Ensemble simulations to investigate the impact of large-scale urbanization on precipitation in the lower reaches of Yangtze River Valley, China

The influence of urban clusters in the lower reaches of Yangtze River Valley (YRV) on a summertime heavy precipitation event is evaluated based on ensemble simulations using the Weather Research and Forecasting (WRF) model. Results show that control runs with realistic representation of urban areas perform better than the sensitivity runs with urban areas replaced by cropland, suggesting that the urban canopy physics in the surface layer, and hence realistic urban land-use classification, is crucial for urban-induced precipitation modelling. Precipitation starts later but is stronger due to the urban effects. Urbanization leads to two adjacent belts of increasing and decreasing precipitation to the north of the Yangtze River. These urban-induced variations in precipitation are attributed to the impact of urban clusters in the lower reaches of the YRV, which act like a single large (non-continuous) city to slow down the precipitation process due to its building barrier effect. Such an impact leads to more precipitation in the north of the precipitation field but provides less moisture for precipitation in the south. The variations of turbulent kinetic energy and vorticity that resulted from the effects of urban clusters show upward propagation during the precipitation event, suggesting that the urbanization impact may be transferred from surface to upper atmosphere, which could exert important influences on precipitation processes.