Urban Impacts on Convective Precipitation in the Osaka Metropolitan Area Using Large Ensemble Simulations

This study presents large ensemble simulations using the WRF model to statistically evaluate the impact of urbanization on precipitation in the Osaka metropolitan area, a megacity located between the sea and mountains that lie close to each other. An ensemble simulation with a total of 640 members is performed for cases prone to convective precipitation without cyclones near the region. By conducting large ensemble simulations, it becomes possible to statistically evaluate the effects of urbanization on precipitation and to reduce the uncertainty regarding urban impacts. The results of the CTRL experiment, which uses real land-use data, are compared with those of the NOURB experiment, in which the Osaka metropolitan area is replaced with forest, to assess the influence of urbanization on precipitation.The CTRL simulations successfully reproduce the spatial distribution of convective precipitation in the Osaka metropolitan area, with a correlation coefficient of 0.83. Urbanization leads to a 14.4% increase in precipitation in the Osaka metropolitan area and a decrease in precipitation over its surrounding mountainous regions. Notably, the increase in precipitation in parts of the Osaka metropolitan area is statistically significant at the 1% level. The precipitation increase over the Osaka metropolitan area results from two key processes induced by the rise in surface sensible heat flux due to urbanization. The first is the destabilization of the atmosphere due to an increase in the mixed layer height. The second is the enhanced water vapor transport into urban areas, resulting from changes in the sea breeze due to a decrease in surface pressure.