Enhanced intensification of hourly rainfall extremes due to urban warming in Phoenix, Arizona

The main cause of flash and pluvial floods in cities is short-duration extreme rainfall events. The built environment can either intensify or weaken extreme rainfall intensity depending on the urban fabric that controls the local environmental and climatic conditions. From 2000 through 2018, we examined how the built area affected hourly extreme rainfall intensities in the large metropolitan area of Phoenix, Arizona, characterized by open low-rise buildings, using a large and dense rain-gauge network of 168 ground stations. We found that hourly extreme rainfall intensities increased both in the city and its surroundings but the increase in the built area was significantly greater (3 times greater) - the mean trend in annual hourly rainfall maximum in the urban area was 0.6 mm h^-1 y^-1 while in the rural surrounding the mean was 0.2 mm h^-1 y^-1. We calculated a negative trend in aerosol concentration (−0.005 AOD y⁻¹) but a positive trend in near-surface air temperature that was considerably larger in the urban areas (0.15 °C y⁻¹) as compared to the rural counterpart (0.09 °C y⁻¹). Even though built surfaces and low-rise buildings contributed to an increase in air temperature, they did not affect air humidity. Generally, rainfall extremes follow the Clausius–Clapeyron relationship with an increase at a rate of 7% °C⁻¹. Our results demonstrate that the warming effect associated with a low-rise urban area can result in increased rainfall extremes that are significantly greater than in the surrounding areas of the city.