Using the Pseudo–Global Warming Method to Study the Interaction between Heat Waves and Urban Heat Islands in a Future Warmer Climate

The interaction between highly urbanized regions and heat waves has been extensively studied to understand the effects of the so-called Urban Heat Island (UHI) on the lower boundary layer. With the growth of mega-cities, the impact of UHIs on energy consumption, environmental conditions and human health has been a concern. Such effects are expected to be compounded in a projected warmer future climate. To better understand this situation and the effectiveness of mitigation strategies, we apply the Pseudo-Global Warming (PGW) method to create forcing data for local climate models that result in better representations of the planetary flow and thermodynamics of a future climate than traditional downscaling methods. In general, we use this method with different Representative Concentration Pathways (RCP) scenarios covering the period from 2030 to 2100 derived from global climate models.

The present study investigates the interaction of the 2006 heat wave in North America with two urban environments (Phoenix and New York City) in current climate and future climate simulations. The future climate conditions were generated using the PGW methodology. Multiple high-resolution (3 km) simulations for each of the metropolitan areas were performed using the Weather Research and Forecasting model (WRF) coupled with the single layer urban canopy model, which improves representation of urban processes and explore how irrigated green roofs and cool roofs can mitigate heat wave amplification by UHIs.

To quantify heat wave intensity, an analytical model is applied to the WRF model output that considers the urban surface heat and water vapor exchanges with the atmosphere. A future, warmer climate is found to amplify the heat wave intensity in both Phoenix (21%) and New York City (48%). This amplification can be almost completely offset by adopting irrigated green roofs in urban areas, and partially offset by adopting cool roofs. These results show that a synergistic interaction exists between the urban heat island and a warming climate, and that the additional warming can be (partially) mitigated by appropriate urban planning.