Observations of urban heat island advection from a high-density monitoring network

With 69% of the world's population predicted to live in cities by 2050, modification to local climates, in particular Urban Heat Islands (UHIs), have become a well studied phenomenon. However, few studies have considered how horizontal winds modify the spatial pattern in a process named Urban Heat Advection (UHA) and this is most likely due to a lack of highly spatially resolved observational data. For the first time, this study separates the two-dimensional advection-induced UHI component, including its pattern and magnitude, from the locally heated UHI component using a unique dataset of urban canopy temperatures from 29 weather stations (3 km resolution) recorded over 20 months in Birmingham, United Kingdom. The results show that the mean contribution of UHA to the warming of areas downwind of the city can be up to 1.2 °C. Using the inverse Normalized Difference Vegetation Index as a proxy for urban fraction, an upwind distance at which the urban fraction has the strongest correlation with UHA was demonstrated to be between 4 and 12 km. Overall, these findings suggest that urban planning and risk management needs to additionally consider UHA. However, more fundamentally, it highlights the importance of careful interpretation of long- term meteorological records taken near cities when they are used to assess global warming