Extreme heavy precipitation events (HPEs) pose a threat to human life but remain difficult to predict. Considerable efforts to improve the skill of the forecasts for such severe events have been made in recent years and significant progress has been realized through the development of convection-permitting numerical weather prediction systems (NWPS). However, our ability to predict such high-impact events remains limited because of the lack of adequate high frequency, high resolution water vapor (WV) observations in the low troposphere (below 3 km). HPEs occurring in small and steep watersheds are responsible for the triggering of flash floods with a sudden and often violent onset and rapid rising time, typically from 1 to 6 h following the causative rainfall. We aim to implement an integrated prediction tool, coupling network measurements of WV profiles and a numerical weather prediction model to precisely estimate the amount, timing and location of rainfall associated with HPEs in southern France (struck by ~7 HPEs per year during the fall). The proposed WaLiNeAs project is a unique, innovative initiative that will for the first time ever allow assimilating high vertical resolution lidar-derived WV profiles in the first 3 km of the troposphere. The benefit of WaLiNeAs to the academic and operational communities is dual: advance knowledge of the complex dynamical and dynamical processes controlling the life cycle of HPEs and enhance the predictability of HPEs in southern France at scales relevant for meteorological studies. Both aspects are dealt with in the framework of WaLiNeAs. A network of 5 autonomous Raman WV lidars will be deployed in the Western Mediterranean to provide measurements with high vertical resolution and accuracy, closing critical gaps in lower troposphere WV observations by current operational networks and satellites. Near real-time processing and ensemble assimilation of the WV data in the French operational Application of Research to Operations at MEsoscale (AROME) model, using a 4DEnVar approach with 15 min updates, is expected to enhance the model capability for kilometer-scale prediction of HPEs over southern France 48 hours in advance. The field campaign is scheduled to start early September 2022, to cover the period most propitious to heavy precipitation events in southern France. The Raman WV lidar network will be operated by a consortium of French, German and Italian research groups. Lidar data will be made available to Météo-France shortly after being acquired up to 96 times per day. Besides demonstrating the potential of WV lidar data assimilation in a near real-time operational context, an ancillary objective of the project is also to show that Raman lidars can be left to operate continuously almost unattended for a period of at least 3 months. It is a prerequisite in the perspective of future deployment of operational Raman lidar systems meant to fulfil the observational gaps in WV in the lower troposphere of the current operational observation networks and satellites. This project will lead to recommendations on the lidar data processing for future operational exploitation in NWPS. This project will contribute significantly to the scientific objectives of CES04 « Innovations scientifiques et technologiques pour accompagner la transition écologique » through the development of all-weather, unattended, continuous operation of Raman lidar systems for smart monitoring of the environment, and WV in particular. This project is highly innovative and will lay the foundation for a future integrated warning tool aiming to prevent natural hazards associated with heavy precipitation events as often experienced along the Mediterranean coastline. Once the proof of concept is validated in the framework of the WaLiNeAs project, similar integrated tools may be applied in other parts of the World to avoid similar natural hazards.

Increasing the amount of urban vegetation is encouraged as an adaptation strategy to countermeasure climate change mainly due to (1) its potential to sequester CO 2 from the atmosphere and to (2) enhance evaporative cooling and positively influence the water cycle in cities. The greening of buildings, that is the implementation of vegetated envelopes on roofs and walls, may contribute to an important part of the urban green infrastructure. However, the carbon sequestration potential of green building envelopes is one of the ecosystem services that has the least been studied due to complexity or lack of data. GREENVELOPES aims at filling this research gap by conducting an in-depth observation and process-based modelling study of building green enve- lopes to better understand their capacity to exchange carbon and water with the urban atmosphere. The research objectives will be addressed by state-of-the-art observations of micrometeorological fluxes and leaf-level measurements at a green roof and green wall site, covering different weather conditions during a two-year period. This is to be complemented by improving an established urban energy balance model so that it accounts for carbon and water exchanges of building green envelopes with the urban atmosphere. Upscaling these exchange processes to the urban scale by the numerical modelling of two application cities, Toulouse (France) and Berlin (Germany), will allow us to quantify and assess the impact of both types of green envelopes on carbon sequestration and evaporative cooling potential in relation to other types of urban vegetation.

This project, which builds on the results of the ANR COCOA project, aims at improving the representation of turbulent ocean-atmosphere exchanges in climate models by taking into account their modulation by waves using ocean-waves-atmosphere coupled modelling systems. Waves form by absorbing momentum in areas of storms and tropical cyclones, and transmit part of it to the ocean, with an impact on deep mixing and the overall heat balance. This energy absorbed by waves can be transported by swell over very long distances, from energetic areas (Southern Ocean, storms) to the inter-tropical band, for example. In this inter-tropical zone, where much of the heat and moisture exchange that drives atmospheric circulation on climatic scales takes place, conditions are met for swell to impact air-sea fluxes. It is therefore important to take into account the impact of waves in energetic zones, where they are formed and a large part of the energy is transferred to the ocean, and in dissipation zones, where they have an impact on surface fluxes. We plan to: 1) quantify the impact of wave-related processes on atmosphere-wave and wave-ocean exchanges at climate scales, based on existing model representations; 2) develop and validate new parameterizations to take into account, in coupled ocean-wave-atmosphere models at climate scales, processes related to tropical cyclones, the effect of waves on ocean mixing, and swell; 3) to build a coupled system around the wave model, ensuring complete consistency of momentum exchanges by resolving the inconsistencies of most of the current coupled systems used at weather prediction scales. This project should pave the way for state-of-the-art consideration of wave effects in climate models.

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).