城市效应对登陆热带气旋妮妲降水影响的模拟

Simulation of the Urbanization Impact on Precipitation of Landfalling Tropical Cyclone Nida(2016)

运用中尺度数值模式WRF耦合城市冠层模式(urban canopy model,UCM),对2016年登陆深圳的热带气旋妮妲(1604)(以下简称妮妲)进行数值模拟。高分辨率数值模拟较好地再现了妮妲登陆前后的强度、路径和累积降水。利用城市化过程当中城市冠层对热带气旋降水的敏感性试验结果表明:城市冠层会减弱对流运动和水汽的输送,导致热带气旋登陆后珠江口城市群区域累积降水量略减少。应用最新的土地利用资料进行的城市下垫面敏感性试验结果表明:由于城市下垫面粗糙度增加,造成登陆地面风的减速,强度减弱,潜热通量与2m高度比湿相应减小;城市下垫面粗糙度增加会加强该区域垂直对流运动以及不稳定能量增加,有利于降水增强,尤其在城市化下垫面处,热带气旋登陆后6h累积降水增加量最大可超过20mm。总体而言,对登陆热带气旋降水而言,耦合城市冠层使城市区域热带气旋降水减少,但在数值模拟中城市冠层影响作用不显著。城市化下垫面对登陆热带气旋暴雨的增幅作用明显,在登陆热带气旋降水预报中应重视。

For more than half a century, changes in atmosphere induced by the land use change associated with urbanization have drawn increasing attention. However, it is still unclear how urbanization affects landfalling tropical cyclone（TC） precipitation, which may complicate precipitation processes during TC landfalls.TC precipitation is always hard to predict accurately,which still deserves further research.Several numerical experiments of tropical cyclone Nida（2016） making landfall in Guangdong Province are conducted using the Advanced Research Weather Research and Forecast system（WRF） to evaluate the urbanization effects on TC precipitation during its landfall. Specifically, WRF is coupled with the urban canopy model（UCM）, and different land use data（new and old） are used for sensitive experiments. The tropical cyclone Nida（2016） landed around Shenzhen, Guangdong Province on 1 August 2016. The model performance on the track, intensity and precipitation of tropical cyclone Nida is evaluated.Both simulated spatial distribution of 6-hour（from 2200 UTC 1 Aug to 0400 UTC 2 Aug） accumulated precipitation are quite consistent with observations, indicating that the coupled UCM model simulation results are credible. Tracks and accumulated precipitation of the typhoon during landfall can be reproduced reasonably well. No significant difference of simulated TC tracks is found between experiments with and without the updated underlying surface and the coupling of the UCM, indicating that the land use change cannot strikingly affect the track. Although simulations fail to accurately capture the post landfalling intensity changes, storms simulated in experiments including the UCM and latest land use data show more rapid weakening rates after landfall, which are closer to observations. Spatial distributions of simulated 6-hour accumulated precipitation are quite consistent with observations. Furthermore, urban canopy tends to reduce TC precipitation in the urban region while the underlying surface change due to urbanization tends to increase TC precipitation. Urban canopy diminishes vapor transports and corresponding convection, resulting in a decrease in accumulated precipitation. By contrast, land use change due to urbanization decelerates the near-surface wind velocity and decreases surface latent heat fluxes, but strengthens updrafts in the urban region and increases convective available potential energy. As a result, the land use change still leads to enhancement of TC precipitation.These results show that land use change due to urbanization（use of urban canopy） tends to limit TC precipitation after landfall. The rainfall enhancement by land use change due to urbanization is partly offset by the suppression due to use of urban canopy. This will significantly affect the precipitation process of landfalling TCs, and should be taken into account in numerical simulations.