Abstract/Summary

To understand the impacts of urbanization on moist convection, we explore how an idealized circular urban island affects the diurnal cycle and spatial distribution of rainfall over urban and surrounding rural areas at the diurnal equilibrium state using large-eddy simulations (LES) performed with the Weather Research and Forecasting (WRF) model. Compared to the control case where the whole domain is covered by grassland, the existence of an urban island significantly enhances the rainfall rate over the urban area as the stronger surface heating creates convergence zones and stronger vertical motions over the urban area. A suite of experiments is then conducted to investigate the effects of soil moisture of the surrounding rural land and the urban size on precipitation. Results show that as the rural soil moisture increases, both urban and rural precipitation rates increase almost linearly. This increase is not attributed to the urban heat island (UHI) effect but rather a stronger moisture deficit effect in the urban area creating a stronger moisture inflow. When the urban area becomes larger but the initial available water remains the same in the domain, the UHI effect and moisture deficit effect increase but the total water supply decreases. As a result, the urban rainfall rate increases first and then decreases as the urban size increases. This suggests that there is an ‘optimal’ scale at which the urban rainfall rate is maximized, at least in our modeling framework. Our simulations further suggest that this optimal scale occurs when the urban fraction lies between 1% and 10%.