Urban Impacts on Convective Precipitation in the Osaka Metropolitan Area: Numerical Simulations over 320 Potential Precipitation Days

Abstract This study conducts numerical simulations using the Weather Research and Forecasting (WRF) Model to evaluate the impact of urbanization on precipitation in the Osaka metropolitan area, a megacity adjacent to the sea and mountains. The simulations were conducted for 320 events on which convective precipitation was likely to occur. The results of the control (CTRL) experiment, which uses real land-use data, are compared with those of the nonurban (NOURB) experiment, in which the urban grids of the Osaka metropolitan area and surrounding regions are replaced with broadleaf forest, to assess the influence of urbanization on precipitation. The model successfully reproduces the general spatial distribution of convective precipitation in the Osaka metropolitan area, with a correlation coefficient of 0.83. Urbanization leads to a 14.6% increase in precipitation in urban areas and a decrease in precipitation over the surrounding mountainous regions. Notably, the slight increase in precipitation east of central Osaka and the decrease in the northern and southwestern parts of Osaka are statistically significant at the 1% level. The increase in precipitation in the Osaka metropolitan area results from two key processes induced by the rise in surface sensible heat flux due to urbanization. The first is an increase in the mixed-layer height. The second is the enhanced horizontal convergence of water vapor flux in urban areas, resulting from changes in the sea breeze due to a decrease in surface pressure over Osaka.