下垫面对郑州城市强降水的影响:城市化及地形影响的数值模拟研究
投稿时间:2018-11-16  修订日期:2019-01-20  点此下载全文
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作者单位E-mail
苏爱芳 中国气象局 农业气象保障与应用技术重点实验室, 河南 郑州 450003
河南省气象台, 河南 郑州 450003 
610061618@qq.com 
施东雷 南京信息工程大学, 江苏 南京 210044  
葛旭阳 南京信息工程大学, 江苏 南京 210044  
基金项目:河南省科技攻关项目(162102310056);国家重点研发计划重点专项项目(2017YFC1502000)
中文摘要:利用WRF-ARW中尺度区域数值模式耦合单层城市冠层模式(slab-UCM),采用ERA-interim 0.5°×0.5°再分析资料作为初始场和边界条件,对2016年6月5日郑州地区发生的一次强对流天气进行模拟,并通过改变下垫面的土地利用类型与地形高度数据设置敏感性试验,探究了城市化及郑州西北部山脉地形对对流性强降水过程的影响。结果表明:1)此次强降水过程主要受高层的低槽系统影响,前期的对流不稳定层结为对流触发提供了有利的热力条件;2)城市热岛效应能够改变下垫面的热力状况,有利于在城市地区激发更强的上升运动,使降水向城区集中;3)郑州西北侧山脉激发的重力波能够使郑州地区对流增强;4)山脉能够阻挡低层干空气向郑州地区输送,因此山脉高度的削减造成郑州地区低层相对湿度减小,使雨滴蒸发冷却增强,进而导致与降水相联系的冷池的增强,雨带移速加快,进一步导致累积降水的减少。
中文关键词:数值模拟  强对流  城市热岛  地形作用
 
Numerical simulation of the influence from urbanization and orography on a severe rainfall event in Zhengzhou
Abstract: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.
keywords:numerical simulation  severe convection  urban heat island  orographic effect
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