Authors Contributions:

Julien Crétat, Yves Richard, Benjamin Pohl, Julita Dudek, Julien Pergaud, Mario Rega and Mélissa Poupelin – Centre de Recherches de Climatologie, UMR 6282 Biogéosciences, CNRS/Université de Bourgogne, Dijon, France

Justin Emery, Damien Roy, Daniel Joly and Thomas Thévenin – ThéMA, UMR 6049, CNRS/Université de Bourgogne et
Université de Franche-Comté, Besançon, France

Eva Marquès and Valéry Masson – Centre National de Recherches Météorologiques, Université de Toulouse, Météo-France, CNRS, Toulouse, France

Abstract:

The influence of topography and land cover on air temperature space-time variability is examined in an urban environment with contrasted topography through simple and multiple linear regression (SLR and MLR) models, ran for each hour of the period 2014–2021, to explain spatial patterns of air temperature measured by a dense network. The SLR models reveal a complementary influence of topography and land cover, with the largest influence during daytime and nighttime, respectively. The MLR significantly improves upon the SLR models despite persistent intensity errors at night and spatial errors in the early morning. Topography influences air temperatures all year round, with temperature decreasing with height during the day and frequent thermal inversions at night (up to 30% of the time). Impervious surfaces are more influential in summer and early fall, especially during the late afternoon for the fraction covered by buildings and during the early night for the distance from the city center. They contribute to increasing air temperature close to the city center and where the fraction covered by buildings is large. By contrast, vegetation contributes to cool air temperatures during the night, especially in spring and early summer for field crops, summer and early fall for forests, and late fall and winter for low vegetation. Our framework proves to be a low-cost and efficient way to assess how strongly and how recurrently the static surface conditions influence air temperature along the annual and diurnal cycles. It is easily transposable to other areas and study fields.

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.

Authors Contributions:

Justin Emery – Université de technologie de Compiègne, AVENUES, Centre Pierre Guillaumat – CS 60 319 – 60 203, Compiègne Cedex, France

Benjamin Pohl, Julien Crétat, Yves Richard, Julien Pergaud, Mario Rega, Sébastien Zito, Julita Dudekand Thibaut Vairet – Centre de Recherches de Climatologie, UMR 6282 Biogéosciences, CNRS/Univ Bourgogne Franche-Comté, France

Thibaut Vairet, Daniel Joly and Thomas Thévenin – UMR 6049 THEMA, CNRS/Univ Bourgogne Franche-Comté, France

Abstract:

This study analyses mobile measurements of urban temperatures in Dijon (eastern France) to quantify the influence of urban form on the micro-scale variability of air temperature. A route was ridden identically on 33 spring and summer evenings on a bike fitted out with measuring instruments (VeloClim). These evenings followed sunny calm days conducive to the formation of thermal contrasts and urban heat islands (UHIs). Two typologies, Corine Land Cover (CLC) and Local Climate Zones (LCZ), are used to assess the impact of urban form and land cover on air
temperatures based on Analysis Of Variance (ANOVA). ANOVA is applied to the mean of runs to maximize the effect of surface states, and to each run individually to maximize the influence of weather conditions.

The results show that both typologies prove relevant and complementary for studying the impact of vegetated and artificialized zones on urban temperature. Temperature variations on intra-urban scales are significantly modulated by urban form and land cover types. Vegetated
areas are systematically cooler than impervious surfaces. Independently of meteorological conditions, urban form has a decisive influence on air temperature and each CLC or LCZ category has
an original air temperature signature.