Impact of heat waves on intra-urban thermal heterogeneity

Extreme heat is one of the deadliest health hazards that is getting more intense and prolonged with every passing year. In the summer of 2018 alone, we witnessed several major Heat Waves that exposed millions to the risk of heat-related casualties. In addition to this, urban populations face increased risk due to the locally exacerbated Urban Heat Island (UHI). Previous studies have investigated the collective impact of a UHI in addition to a heatwave and observed a synergistic impact in select cities. These studies used the UHI intensity (the difference between mean urban and rural temperatures) as a metric. However, such a metric is inadequate in capturing the spatio-temporal dynamics of the intra-urban thermal heterogeneity. In this work, we introduce a novel framework that identifies multi-scale clusters of extreme heat for various thresholds within cities. Using this framework, we analyze three major heatwaves that occurred in the summer of 2018 over three major metropolitan regions: Los Angeles, Paris, and New Delhi. These cities are diverse in their urban form and functions as well as background climate. The Weather Research Forecast model (WRF) is the primary modeling environment to simulate the heat waves. We compare the power spectral density and Hurst exponent of urban temperatures as well as the associated thermal energy for a heatwave and non-heatwave scenario. Such a multi-scale framework allows for the identification of the length scale of maximum thermal impact. Finally, we discuss the role of the spatial configuration of urban form and functions in modifying the impact of a heatwave on a city.