Heatwaves and Summer Urban Heat Islands: A Daily Cycle Approach to Unveil the Urban Thermal Signal Changes in Lisbon, Portugal

Abstract

Lisbon is a European Mediterranean city, greatly exposed to heatwaves (HW), according to recent trends and climate change prospects. Considering the Atlantic influence, air temperature observations from Lisbon’s mesoscale network are used to investigate the interactions between background weather and the urban thermal signal (UTS) in summer. Days are classified according to the prevailing regional wind direction, and hourly UTS is compared between HW and non-HW conditions. Northern-wind days predominate, revealing greater maximum air temperatures (up to 40 ˚C) and greater thermal amplitudes (approximately 10 ˚C), and account for 37 out of 49 HW days; southern-wind days have milder temperatures, and no HWs occur. Results show that the wind direction groups are significantly different. While southern-wind days have minor UTS variations, northern-wind days have a consistent UTS daily cycle: a diurnal urban cooling island (UCI) (often lower than –1.0 ˚C), a late afternoon peak urban heat island (UHI) (occasionally surpassing 4.0 ˚C), and a stable nocturnal UHI (1.5 ˚C median intensity). UHI/UCI intensities are not significantly different between HW and non-HW conditions, although the synoptic influence is noted. Results indicate that, in Lisbon, the UHI intensity does not increase during HW events, although it is significantly affected by wind. As such, local climate change adaptation strategies must be based on scenarios that account for the synergies between potential changes in regional air temperature and wind.

Urban Heat Island and Its Interaction with Heatwaves: A Review of Studies on Mesoscale

“…In contrast, some studies report that the UHII is not amplified during the HW periods [25,30,128,133]. The diurnal variations of the UHII during the HW period are also similar to those during normal days [25], with the peak UHII recorded at midnight.…”

“…Maximum air temperature is widely used for characterizing HWs in the literature [28,34,53,58,90,99,103,113,118,122,123]. Another temperature indicator is the mean temperature, which is adopted in [30,110]. Combined maximum and minimum air temperatures are also used to characterize HWs [48,112,124].…”

“…On the other hand, HWs may strongly affect the spatial and temporal variability of UHI by altering sensible and latent heat transfer and changing wind conditions [4,27,28]. While many researchers [27][28][29] have reported synergistic interactions between UHI and HW, other researchers [25,30] found no synergistic effect. Nevertheless, the interaction between UHI and HW is complex and remains a subject of debate.…”

Urban heat island data by local weather types in Lisbon metropolitan area based on Copernicus climate variables dataset for European cities

“… The maps of UHI intensity by LWT provide new insights to previous research about Lisbon's UHI undertaken during the last 40 years [1] , [2] , [3] , [4] , [5] , [6] , especially concerning the influence of local weather conditions on UHI intensity and pattern. The data can be quite useful for stakeholders, especially urban planners, designers and architects that are involved on the identification of critical heating urban canyons and the weather conditions most prone to higher UHI values and, hence, thermal discomfort conditions, considering also the negative of future climate projections.…”

“…The maps of UHI intensity by LWT provide new insights to previous research about Lisbon's UHI undertaken during the last 40 years [1] , [2] , [3] , [4] , [5] , [6] , especially concerning the influence of local weather conditions on UHI intensity and pattern.…”