On the urban impacts on summer precipitation in New York City using a high resolution urban-mesoscale model

The Weather Research and Forecasting (WRF) model, coupled with a multi-layer urban canopy model, is used to evaluate impacts of the urban canopy on summer precipitation in New York City (NYC). The urban canopy model considers thermal effects like heat fluxes from roofs, streets, and walls, as well as the reflection and trapping of radiation in street canyons. Mechanical effects like the momentum sink by friction are also included. High resolution (1 km.) urban canopy parameters from the National Urban Database were employed to initialize a multi-layer urban parameterization. Simulations were performed for several summer days in 2007 for the NYC area taking into account the direct and indirect effect of aerosols on the radiation budget and cloud formation. Urban building structures affect thunderstorms movement and formation. The bifurcation of moving thunderstorms caused by the building barrier effect and a deeper boundary layer with more mixing of water vapor produces a rainfall reduction over the metropolitan area and a precipitation increase on its lateral surroundings. The evolution of urban heat island formation was also analyzed. Nocturnal warming due to wall and road effects develops a stronger urban heat island during nighttime hours.