Unveiling the Thermo-Hygrometric Influence of Summer Sea and Estuarine Breezes (SEBs) in Lisbon (Portugal)

Local wind, such as sea breezes, play a crucial role in cooling coastal cities. This study presents new insights about the thermo-hygrometric influence of the Tagus and Atlantic Ocean breezes (sea and estuarine breezes [SEBs]) in Lisbon's urban climate (Portugal). SEB events were identified in the summer of 2022 according to a wind rotation criterion: the interruption of prevailing North and Northwest (Nortada) winds during the morning, the wind shift to Northeast/East/Southeast and, sometimes, to further South/Southwest/West (rotation between 22.6 degrees and 292.5 degrees) and the return of the regional flow at late afternoon. Additionally, air temperature and absolute humidity anomalies (Delta T/Haurb) were calculated according to the distance to the riverfront area. Results show that SEB occurred on 37 (31%) out of 120 days, mainly in July (43%) and August (32%), between, on average, 10:00 AM and 4:00 PM, and average wind speeds of 3.4 m/s. According to the daily thermo-hygrometric cycle, the areas up to 4 km of the Tagus estuary were, on average, cooler than northern Lisbon during SEB events, especially the areas up to 500 m (average Delta Turb reached -1.7 degrees C). Additionally, there was a significant increase in the moisture content during SEB hours across the city but especially close to the riverfront area: the areas up to 500 m registered, on average, Delta Haurb of 4.2 g/m3 on SEB events (12:00 PM) against 2.1 g/m3 during typical Nortada days. This research is a starting point for a future delimitation and preservation of SEB penetration zones in Lisbon to address outdoor thermal discomfort during summer.