“Tierce Forêt”: Measuring the Cooling Effects from Greening a Parking Lot

Sophie Parison1,2, Maxime Chaumont1, Raphaëlle Kounkou-Arnaud3, Andrej Bernik4, Marcos Da Silva4, Martin Hendel1,5 1 Univ Diderot, Sorbonne Paris Cité, LIED, UMR 8236, CNRS, F-75013, Paris, 2 Paris City Hall, Paris, France 3 Météo-France, Direction Inter-régionale Ile-de-France/Centre, Paris, France 4 Fieldwork Architecture, F-75010, Paris, France 5 Université Paris-Est, ESIEE Paris, département SEN, F-93162 Noisy-le-Grand, France Abstract A parking lot in the municipality of Aubervilliers () is currently under study for conversion from a dark mineral area to an open green space with permeable pavement as an urban heat island countermeasure. In this paper, we present the framework of the project and the methodology adopted to estimate the impact of the countermeasure once the site is converted. A preliminary campaign of heat stress measurements was also conducted with a travelling weather station. Before conversion, high thermal stress is exhibited under direct insolation in the parking lot while no stress is observed when under shading. Keywords: urban heat island countermeasure, permeable concrete, urban greening Introduction Urban greening is among the best-known countermeasures to the urban heat island (UHI) phenomenon [1]. Parks in particular have been the focus of many studies, but many studies only conduct measurements after park construction [2]. However, identifying sites sufficiently ahead of time to prepare a measurement campaign before and after park construction can be difficult. In the Paris Metropolitan Area, a site in the municipality of Aubervilliers is under study for conversion into an urban green area. The zone under study is currently used as a parking lot for the occupants of a home for young workers. It currently offers no urban amenities for pedestrians and inhabitants and its dark-colored asphalt concrete strongly absorbs solar radiation. The conversion will create a green open space with new functionality for pedestrians and inhabitants, including several trees and permeable concrete paving. This site and its construction work schedule offer an opportunity to study the site before and after greening. Microclimatic measurements are under way to determine the site’s microclimatic characteristics before and after site conversion. This paper proposes a methodological description of the measurement campaign, preliminary results and discusses expected results.

Parking Lot Configuration The site chosen for the study is a parking lot of about 1900 m² for the occupants of a home for young workers in the municipality of Aubervilliers, located in the Parisian northern periphery. It is currently surrounded by tall buildings and offers no amenities for pedestrians, such as benches, shade or uncirculated paths. The ground is made of a bad-shape dark-coloured impervious asphalt with albedo of 0.13, favouring the absorption of solar radiation and thus damaging thermal comfort. Furthermore, the site strongly lacks vegetation apart from peripheral flowerbeds and sparse trees. A photograph of the parking lot in its current state is provided on Figure 1.

Figure 1: Photograph of the parking lot before conversion

Considering those characteristics, the site thus represents a good candidate for conversion to an open green area as an urban heat island (UHI) countermeasure. The conversion project carried out in this regard completely rethinks this urban area. It will create a friendly un-driven space open for pedestrians and inhabitants. Trees will be planted with an increased density in the sunniest areas in order to create a “tertiary forest”, and the ground will be replaced entirely with a permeable light- coloured concrete in order to retain rainwater in the sub-layers, thus allowing trees to properly grow and underneath soil to be fertile.

In order to estimate the impact on microclimate and thermal comfort of such conversion, measurements before the latter must be conducted sufficiently ahead of time to ensure reliable representative data. This site and its construction work schedule offer an opportunity to study the site before and after greening. In this regard, to determine the site’s microclimatic characteristics, two weather stations were installed by Météo-France. One of them is placed in the parking lot in the future converted zone while the other one is located onto a rooftop, outside the future converted zone, but close by nonetheless. The configuration of the site is illustrated on Figure 2.

Figure 2: Configuration of the site under study and weather stations’ positions Instrumentation of the Site The installed weather stations allow the measurement of numerous microclimatic indicators, all useful to determine impact on heat stress: air temperature, relative humidity, black globe temperature (all at 1.5 m high), wind speed and long and shortwave upward and downward radiation. Stations are power supplied with solar panels and measure continually. A rain gauge is also placed next to the rooftop station. A close-up to the stations is shown on Figure 3.

Wind speed

Air temperature and relative humidity

Black globe temperature

Solar panel

Long and shortwave upward and downward radiation

Figure 3: Rooftop (left) and parking lot (right) weather stations Methodology of Future Analyses Microclimatic measurements are under way to determine the site’s microclimatic characteristics before and after site conversion. Previous work by the authors has shown that direct comparison between case and control sites were unsuited to determining the impact of the considered UHI countermeasure [3]. Instead, following the Lowry approach [4], the measured meteorological parameter M can be expressed as the linear combination of the “background” climate C, the effect of local landscape L, and the effects of local urbanization E. Before conversion, for a given weather type (i.e. hot summer days), control and case weather stations, respectively rooftop and parking stations here, thus measure the following:

푀푏푒푓표푟푒, 푟표표푓푡표푝 = 퐶푏푒푓표푟푒, 푟표표푓푡표푝 + 퐿푏푒푓표푟푒, 푟표표푓푡표푝 + 퐸푏푒푓표푟푒, 푟표표푓푡표푝 (1)

푀푏푒푓표푟푒, 푝푎푟푘푖푛푔 푙표푡 = 퐶푏푒푓표푟푒, 푝푎푟푘푖푛푔 푙표푡 + 퐿푏푒푓표푟푒, 푝푎푟푘푖푛푔 푙표푡 + 퐸푏푒푓표푟푒, 푝푎푟푘푖푛푔 푙표푡 (2)

After conversion of the site, stations will then measure:

푀푎푓푡푒푟, 푟표표푓푡표푝 = 퐶푎푓푡푒푟, 푟표표푓푡표푝 + 퐿푎푓푡푒푟, 푟표표푓푡표푝 + 퐸푎푓푡푒푟, 푟표표푓푡표푝 (3)

푀푎푓푡푒푟, 푝푎푟푘푖푛푔 푙표푡 = 퐶푎푓푡푒푟, 푝푎푟푘푖푛푔 푙표푡 + 퐿푎푓푡푒푟, 푝푎푟푘푖푛푔 푙표푡 + 퐸푎푓푡푒푟, 푝푎푟푘푖푛푔 푙표푡 (4)

The impact of the countermeasure I, i.e. the conversion of the parking lot to a green area, is contained in the following term:

퐸푎푓푡푒푟, 푝푎푟푘푖푛푔 푙표푡 = 퐼푐표푛푣푒푟푠푖표푛 + 퐸푏푒푓표푟푒, 푝푎푟푘푖푛푔 푙표푡 (5)

The configuration of the considered site is illustrated on Figure 4.

Figure 4: Control and case station sites, before and after UHI countermeasure

Assuming that no changes occurred at control site, in our example 푀푏푒푓표푟푒, 푟표표푓푡표푝 is thus equal to 푀푎푓푡푒푟, 푟표표푓푡표푝. What is more, apart from E, before conversion and at a given time, the effect of local landscape on the one hand and the background climate on the other hand are equal two-by- two between the rooftop and parking lot stations (i.e. control and case). Therefore, to isolate the effects of the UHI countermeasure, here, the conversion of the site, one has to consider the following:

∆푀푎푓푡푒푟 − ∆푀푏푒푓표푟푒 = (푀푏푒푓표푟푒, 푝푎푟푘푖푛푔 푙표푡 − 푀푏푒푓표푟푒, 푟표표푓푡표푝) − (푀푎푓푡푒푟, 푝푎푟푘푖푛푔 푙표푡 − 푀푎푓푡푒푟, 푟표표푓푡표푝) (6)

Hence:

∆푀푎푓푡푒푟 − ∆푀푏푒푓표푟푒 = 퐼푐표푛푣푒푟푠푖표푛 (7)

Therefore, both interstation average profile before conversion and after conversion must be considered in order to eliminate the impact of the local climate and landscape on acquired data. For additional information regarding the methodology, a thorough description of the method is provided in Hendel [5]. This method will be used after the conversion of the site to determine its impact. Measurements are currently undergoing to characterize the site before the conversion (equations 1 and 2) during the summer.

Estimation of the Heat Stress on Site Before Conversion In the meantime, a preliminary measurement campaign was conducted in July 2018 to estimate thermal stress on site before conversion. To do so, a travelling weather station was used on eight different locations in the parking lot to collect microclimatic measures. It was equipped at 1.5 m with a black globe and air temperature thermometers, relative humidity probe and a hot wire anemometer. These were performed on a typical summer day, i.e. with low wind speeds (<3m/s), clear sky and three-day averaged minimum and maximum daily air temperature respectively greater than 16° and 25°C. These weather conditions match for days of Pasquill Stability Class A or A-B [6]. A photograph of the travelling station is provided on Figure 5.

Figure 5: Travelling weather station used for local measurements From these measurements, the Universal Thermal Climate Index (UTCI) equivalent temperature were calculated to investigate heat stress on site in its current state. A cartography of the results is shown on Figure 6.

Figure 6: Thermal stress on site before conversion measured on eight different locations Large discrepancies are observed. All measurements performed under trees or building shading exhibit no thermal stress. On the other hand, measurements conducted under direct insolation show a high level of thermal stress, all exceeding 35°C equivalent temperature. The highest thermal stress was recorded inside the parking-lot, under direct insolation and far from vegetation. Perspectives A parking lot under study for conversion to a green space as a UHI countermeasure in the Paris metropolitan area was equipped with two weather stations in order to follow the evolution of the microclimatic indicators before and after conversion of the site. A methodological approach has been proposed to precisely estimate the impact of the conversion of the site. After conversion, strong impact is expected to be found during typical summer days as a consequence of the presence of trees and, to a lesser extent, to the permeable pavement. Parallel to that, a preliminary measurement campaign has been conducted to investigate heat stress on site before conversion on eight different locations, using the UTCI equivalent temperature. Results show that no thermal stress is observed under shading whereas high stress is exhibited under direct insolation, particularly far away from vegetation. Once the site is converted, the same campaign will be conducted again under same meteorological conditions and heat stress conditions on same locations will be compared before and after countermeasure. References [1] H. Akbari, M. Pomerantz, and H. Taha, “Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas,” Sol. Energy, vol. 70, no. 3, pp. 295–310, Jan. 2001. [2] D. E. Bowler, L. Buyung-Ali, T. M. Knight, and A. S. Pullin, “Urban greening to cool towns and cities: A systematic review of the empirical evidence,” Landsc. Urban Plan., vol. 97, no. 3, pp. 147–155, Sep. 2010. [3] M. Hendel, P. Gutierrez, M. Colombert, Y. Diab, and L. Royon, “Measuring the effects of urban heat island mitigation techniques in the field: Application to the case of pavement- watering in Paris,” Urban Clim., vol. 16, pp. 43–58, Jun. 2016. [4] W. P. Lowry, “Empirical Estimation of Urban Effects on Climate: A Problem Analysis,” Journal of Applied Meteorology, vol. 16. pp. 129–135, 1977. [5] M. Hendel, S. Parison and L. Royon, “An Improved Method for Quantifying the Field Effects of Urban Heat Island Mitigation Techniques,” in ICUC10 & 14thSymposium on Urban Environment, Aug. 2018. [6] F. Pasquill, “The estimation of the dispersion of windborne material,” Meteorol. Mag., vol. 90, no. 1063, pp. 33–49, 1961.