Uncertainties in the Impact of Urbanization on Heavy Rainfall: Case Study of a Rainfall Event in Beijing on 7 August 2015

The impact of urbanization on precipitation was investigated using a regional model with different artificial scenarios, which included unavoidable errors resulting from uncertainties related to the initial value and boundary conditions. We studied a typical summer rainfall event that occurred in Beijing on 7 August 2015 to determine whether the uncertainty in parameterization schemes and the initial field affected the urban impact on rainfall. We also quantitatively analyzed the uncertainty in the impact of urban land use on heavy rainfall caused by the initial field and parameterization schemes. The results showed that the microphysics scheme had higher levels of uncertainty than the planetary boundary layer scheme in forecasting precipitation. Urban land use led to an increase in precipitation in the downwind suburban area, consistent with the results of a previous case study. The magnitude of the impact of urban land use on precipitation was about 8 mm, much smaller than the 20 mm reported in the earlier case study. We suggest that the case study probably overestimated the impact of urban land use on rainfall by about 100% because of model uncertainties. Ensemble forecasting is valuable for studies of the impact of urban land use because there is a positive correlation with precipitation and its spread. The ensemble mean analysis increased the significance of the spatial distribution of the impact of urban land use relative to the earlier case study, although the magnitude of the impact was reduced.