The influence of daily weather types on the development and intensity of the urban heat island in two Mediterranean coastal metropolises

In this study we investigated the association between daily weather types (WTs) and the Urban Heat Island (UHI) in two Mediterranean coastal metropolises. For this purpose, we employed an existing weather type classification scheme and examined which WTs influence or drive the intensity of the UHI. We used the gridded weather typing classification (GWTC), in which meteorological conditions at a single location are categorized in daily WTs. We compared these WTs with the maximum temperature differences between urban centers and rural areas in the two major metropolises of Greece (Athens and Thessaloniki). These metropolises have dissimilar geography and spatial planning as well as their urban climate characteristics have differences. We used two groups of temperature time series on a daily basis. One with high and the other with low temperature differences (upper 5% and lower 5% of the maximum temperature differences (ΔΤmax)) which reported the WTs that contribute to increase of UHI Intensity. We found that urban overheating was amplified during daytime under Humid, Humid Warm and Warm conditions in both Athens and Thessaloniki. As for nighttime, urban overheating is associated with Warm, Dry and Dry Warm conditions in Thessaloniki while in Athens increased under Humid, Humid Warm and Warm conditions.