Research On Urban Canopy Heterogeneity And Development And Application Of Wind Field Parameterization Schem

It is heterogeneous in urban area with various layout of urban buildings,which leads to the significant spatial inhomogeneity of the wind field in and over the canopy of urban street canyons.However,most of the current urban canopy models do not fully consider the heterogeneity of the urban canopy,where large discrepancies thus exist between simulations and observations among urban areas.In this study,the observations in Nanjing Caihongqiao,a residential area with high-density buildings,were analyzed to uncover the fact of heterogeneity,which is that the observations of wind speed and direction and air pollution concentration are significantly different in scale and variation at two sites with a distance of around 80 m.The corresponding computational fluid dynamics（CFD）simulation also confirmed that the spatio-temporal differences are mainly caused by the different spatial positions of the building distribution pattern and emission source distribution pattern relative to different sites.Based on the above observations and mechanism analysis,Inhomogeneous Wind Scheme for Urban Street（IWSUS）is developed based on the layout of typical urban street canyons.The outputs are the wind profile results at six typical positions inside the long street canyons with different vertical distribution characteristics,which were obtained via CFD method and the relevant site observation data.The results are not only different in the description of physical laws of the exponential logarithmic wind profile scheme applied in the traditional urban canopy model,but also can reflect the difference in the distribution characteristics of the wind field at different typical points in the street canyon,which is distinguished from the exponential logarithmic scheme mentioned above that only gives the average wind profile in the street canyon.The comparison with the observation showed significant improvement in the values of wind speed especially at levels lower than the street height,with the normalized mean bias decreased by 75.5%compared with the exponential logarithmic scheme.Therefore,for the observation sites inside street canyon,the IWSUS scheme can output results that are more consistent with the distribution of surrounding buildings,so that the spatio-temporal representation and numerical values of the simulation and observation have a better correspondence,and further improved the accuracy and verifiability of urban canopy models.Finally,the IWSUS is successfully coupled with the mesoscale atmospheric model,and is applied to conduct a numerical simulation experiment on the PM_2.5 pollution process in Nanjing in January 2022,and compared with the original WRF-Chem control group results.The results show that the deviation between the simulation results of the urban surface meteorological environment variables from the experimental group and the observation is smaller than that of the control group,and the urban heat island intensity from the experimental group is stronger than that of the control group.In addition,the stratification of urban boundary layer simulated by the experimental group is more unstable than that of the control group,resulting in a stronger vertical transport of PM_2.5in experimental group.For different transport scenarios,the PM_2.5transport mechanism from the simulation of experimental group and the control group is also different.It can be seen that the implantation of IWSUS can further improve the simulation performance of the meteorological model on the urban boundary layer and pollution process.This study focus on the inhomogeneity of urban canopy and urban boundary layer,which develops a new parameterization scheme of near-ground wind field for urban canopy model and the advances is confirmed by comparing and validating with observation and CFD.The thesis related to urban boundary layer is enriched by this study.On the other hand,the ability of precisely simulating the wind and turbulence field in sub-grid of urban canopy model is also greatly enhanced.This study can also provide the basis and numerical tool for the application in the field of urban climate and urban meteorological environmental in the future.