Description of the thesis topic

Mission:
This project will assess the local cooling potential of urban greening solutions through a novel framework that characterises systematically the influence of atmosphere dynamics. This systematic framework then allows for local, surface-induced effects to be identified more precisely so that more meaningful conclusions can be drawn about the contrasts between vegetated areas and built-up surfaces. Atmospheric dynamics will be quantified through six indicators that are derived from novel ground-based atmospheric remote sensing profile observations.

Heat risks to human health and mortality are exacerbated in urban areas, where heat waves are more intense and last longer than in rural areas. Heat stress for humans is partly related to the surrounding air temperature, however, other factors such as radiation (solar and thermal radiation), humidity, and wind can have severe implications. Depending on the conditions, these processes can either contribute to an elevation or exacerbation of the discomfort and health risk. One of the most widely deployed means of combating urban overheating is the creation of green spaces and the greening of streets. Vegetated areas can provide local cooling through shading and evapotranspiration and further reduce the surface's capacity to store heat in comparison to impermeable building materials (such as concrete or asphalt) that are arranged in a three-dimensional morphology of streets and canyons. These mechanisms alter the energy exchanges between the surface and the atmosphere (solar and longwave radiation; turbulent fluxes sensible and latent heat; momentum exchange) and are influenced by the specific design (e.g. the choice and layout of plant species and types) and management (e.g. tree size; irrigation requirements) of greening solutions, as well as by the built environment (e.g. urban density of the neighbourhood; direct emissions of anthropogenic heat). In addition to surface-driven processes, synoptic background weather conditions and atmospheric boundary layer (ABL) dynamics also affect contrasts in microclimate conditions observed between built-up areas and green spaces. All these factors play a role in the specific cooling potential of urban vegetation that varies considerably over time and space. To explain these variations systematically and to better quantify the climate benefits of greening solutions, an improved understanding of the interactions between atmospheric boundary layer dynamics and surface-induced effects is required at different spatial scales.

Although significant progress has been made in understanding and predicting urban microclimates, the implications of atmospheric dynamics in heterogeneous environments remain an important area of research. To characterise dynamics and energy exchanges in the atmospheric boundary layer, six indicators will be derived from continuous atmospheric remote sensing profile observations, namely horizontal advection, thermal stratification, vertical mixing, entrainment, cloud effects, and boundary layer heights. These indicators help to describe local cooling and warming in the context of radiation exchanges, as well as vertical and horizontal transport of heat, moisture, and momentum. By including information from the surface up to the top of the atmospheric boundary layer, the influence of synoptic processes can be isolated from those driven by surface conditions.

Thanks to advances in ground-based remote sensing technology and algorithm development, those profile observations can now be obtained from multi-sensor networks across urban areas. A novel synergy approach will be developed that combines observations of thermal stratification from microwave radiometers, profiles of wind and turbulence from Doppler wind lidars, as well as clouds characteristics and aerosol layer heights from automatic lidars and ceilometers. The study area for this project is the Île-de-France region.

Main activities:

- This research is based on a detailed analysis of atmospheric measurements from a multi-sensor network (remote sensing and surface stations) using statistical analysis and a physical understanding of atmospheric processes.
- The student is expected to identify how observed conditions align with the current state of the art description of the urban atmosphere and how the measurement synergy can help to advance the detailed process-understanding of surface-atmosphere exchanges in urban settings.
- Results should be published in concise scientific publications (journal articles) and presented at scientific conferences and project meetings.
- In the context of the project, the student will interact with a large, interdisciplinary academic community and also contribute to outreach events and discussions with municipalities and other stakeholders (including the private sector). We except the student to be pro-active in these activities of science communication.
- The project involves practical work with the remote sensing instrumentation, with a particular focus on Doppler wind lidars as well as automatic lidars and ceilometers. Responsibilities include the design of scan strategies, instrument maintenance and trouble-shooting, documentation, as well as the development of processing software for advanced measurement products.

Work Context

Contexte : Situation de l'emploi et conditions :
The PhD project is associated with the Paris RÉUSS-I project: inteGREEN, funded by ANR France 2030 PEPR Ville Durable et Bâtiment Innovant (VDBI). inteGREEN aims to provide an integrated assessment of urban vegetation from multiple perspectives, including its social functions and usage requirements, the ecosystem health of green spaces linked to plant ecophysiology, soil health and the water cycle, as well as its impact on human health in relation to microclimates and air quality. The thesis is an important component of this multidisciplinary project and will be linked to other theses and postdoctoral research activities that use the results of this work and will also make contributions, for example on the evaporative response of plants.

The regional research federation Institut Pierre Simon Laplace (IPSL) is a university research institute that brings together nine laboratories with approximately 1,400 members, equally divided between researchers, engineers and students. The position will be based at the Laboratoire de Météorologie Dynamique (LMD) at the École Polytechnique in Palaiseau, within the team responsible for the SIRTA atmospheric observatory. The student will work in close collaboration with students and researchers from other IPSL scientific laboratories. The LMD-IPSL and SIRTA have diverse expertise in urban climate processes and atmospheric measurements taken from numerous platforms. Data processing takes place at the AERIS national data centre, part of which is integrated into the IPSL.

The position is located in a sector under the protection of scientific and technical potential (PPST), and therefore requires, in accordance with the regulations, that your arrival is authorized by the competent authority of the MESR.