Urbanization counteracts large-scale climatic trend in earlier rainfall in the East-Mediterranean

Changes in the Mediterranean climate, particularly shifts in the wet season’s peak and increasing dry season intensity, have been extensively studied due to their direct implications for the local environment, agriculture, and populations. Comparatively less attention has been given towards changes in the transitions between these seasons, specifically the onset and conclusion of the dry and wet seasons. In this study, we analyzed over 80 years of historical daily rainfall records from twelve meteorological stations across Israel, spanning a north-south climatic gradient. We found a consistent, statistically-significant shift toward an earlier end to the dry season driven by an increase in frequency of synoptic systems that are locally associated with rain, effectively shortening the dry season in nearly all locations. One region was excluded: the growing urban center, where no trend change was observed. To investigate this anomaly, we used high-resolution three-dimensional mapping of the urban area to quantify the change in building surface area and associated energy balance. The rapid urban expansion heavily influences the Bowen ratio, especially in comparison to adjacent rural regions, driving an urban dry-island effect towards the end of the dry season, particularly at nighttime. We show how the relationship between increased urban surface area, sensible heat load, and boundary layer dynamics can be quantitatively linked to changes in the lifting condensation level, which provides a mechanistic explanation for rainfall suppression over the urban region.  Our results highlight the dual influence of large-scale climatic trends and localized urban influences on rainfall during the Mediterranean transition from dry to wet season, offering a glimpse into both long-term changes in regional rainfall dynamics, and a pathway through which urbanization may be reshaping these patterns, during seasonal transition periods.