A comprehensive understanding of urban heat island (UHI) can help to evaluate the potential heat risk and to design the mitigation strategies efficiently. This study quantifies the UHI in the city of Berlin and investigates the underlying physical mechanism and influencing factors using the integration of in-situ observation, remote sensing (MODIS), and numerical modelling (WRF). Berlin shows UHI for both observations and simulations. Up-to-date land cover data can contribute to improving the model performance in simulating the UHI, especially in the rural areas. The urban expansion increases the spatial extent of UHI in Berlin but has little impact on the intensity of UHI based on the modelling results. The MODIS LST shows a highly positive correlation with the ISA regionalized by a Kernel Density Estimation method (ISAKDE). The linear functions of LST against ISAKDE are fitted well at both annual and daily scales. The slope of the linear function represents the increase in LST from the natural surface in rural areas to the impervious surface in urban areas and is defined as SUHI intensity. The derived surface UHI intensity shows high values in summer and during the day than in winter and at night. The WRF/UCM simulated 2 m air temperature (T2m) shows a good relationship with the ISA in the WRF grids (ISAWRF). Similarly, the UHI intensity is quantified using the slope of the ISAWRF-based linear functions of the simulated T2m. The derived UHI intensity shows U-shaped diurnal variations, with large values at nighttime. The daytime UHI intensity is low and even negative, due to the shading of the buildings. UHI and urban aerosols can interact with each other. UHI could promote the turbulent dispersion of aerosol particles, decreasing the urban-rural difference in near-surface aerosol concentrations. Urban aerosols affect UHI by reducing the incoming solar radiation and increasing the atmospheric longwave radiation in the urban areas. The response of the surface to the change of the absorbed energy is strong at night and weak during the day. As a result, the surface UHI is enhanced by the urban-rural difference of incoming longwave radiation at night, while is negligibly altered in the daytime, due to the offset of the opposite change of solar radiation and atmospheric longwave radiation and the weak response of the surface. It is hoped that the study results can contribute to a better understanding of the urban climate in Berlin and can provide useful information for designing the UHI mitigation strategies and urban planning for Berlin.
