Topoclimatic characteristics of Zahle (Eastern Lebanon): Thermal breezes and urban heat island phenomenon - Preliminary results

This work examines the topoclimatic characteristics of Zahle, specifically thermal breezes and the Urban Heat Island (UHI) phenomenon. Three-hourly data from 1994 to 2021 from conventional weather stations shows a frequency of breezes in the summer season of 74%. In Rayak, mountain and valley breezes have been noticed alternating between day and night, channeled by the topography of the Bekaa Valley. In Houch El Oumaraa, valley-sea breezes dominate during the day and mountain breezes occur at night. These breezes affect the distribution of air temperature and humidity, creating an average thermal contrast of 4.3 degrees C between the city and its surrounding countryside at night. Data from weather stations implanted in 2022 and car surveys at a fine scale (half hourly data) level has confirmed the previous findings: During nighttime, the mountain breeze dominates and brings cool and moist air from the mountains into the valley at a low speed (<1 m/s). During the daytime, a valley breeze dominates and can reach speeds of up to 4 m/s. Additionally, a sea breeze with faster speeds is detected. The car surveys indicate a temperature variation of 9 degrees C between the valley and the surrounding urban areas during early morning.