Linking Urban Heat Island Intensification with Rainfall Variability in Tropical Archipelagic Indonesia

Urbanization in humid tropical archipelagic environments alters surface energy balance and atmospheric moisture dynamics, yet the coupled behavior of Urban Heat Island (UHI) intensity and rainfall extremes in such settings remains poorly quantified. This study investigates the interactions between urban thermal amplification and precipitation variability using quality-controlled daily observations (2009–2025) from three BMKG stations in South Sulawesi, Indonesia, representing an urban–airport site (Hasanuddin), a coastal urban site (Paotere–Makassar), and a rural inland reference (Masamba). While no statistically significant long-term trend is detected in mean rainfall across stations, extreme precipitation exhibits pronounced spatial differentiation. Generalized Extreme Value modeling reveals that the 50-year return level at the urban–airport site reaches approximately 330 mm, substantially exceeding the ~210 mm estimated at the rural station, indicating urban-enhanced rainfall extremes. Concurrently, a persistent positive UHI signal (+1.4 to +1.8 °C in annual mean temperature) is observed at the urban–airport site relative to the rural reference, whereas the coastal urban station shows near-neutral or slightly negative anomalies consistent with sea-breeze moderation and high moisture availability. Daily-scale analyses demonstrate robust negative associations between UHI intensity and both rainfall and relative humidity (Spearman ρ ≈ –0.54 to –0.66, p < 0.001), suggesting that enhanced latent heat fluxes and cloud-related radiative effects suppress urban thermal contrast under wet conditions. These results indicate the coexistence of urban-amplified rainfall extremes (“rain island”) and a moisture-suppressed UHI regime, highlighting nonlinear feedbacks between thermal forcing and hydrological processes in tropical coastal cities. By providing one of the first integrated observational assessments of UHI–rainfall coupling in an archipelagic Southeast Asian context, this study advances understanding of urban hydroclimatic feedbacks and underscores the importance of incorporating moisture–thermal interactions into climate adaptation strategies, urban design, and flood-risk management in rapidly urbanizing tropical regions.