest-generation atmospheric models like the hydrostatic icosahedral dynamical core DYNAMICO carry the promise of addressing scientific issues that remain out of reach with current operational models. Putting these latest-generation models to work to answer scientific questions nevertheless requires significant effort and collaboration between experts of the numerics, computing, and scientific use of such models. HEAT sets up such a collaboration in order to address extreme atmospheric modelling applications and remaining numerical and computational challenges. We will pioneer numerical modelling of the general atmospheric circulation of gaseous giant planets and achieve significant milestones towards millenial-scale Earth system simulations relevant for palaeoclimatology. In terms of numerical and computational challenges, our objectives are to address the higher-order extension of highly scalable numerical methods for transport and dynamics, and bottlenecks for non-hydrostatic modelling, especially elliptic problems. Jupiter and Saturn have fast-rotating atmospheres, prone to powerful global winds and intense convective and wave activity. The ambition of HEAT is to put together a team of experts able to design and implement unprecedented high-resolution global circulation experiments for Saturn and Jupiter developing significant wave activity and elucidate how this wave activity forces the global circulation, a dynamical process that is also key to the terrestrial climate. In a longer perspective, our effort will equip France and Europe with a state-of-the-art operating model to interpret the results from the NASA mission JUNO and the ESA mission JUICE towards the Jovian system. In order to understand large climate reorganizations that occured during the last deglaciation we will gradually develop a new ESM based on DYNAMICO, the land surface model ORCHIDEE, the aerosols model INCA and the ocean/sea-ice model NEMO, with a careful analysis of key conservation constraints. In addition to long equilibrated simulations, short global 30 km resolution simulations of key periods in the past will offer new possibilities to refine the comparison of model results with regional paleoclimate observations. On the numerical side we shall explore WENO approaches to non-oscillatory, accurate finite volume transport as well as the discontinuous Galerkin method in order to offer a range of accuracy/efficiency trade-offs for DYNAMICO's transport scheme that may be important in the presence of sharp fronts and strong nonlinear interactions. For the dynamics formal accuracy is crucial for reducing the imprinting caused by mesh irregularities. We shall pursue the mixed finite element approach, which yields excellent discrete conservation properties. Finally, as exascale capability will approach, global modelling will become possible at fine spatial scales where the hydrostatic approximation becomes problematic. On one hand, we aim at extending DYNAMICO to non-hydrostatic dynamics while maintaining scalability, ability to relax certain terrestrial approximations and exact conservation of mass, energy, vorticity, for long-term simulations. On the other hand we will explore innovative solutions aimed at improving the scalability and feasibility of long-time-step approaches. Beyond HEAT, the work produced by all tasks, and the expertise gained, will contribute to strengthen the world-class capability of the French climate and weather modelling community to address an extended range of future scientific challenges related to past, present and future climate dynamics at global and regional scales. By bringing together geoscientists and applied mathematicians, and by contributing to events such as the CEMRACS summer school and the PDEs on the Sphere workshop, HEAT will strengthen the French applied maths community addressing atmospheric modelling in the long run and contribute to its insertion in the corresponding international community.

Tropical cyclones are a major threat for many countries. They generate strong winds and deadly floods that damage buildings and infrastructure and severely disrupt civil and military organization. A precise forecast of the trajectory and intensity of tropical cyclones makes it possible to take the necessary measures to protect people and property while avoiding false alarms. The quality of the forecasts, however, depends largely on the quality of the observations. For the moment, only airborne measurements provide accurate in situ observations for the initialization of tropical cyclone prediction models. For this reason, the United States Air Force (USAF) and the National Oceanic and Atmospheric Administration (NOAA) have been deploying considerable resources for the observation of cyclones in the North Atlantic and East Pacific for several decades. These airborne measurements, carried out quite systematically by the USA and sporadically by Taiwan, are however exceptions worldwide. No airborne measurements are made in the South Indian Ocean, where the island of La Reunion is however regularly exposed to cyclones. Most of the real-time information collected by the Regional Specialized Meteorological Centers (RSMCs) therefore comes from satellite observations. These observations are irreplaceable, but they are sometimes inaccurate and are also too sporadic (a measurement every 12 hours for polar satellite instruments). In particular, the central pressure is a fundamental parameter for estimating the intensity of a cyclone, but satellite remote sensing approaches to assess the central pressure of cyclones are highly indirect and lack precision. Aeroclippers are balloons connected by a guide rope to the surface of the ocean. They evolve in the atmospheric surface layer, typically at a height of 30 to 50 meters. These balloons are carried away horizontally toward the eye of cyclones and then remain captured. The Aeroclipper is currently the only vector capable of giving an in situ measurement of the surface wind as it passes through the eyewall, then to provide continuous and real time measurements of the surface pressure in the eye until the cyclone dissipates. This balloon thus provides a unique possibility: (i) to follow the position and intensity of the cyclones; (ii) to improve the forecast of cyclones by assimilating the evolution of the central pressure; (iii) to evaluate and improve satellite approaches; and (iv) to correct possible biases in historical databases and thus to better detect a possible trend of the cyclonic characteristics during the last decades. The unique measurements given by Aeroclippers should also help improve our knowledge of cyclones. This is necessary to better predict the evolution of their characteristics in a warmer climate, in particular their poleward migration and their interaction with mid-latitude storms. Two Aeroclippers were already captured in Cyclone Dora in the Indian Ocean in 2007. After an interruption, developments resumed in 2015 with technical and financial support from the French center for space studies (CNES, Centre National d’Études Spatiales). Due to lack of human and funding resources, CNES must now suspend this development for an indefinite period of time (several years). The Aeroclipper system is however perfectly mature and it is not justified to postpone the first campaigns. The aim of this proposal is to validate the entire Aéroclipper system by conducting a test campaigns in 2020 and in 2021 with a new mechanical system and a new gondola currently under development at the Laboratoire de Météorologie Dynamique (LMD).

The present project makes use of recent Monte Carlo advances to design a numerical tool addressing a quite extreme climate-science calculation: evaluating the radiative forcing of an ensemble of greenhouse gases, at the global scale, integrated over climatic durations, using a reference radiative transfer model with high-resolution spectral data and 4D atmospheric fields (space and time). Starting from the null-collision concept, the feasibility was recently established and the present challenge consists in defining an efficient strategy for sampling the space of molecular transitions combined with all the geometrical and temporal complexity of atmospheric variables. For this purpose, we gather climatologists, spectroscopists, specialists in the statistical-engineering of complex systems and specialists in geometry-modelling for computer graphics applications. MCG-Rad benefits of a prototype-algorithm available and already tested for clear-sky atmospheres. This algorithm will be implemented at the launching of the project, allowing first climatic evaluations, in particular within the Radiative Forcing Model Intercomparison Project. But outside this first practicability level, a major issue is convergence-enhancement: the size of the community interested by such reference radiative-transfer computations at the climatic scale will be highly dependent on the required computational-resource. We therefore concentrate an essential part of the present research on the theoretical exploration of all means to reduce the variance of the Monte Carlo estimate, i.e. using the most advanced propositions of both the transport-physics and computer-graphics communities to reduce the computational-resources. These theoretical considerations extend widely the objective of the original algorithm (that was restricted to clear-sky single atmospheric columns). The question is scalability when thinking of: - the 4D fields produced by climate General Circulation Models (GCM), including multiple-scattering in 3D clouds below the column-scale (clouds reconstructed from the GCM outputs); - the set of all radiative-observables of interest, as elaborated by the climate-change community, i.e. with an accuracy-requirement formulated in terms of integrated radiative-forcing (a quantity that is small compared to the involved infrared and shortwave fluxes which makes it difficult to evaluate) and including sensitivities to the concentration of absorption gases. Three neighbor communities will be connected to the project as they face similar challenges: heat-transfer/combustion engineering, planet atmospheric-studies, cloud dynamics. A second circle of scientific expertise will therefore join the project via annual meetings and workshops. They will contribute to our software-design choices, structuring the libraries so that the present work addresses a broader audience as an example of jointly sampling large spectroscopic databases and complex 3D/4D fields for accurate radiative-transfer objectives.