Authors Contributions:
Julien Crétat, Yves Richard, Olivier Planchon, Melissa Poupelin, Mario Rega, Julien Pergaud, Julita Diallo-Dudek and Benjamin Pohl – Centre de Recherches de Climatologie, UMR 6282 Biogéosciences, CNRS/Université de Bourgogne, France
Justin Emery and Ludovic Granjon – ThéMA, UMR 6049, CNRS/Université de Bourgogne, France
Daniel Joly and Damien Roy – ThéMA, UMR 6049, CNRS/Université de Franche-Comté, France
Abstract:
Impact of topography and atmospheric circulation on the urban heat island under heat waves (Dijon, France). Heatwaves and hot days lead to increased thermal stress, and the latter is potentially exacerbated in urban areas. We examine here the combination of these phenomena using a dense network of air temperature observations in Dijon (northeastern France) over the 2014-2021 period. To that end, we analyze (i) local-scale to synoptic-scale atmospheric circulation and (ii) local factors (land use and topography) influencing the temperature. The five heatwaves that occurred during the period last 4 to 5 days and are associated with large-scale atmospheric blocking, that also favor thermal inversions. Out of the 24 nights under study, 60% are characterized by an urban heat island (UHI) above +3°C and a thermal inversion often exceeding 0.5°C/100 m under calm wind conditions (<2 m/s); 30% by an UHI below +2°C and an adiabatic gradient under windy conditions (>2 m/s); and 10% by a weak UHI, a weak thermal inversion, and variable wind conditions. Similar statistics are obtained for the 105 hot days of the period. Heatwaves and hot days are conducive to two contrasted spatial patterns depending on wind conditions. Windy conditions (>2 m/s) act to ventilate urban heat excess and limit topographic influence. This results in homogeneous air temperature across the study area. In contrast, calm conditions (wind <2 m/s) act to maximize the influence of land use and topography, leading to heat excess in the city and over the plateau west of it and relative coldness in the plain, east of the city, and along the river valley crossing the city from north-west to south-east. Mobilizing this natural cold axis provides an opportunity to damp UHI or, at least, promote cold islands in the city center. This study points out the complementarity of observational networks monitoring the urban climate with atmospheric circulation and surface properties, to quantify the influence of the various driving mechanisms that modulate air temperature and its spatial variability.