WRF Simulation of the Urban Heat Island and its Impact on Wind Dynamics and Air Pollution Transport Mechanisms in a Densely Populated Urban Valley of the Tropical Andes

Some of the main environmental impacts of cities occur through the Urban Heat Island (UHI) phenomenon. The UHI can exert strong effects on wind flow and, therefore, on the transport of air pollutants emitted from urban areas. This transport is critical for air quality, especially when cities are located in complex terrains where orographic barriers can largely restrict ventilation. In this study, we use the WRF model to simulate the diurnal variability of the UHI in the Aburrá valley, which is one of the most densely populated regions in Colombia, and a region where severe air pollution episodes have been recently observed. Further, we study the wind fields and transport mechanisms related to the UHI dynamics during severe air pollution episodes. The model configuration consists of 3 nested domains forced by boundary conditions from the Global Forecasting System (GFS at 0.25^o), with horizontal resolutions ranging from 12 km in the external domain to 1 km in the innermost domain. Comparison between simulated and ground-based observed data of temperature was carried out through standard statistical metrics, indicating that the model simulates the UHI dynamics. The UHI has a typical diurnal cycle in which its intensity is higher around sunset and nighttime and lower during the early morning. To further investigate the UHI impact on the valley ventilation we compare simulations with different UHI-related properties of the urban area. The simulated spatial and temporal variability of surface temperature allows to identify strong effects of the city on the temperature field in the Aburrá river valley. Collectively, our results highlight important effects of the UHI on the diurnal dynamics of the transport mechanisms that are determinant for air quality. More generally, our results suggest that the UHI dynamics, as well as their potential alteration via modification of the properties of urban areas, should be thoroughly considered in decision-making processes oriented toward improving the urban environment.