Abstract
Heat waves are projected to become more frequent, longer-lasting, and intense. At the same time, urban areas are confronted with the urban heat island (UHI) phenomenon, which adds to the thermal stress experienced during hot spells. Focusing on the Paris area during the hot summer of 2003, we investigated the influence of heat waves on UHI intensity, i.e., the urban-rural temperature contrast. In a first step, this was done based on observed temperatures from an urban and a rural site, showing that per C increase in the daytime temperature, the nighttime UHI intensity increased by 0.086 C. Recognizing the limited spatial representativeness of the urban experimental site, located in a park, we then performed simulations with an urban climate model, covering the wider Paris area for the summer of 2003. First, a validation was done using the aforementioned temperature measurements to do so. Subsequently, we estimated the sensitivity of the nighttime UHI intensity with respect to the daytime temperature, this time using simulated temperatures of the densely built-up areas in the center of Paris, yielding an increase of UHI intensity of 0.19 C per C increase in the daytime temperature. While these results only apply to the domain and period studied, they do confirm recent reports that the UHI intensity increases during heat waves. The results also show that for the cooler parts of the urban fabric (e.g., parks), the UHI intensification during heat waves is around half of that of the dense urban fabric, thus providing some insights into possible mitigation strategies for the future.
Highlights
Global climate projections that consistently point towards an increase in the number, frequency, and intensity of heat waves [1,2,3] have shown that extremely hot summers such as the one of 2003 in Europe are likely to become fairly common towards the end of the century
It has been shown that, whereas minor heat wave episodes do induce a fair share of harvesting, this effect decreases as a function of the heat wave strength [7]
The is located at 48.61◦ N and 2.68◦ W, which is near a small airfield 8 km north of the center of Melun, large-scale “ambient” temperature, as a function of which we seek to express the daily evening-time which itself is 35 km from the city center and 20 km from the outskirts of Paris. It is located in the urban heat island (UHI) intensity, was taken as the daily temperature occurring in Melun at 2 p.m
Summary
Global climate projections that consistently point towards an increase in the number, frequency, and intensity of heat waves [1,2,3] have shown that extremely hot summers such as the one of 2003 in Europe are likely to become fairly common towards the end of the century. The authors of [21] investigated the impact of the heat wave occurring in the second half of July 1999 in the Midwestern US, with temperatures rising well above 32 ◦ C, which resulted in several hundred excess deaths During this event, the cities of Chicago and St. Louis were found to be disproportionally hotter than their rural surroundings, i.e., during the heat wave the UHI intensity of these cities was higher than average. We tackle a similar research question, i.e., we investigate whether the urban-rural temperature increment gets enhanced during heat waves or not, and to what extent This is done for the area of Paris (France), using both in situ measurements and model simulation results, for the extended summer (May–September) of 2003.
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