Based on the 0.5°×0.5° ERA-interim reanalysis data,a heavy rainfall event that occurred on June 5,2016 in Zhengzhou was simulated using the Advanced Research Weather Research and Forecasting Model (WRF-ARW) coupled with a slab urban canopy model (UCM).Sensitivity experiments with the modified land-use categories and terrain height are conducted to investigate the impacts from urbanization and topography of the mountain to the northwest of Zhengzhou on this heavy rainfall event.The conclusions can be summarized as follows:
1)The results show that the severe rainfall events are mainly favored by the ageostrophic-forced ascending motion associated with an eastward-migrating upper-level trough and the unstable convective stratification in the vicinity of Zhengzhou.
2)The precipitation pattern simulated in the control experiment is well consistent with the observational data.The sensitivity experiment (NO_CITY) in which the land-use category of Zhengzhou is changed from urban land to grass features significantly lower rainfall amount in the urban area comparing with the control experiment (CTL).Further analysis shows that the urban surface temperature in NO_CITY are clearly decreased comparing to that in CTL since the modified land-use type weakens the thermodynamical effect of urban surface.The decreased surface temperature in turn suppresses the sensible heat flux and leads to weak ascent in the center of Zhengzhou.It is indicated that the effect of urban heat island tends to enhance upward motion and thus convection in the urban area,which increases the rainfall amount and results in the precipitation more concentrated in the city.
3) The sensitivity experiment in which the mountain to the northwest of Zhengzhou has a reduced orographic height (LOW_MONT) also shows lower rainfall amount than CTL.The results show that this remarkable difference can be attributed to two main factors.Firstly,the gravity wave excited by the orographic effect can promote the convection in Zhengzhou region and thus increase the rainfall in urban area.The lower the mountain is,the weaker the gravity wave excited will be.Hence,the convection in Zhengzhou in LOW_MONT is suppressed,which results in a lower rainfall amount.Secondly,through examining the low-level relative humidity field,it is found that the mountain to the northwest of Zhengzhou plays an important role in hampering the northward transport of the low-level dry air.Therefore,as the height of the mountain is artificially lowered,a larger amount of low-level dry air is advected northwards and thus lowers the relative humidity in Zhengzhou region.The decreased low-level relative humidity enhances the evaporation cooling,and thus the cold pool is associated with the precipitation.The strong cold pool helps the rain band to move fast.The faster the rain band moves,the less time the rainfall will sustain.Therefore,this effect leads to a further decrease in the total rainfall in LOW_MONT experiment.