Numerical Simulation of Impact of Urbanization on a Precipitation Process: A Case Study of Heavy Rainfall in Kunming

doi:10.13209/j.0479-8023.2016.128

Abstract

Abstract:

A rainstorm process over west of Kunming city during the night of May 24, 2012 is numerically simulated to study the effect of urbanization on the precipitation by using the weather research forecasting model (WRF) numerical model with single-layer urban canopy model. It is found that the precipitation process is well simulated by comparing the spatial distribution of precipitation between the satellite observation and simulated result. The presence of the city changes the latent heat and low level vertical motion over urban area significantly, resulting in slight reduction of the precipitation intensity in the precipitation center and enhancement of the downwind precipitation. The area where precipitation increases coincides with the area where water vapor flux convergence at 750 hPa increases. The reduction of precipitation in center may be due to restraint of the moisture supply in the lower atmosphere by urban underlying. While the increase of precipitation over downwind area may result from the relatively strong vertical motion which transports more water into upper atmosphere. If the urbanization continues expansion from its current level, not only the vertical motion and atmospheric instability but also the limiting of the moisture supply in the lower atmosphere will be enhanced. The combination of these two effects will enlarge the uncertainty of precipitation over downwind area.

Key words: urbanization, precipitation process, WRF model

摘要：

为了探究昆明地区城市化对降水的影响, 利用中尺度气象模式 WRF (the weather research and forecas-ting model)耦合单层城市冠层模块, 对2012年5月24日夜间发生在昆明城区西部的一次强降水过程进行数值模拟研究。通过与卫星观测得到的降水空间分布进行对比, 发现WRF模式能较好地再现此次降水过程。城市的存在会较显著地改变城市地区的潜热和低层垂直运动, 使得位于城区的降水中心降水强度略有减小, 而城区下风向的降水有所增加, 降水增加的区域与 750 hPa 高度上的水汽通量辐合加强的区域相对应。城区降水中心降水量减少可能是由于城市下垫面抑制了低层大气的水汽供给。城区下风向降水增加的可能原因是该区域相对较强的垂直运动使得更多的水汽被输送到高层大气中, 有利于降水的增强。在目前的城市化水平上, 若昆明地区的城市进一步扩张, 城区下风向地区的垂直对流运动和大气不稳定度均会增强, 但城市下垫面对低层水汽供给的抑制作用也会增强, 这两种作用的同时存在会使得城市下风向地区降水的变化存在不确定性。

关键词: 城市化, 降水过程, WRF模式

CLC Number:

P404

Yijia ZHENG, Shuhua LIU, Ping HE, Yucong MIAO, Shu WANG. Numerical Simulation of Impact of Urbanization on a Precipitation Process: A Case Study of Heavy Rainfall in Kunming[J]. Acta Scientiarum Naturalium Universitatis Pekinensis, 2017, 53(4): 627-638.

郑亦佳, 刘树华, 何萍, 缪育聪, 王姝. 城市化对昆明一次强降水过程影响的数值模拟研究[J]. 北京大学学报自然科学版, 2017, 53(4): 627-638.

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URL: https://xbna.pku.edu.cn/EN/10.13209/j.0479-8023.2016.128

https://xbna.pku.edu.cn/EN/Y2017/V53/I4/627

Figures/Tables 10

Fig. 1 Nested computational domains in the WRF model simulation

Table 1 Run-time, domain configurations and physics options used in the WRF simulation

模式参数	WRF模式参数设置
模拟时间	2012?05?24 14:00至2012?05?25 20:00
水平分辨率	27, 4.5, 1.5 km
水平格点数	120×120, 131×131, 100×100
土地利用类型数据	MODIS
微物理参数化方案	Thompson graupel方案
辐射参数化方案	RRTMG方案
积云对流参数化方案	Kain-Fritsch方案
边界层参数化方案	YSU方案
陆面过程参数化方案	Noah陆面过程方案耦合UCM城市冠层方案

Table 2 Landuse in different WRF model simulation

模拟试验	土地利用类型
CTL试验	使用MODIS地表利用类型数据
Nourb试验	将城市下垫面替换为农田
Urban试验	将城市周围的农田改为城市

Fig. 2 Landuse of the innermost domain in different WRF model simulation

Fig. 3 Accumulated precipitation for CTL, Nourb, Urban and TRMM data from 21:00 May 24 to 05:00 May 25, 2012

Fig. 4 Difference in accumulated rainfall from 21:00 May 24 to 05:00 May 25, 2012 superposed on mean 10 m wind vector for the precipitation period

Fig. 5 Difference distribution for CTL-Nourb and Urban-CTL of water vapor flux divergence at 750 hPa for the precipitation period

Fig. 6 Time series of different simulation for surface meteorological variables over the urban region from 21:00 May 24 to 12:00 May 25, 2012

Fig. 7 Difference of water vapor flux superposed with vertical velocity ((a) and (c)) and CAPE ((b) and (d)) in height-latitude cross section at 00:00 LST on May 25, 2012

Fig. 8 Difference of water vapor flux superposed with vertical velocity ((a) and (c)) and CAPE ((b) and (d)) in height-latitude cross section at 00:00 LST on May 25, 2012

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