Urban Heat in Motion - How Expansion and Thermal Advection have Shaped Phoenix’s Warming Trends

Urban expansion is a key contributor to near-surface temperature increases in modified landscapes. While much of past research has examined the impact of urban areas on downstream non-urban regions, the influence of thermal advection from upstream urban development on existing urban areas remains underexplored. In this study, we analyze historical temperature records for rapidly and non-rapidly expanding cities in the southwestern U.S. to determine the primary drivers of warming trends.

Our findings reveal that Phoenix, AZ, and Las Vegas, NV, exhibit significant warming trends in minimum temperatures (Tmin), with rates of 0.835°C/decade and 0.802°C/decade, respectively, alongside reductions in diurnal temperature ranges of -0.506°C/decade and -0.664°C/decade. Conversely, non-rapidly expanding cities such as Williams, AZ, and Flagstaff, AZ, show minimal Tmin increases of 0.005°C/decade and 0.21°C/decade, with much smaller diurnal cycle changes of +0.035°C/year and -0.0073°C/decade, respectively.

We quantified the impact of thermal advection using the Weather Research and Forecasting (WRF) model and semi-Lagrangian simulations to assess the role of upstream urban expansion on observed temperature trends at Phoenix Sky Harbor International Airport. Simulations for June 2002 and June 2019, under various urban extent scenarios, align with long-term observational data, confirm upstream urban expansion as the dominant driver of warming. Simplified semi-Lagrangian simulations further validated these results, demonstrating that thermal advection contributes to rising Tmin and declining diurnal cycles.

This study highlights the utility of the semi-Lagrangian model as a computationally efficient tool for prediction of future Tmin trends driven by urban growth. The findings have broad implications for managing warming in semi-arid cities worldwide.