Challenges associated with adaptation to future urban expansion

The most populated state in the United States, California, is projected to add millions of new inhabitants through the end of the current century, requiring considerable landscape conversion to the built environment. A suite of continuous multiyear, medium-range resolution (20-km grid spacing), ensemble-based simulations is examined to assess urban expansion climate effects on California in 2100, and potential strategies to alleviate them. Summertime [June-August (JJA)] warming due to urban expansion of 1°-2°C is greater relative to any other season, and is completely offset by a range of adaptation strategies: green roofs (highly transpiring), cool roofs (highly reflective), and hybrid roofs (with combined biophysical properties of green and cool roofs). After offsetting of urban-induced warming, cool and hybrid roofs lead to a further 1°-2°C reduction in JJA 2-m temperature, highlighting enhanced efficacy of these adaptation strategies. Guided by medium-range-resolution results, additional high-resolution (2-km grid spacing) experiments are conducted for a subset of the JJA periods conducted on a coarser scale. Urban-induced 1°-2°C warming (local maximum warming exceeds 4°C) is simulated, and is offset by cool and green roof deployment. In agreement with coarser-resolution results, maximum near-surface cooling is greater for cool roofs relative to green roofs. Reduced daytime warming associated with both cool and green roofs also modifies the convective mixed layer, reducing turbulent kinetic energy and planetary boundary layer height, although this impact is less for green roofs than for cool roofs. The results presented here demonstrate the importance of future urban expansion in California and illustrate climatic consequences with implications for regional air quality.