Impact of urbanization on precipitation: a multi-site observation and modelling approach over the United States of America.

Extreme climatic events are increasing on a global scale. Intense rainfall can induce floods, especially on impervious soils such as the ones we find in cities. Understanding how cities impact precipitation can help us to manage the risks generated by rainfall. Thus, numerous articles aim to investigate the potential effect of urban areas on precipitation. While many aspects of this effect are intensively studied, general conclusions are difficult to reach since the studies generally focused on one or a few cities and methodologies are often different preventing us to compare the role of different cities. In this study, we analyze the urban impact on precipitation at a country scale, 38 cities over the United States of America for 11 years (2002-2012). With an observation dataset (radar and rain gauges) and a convective-permitting climate simulation dataset (WRF) both at 4km resolution, we first examine the hourly precipitation variation according to wind directions. In particular, we assess precipitation means, maxima, and quantiles, as well as diurnal and seasonal effects downwind and upwind of each city. In a second step, we investigate the diversity of the results over our city set following three parameters: the urban sprawl, the altitude gradient, and the surface roughness.