Contrasting meteorological factors affecting the summer urban heat island - the case of the Mediterranean coastal city of Tel Aviv, Israel

In areas with a long summer season, especially where heat stress prevails throughout the day, such as Mediterranean coastal cities, Urban Heat Island (UHI) causes further thermal discomfort and an increase in energy consumption for air conditioning. The intense warming of the area, which is greater at night compared to the day, significantly enhances energy consumption from air conditioning during these hours. Previous studies have indicated a pronounced UHI in stable winter nights, whereas most studies on the UHI in the warm season have focused on individual case studies rather than examining hot days and the warm season.

The study identifies the summer UHI characteristics of Tel Aviv under different weather conditions and develops a statistical downscaling model that predicts the UHI intensity, based on synoptic- and meso-scale variables. It focuses on extreme summer nights, exhibiting elevated temperatures and heat stress. Meteorological data was collected over five summer seasons (2020-2024). Detailed (10-minute) urban and rural meteorological measurements for the mid summer months (July and August), including temperature, relative humidity and wind velocity, together with radiosonde data from a nearby rural station.

We show that the Tel Aviv nocturnal UHI is quite variable during summer days, depending on meso-scale variations under the semi-permanent synoptic seasonal conditions. During daylight hours, the UHI is weak (under 2°C) and occasionally negative, i.e., temperatures at the urban stations are lower than at the rural location. The nighttime UHI ranges from negligible (below 1°C) to extreme (9°C), and is typically between 4-7°C.

Our study shows that the intensity of the UHI is significantly dependent on the westerly wind component (the sea breeze, enhanced by the synoptic-scale Etesian winds), the height of the persistent seasonal marine inversion and the existence and intensity of a surface inversion, observed near the city. A prediction equation for the UHI, based on these predictors, yields 0.85 correlation with the observed values. A surprising finding is an inverse relationship between the nocturnal temperature and the UHI intensity.

To investigate the finding mentioned above, several cases of excessive nocturnal heat events, with minimum temperatures exceeding 26°C, with high humidity were examined. In some of them the nocturnal UHI disappeared or remained very weak (0-2⁰C), whereas during others it was distinct and occasionally extreme (5-9⁰C). We found that extreme hot nights with minimal UHI are often characterized by the absence of the night land breeze and increased nocturnal westerly winds from the warm Mediterranean Sea. These events are characterized by a relatively high base of the marine inversion accompanied by clouds. Conversely, clearer nights with light land breeze and a pronounced surface inversion, or a low marine inversion base, result in a distinct and strong UHI.