Investigation of urban park cooling efficiency during summer in Paris with drones, sondes, and ground measurements

Urban green infrastructures can regulate microclimate and mitigate urban heat island (UHI) effects. In summer, urban parks offer cooler spots where citizens are protected from the heat of the city, more particularly in the evening and the night, when the UHI arises. During heatwaves, urban parks can therefore play a key role in addressing health risks. Previous research shows that urban vegetation has the potential to mitigate temperatures at the pedestrian level depending on park size, vegetation type, topography or local climate. However, little is known concerning the impact of meteorological conditions (wind, turbulence, cloudiness, UHI intensity, atmospheric stability) on the urban park cooling effect. The cooling along the vertical dimension remains also sparsely quantified. 

One of the aims of the PANAME (for PAris region urbaN Atmospheric observations and models for Multidisciplinary rEsearch) intensive measurement campaign is to better understand and quantify the role of urban parks in regulating the microclimate during summer periods. The July 2023 special observation period (SOP) focused more particularly on investigating the horizontal and vertical cooling potential of four urban parks and woods of various sizes from late afternoon to early part of the night. The experimental set-up combined in situ surface measurements in parks and urban areas, such as profiling of the surface layer using quadcopter drones and soundings. For drone measurements, each urban park was associated with a nearby urban site, consisting in semi-enclosed to enclosed paved courtyards surrounded by buildings. Profiles were simultaneously undertaken in both urban/park environments from 16 to 20 UTC for each of the 21 days of measurements that took place for the 2023 SOP. This resulted in an innovative multi-source dataset currently being used to quantify and analyse the differences in surface and near-surface atmospheric cooling rates between an urban park and the nearby urban environment, as well as between different parks, but also to study the cooling potential related to meteorological conditions.

The first results show that the evening cooling is highly dependent on the meteorological conditions, which is in agreement with recent work establishing a link between park cooling efficiencies and weather types with contrasting UHI regimes. For some evenings, the surface park cooling between the late afternoon and the sunset can reach more than 3 K while it is often less than 1 K in the urban environment. For other meteorological conditions, the cooling rate is similar between park and urban environments. When the cooling is different between parks and urban environments, the park cooling extends vertically over several tens of meters, where a local inversion is found. For those types of meteorological conditions, significant cooling is also found in very small urban parks (less than 1 ha).