Intensification of sub-daily rainfall extremes in a low-rise urban area

Short-duration extreme rainfall events are the main trigger of flash and pluvial floods in cities. Depending on the local climate zone and urban fabric that affect meteorological variables such as air temperature, humidity, and aerosol concentration, the built environment can either intensify or reduce extreme rainfall intensity. This study examined how urbanization in a large metropolitan area characterized by open low-rise buildings, affected sub-daily extreme rainfall intensities over the period between 2000 and 2018. The research was conducted in the metropolitan region of Phoenix, Arizona, which is supported by a large and dense rain-gauge network (168 stations). The built area increased by 6% between 2001 and 2016 and the number of residences by 300,000. Over the study period, sub-daily extreme rainfall intensities intensified both in the urbanized area and in its rural surroundings but the intensification trend within the built area was considerably larger (3 times larger). 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 degrees C y(-1)) as compared to the rural counterpart (0.09 degrees C y(-1)) for the period between 2005 and 2018. Although built surfaces and open low-rise buildings contributed to an increase in air temperature, they did not affect air humidity. Changes in rainfall extremes approximately follow the Clausius-Clapeyron relation within the urban area with an increase at a rate of 7% degrees C-1. These results demonstrate that the warming effect associated with a low-rise urban area can cause an intensification of sub-daily rainfall extremes that is significantly larger than in nearby rural areas.