Urban Cooling by Trees – Assessing and Modeling for Designing Sustainable Cities – COOLTREES

The COOLTREES project relies on an ongoing experiment since 2013 in the city of Strasbourg where both urban climate modellers and ecophysiologists have already gathered forces, 3 scientific work pack ages (WPs 2-4), 1 project management package ( WP1) and 1 work package ( WP5) dedicated to the dissemination of the project outputs (Fig. 1). The WP2 will be dedicated to the xperimental part of the project. It is decomposed in to two tasks: the T2.1, dedicated to the 3D reconstruction of urban trees a nd buildings in term of both their architecture and their spatial organization in urban areas to acquire a 3D numerical model of a canyon, and the T2.2 task on monitoring fluxes and meteorological variables and post processing to fit WP3 and WP4 requirements. WP3 will be dedicated to the modeling of tree shading and latent heat effects of an urban canyon through the development of modules dedicated to trees and to thermal comfort indexes in a 3D simulation tool: LASER/F. The WP4 will focus on the validation of the Meso-NH surface scheme (SURFEX) on several districts containing more or less vegetation. At last this project is based on the complementary and the perfect match between skills of participants and project’s objectives: from plant biology to human bioclimatolog y through urban climate modeling and remote sensing. Even if all partners know each ot her and already worked together, a postdoctoral researcher will be hired in mid-year 1 for two years to ensure a good interconnection between the experimental and the modeling works.

The project has finalized the thesis of Miss Elena Bournez defended in November 2018 on the «Study of the role of vegetation in the creation of urban microclimates. Combined approach of measurements and modeling at different scales. « with the implementation of a point cloud processing chain to reconstruct a 3D street including trees until the simulation of their functioning. This work led to a better appreciation of the links between the representation of the leaf surface and the simulation of transpiration and shading. All the data measured (2014 to 2018) on the experimental site were structured and consolidated (description of variables, calculation of elaborated variables) in order to make it available as open data. This work was partly done through the internship of Miss Alice Maison (AgroParisTech) and these data are now hosted on the INRAE Data portal. They will be published, and thus made available to the public, at the end of 2021. They will constitute a unique database gathering 3D structure and typology of the elements of an urban landscape (trees, soils, buildings) and the dynamics of local flows (radiation, wind and evapotranspiration of trees) which can be used to validate models of urban microclimate or functioning of urban trees. Finally, the development of a 3D numerical model of surface temperature taking into account the functioning of trees in an urban environment and the thermo-radiative exchanges between the elements of a street and the atmosphere (Figure 1b) has been done. It allows to apprehend the structural and functional complexity of a street and its use will allow to predict the consequences of a local development on the temperatures felt by the inhabitants.
Dissemination of the results of the project to professionals managing trees in urban areas via the collaboration of the association Plante et Cité.

1. Understand the urban structural and functional diversity (multi-species) of trees in the city using the LASER/F model
2. Collect data on the physiology of urban tree species
3. Integrate the combined effects of hydric and thermal stresses on the functioning and survival of urban trees (see Elder model - ANR Hydrauleaks)

1. Bournez, et al. (2019). Sensitivity of simulated light interception and tree transpiration to the level of detail of 3D tree reconstructions. Urban Forestry and Urban Greening, (38), 1-10. , DOI : 10.1016/j.ufug.2018.10.016).
2. Bournez E., Landes T., Kastendeuch P., Najjar G., Saudreau M., Colin J., Ngao J., 2018. Simulation of urban vegetation impact based on the microclimate model LASER/F: the case study of a park in Strasbourg. International Conference on UrbanClimate (ICUC 10), New-York, 6-10 août.
3. Bournez E., Kastendeuch P., Landes T., Najjar G., Saudreau M., Colin J., Ngao J., 2018. Simulation du rôle de la végétation d’un parc urbain à partir du modèle microclimatique LASER/F : le cas du jardin du Palais Universitaire à Strasbourg. Colloque AIC, Nice, 4-7 juillet 2018, 6 p.
4. Bournez E., Kastendeuch P., Landes T., Saudreau M., Najjar G. 2018. Reconstruction 3D d’un environnement urbain à partir de données photogrammétriques et lasergrammétriques acquises par voies aériennes pour la simulation du microclimat d’un parc urbain. Atelier TEMU, Strasbourg, 19-20 mars 2018.
5. Saudreau, Marc; Ngao, Jérôme; Améglio, Thierry; Kastendeuch, Pierre; Najjar, Georges; Landes, Tania; Colin, Jérôme; Maison, Alice, 2020, «An Extensive Dataset of 3D Geometry, Microclimate fluxes and Vegetation Functioning of an Urban Site in France«, doi.org/10.15454/J5HAZR, Portail Data INRAE, DRAFT VERSION
6. Woussen M., Kasdenteuch P., Benhoussa S., Rondeaux C., Najjar G. Ngao J., Améglio T., Landes, T. and Saudreau M, 2021, The improvement of the thermo-radiative model LASER/F by including interaction with 3D trees and their functional responses.

Among adaptation strategies of cities to climate change and especially to extreme heat events, « greening » cities, i.e. reintroducing vegetation within cities is probably a promising way since vegetation through transpiration and shading has a significant impact on the heat balance of the surrounding atmosphere. This positive impact is strengthened by ecosystem services that could provide vegetation and especially trees, to inhabitant: environmental clean-up (air and soil), carbon sequestration, social well-being, and biodiversity. Trees by their structure could provide a high level of service in terms of shading and cooling via the evapotranspiration process. Several kinds of models have been developed so far to simulate the interactions between trees and urban climate by adding a specific module to urban climate models. However the resolved spatial scale of urban climate models is usually larger than 100m. This disables an accurate representation of the spatial arrangement of trees and buildings at the canyon scale and theirs interactions with the atmosphere. As a consequence, and because of the sensitivity of the latent heat flux to the representation of the tree structure, the latent heat flux is not properly simulated in urban climate models.
The general objective of COOLTREES is to assess and model the evapotranspiration of urban trees in relation with their environments, and to relate their role in the urban climate with their structural and functional traits. To scale-up the latent heat flux from the tree scale to urban scale, three different models, associated to that three spatial scales, will be investigated and interconnected through the project: the RATP model (the tree scale), the LASER/F model (the canyon scale) and the SURFEX model from MESO-NH model (the city scale). The simulation outputs will be assessed from data acquired from on ongoing experiments in the city of Strasbourg. The duration of the project is 36 months and will gather four complementary partners. Two research teams, UMR PIAF INRA/Université Clermont Auvergne, and UMR ICUBE CNRS/ Université de Strasbourg/INSA/ENGEES, and the engineer services of the city of Strasbourg. Together, they cover all the scientific and technical fields of the project: from plant biology (UMR PIAF) to urban climate modeling and remote sensing (UMR ICUBE), and urban planning through local policy-makers (mairie de Strasbourg). The “Plante et Cité” association is also involved in the project and will promote interconnexion of the project and dissemination of the results to other actors involved in the topic of the development of urban vegetation.