Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The...Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind vel展开更多
A strong urban heat island (UHI) appeared in a hot weather episode in Suzhou City during the period from 25 July to 1 August 2007. This paper analyzes the urban heat island characteristics of Suzhou City under this ...A strong urban heat island (UHI) appeared in a hot weather episode in Suzhou City during the period from 25 July to 1 August 2007. This paper analyzes the urban heat island characteristics of Suzhou City under this hot weather episode. Both meteorological station observations and MODIS satellite observations show a strong urban heat island in this area. The maximum UHI intensity in this hot weather episode is 2.2℃, which is much greater than the summer average of 1.0℃ in this year and the 37–year (from 1970 to 2006) average of 0.35℃. The Weather Research and Forecasting (WRF) model simulation results demonstrate that the rapid urbanization processes in this area will enhance the UHI in intensity, horizontal distribution, and vertical extension. The UHI spatial distribution expands as the urban size increases. The vertical extension of UHI in the afternoon increases about 50 m higher under the year 2006 urban land cover than that under the 1986 urban land cover. The conversion from rural land use to urban land type also strengthens the local lake-land breeze circulations in this area and modifies the vertical wind speed field.展开更多
An urban canopy model is incorporated into the Nanjing University Regional Boundary Layer Model. Temperature simulated by the urban canopy model is in better agreement with the observation, especially in the night tim...An urban canopy model is incorporated into the Nanjing University Regional Boundary Layer Model. Temperature simulated by the urban canopy model is in better agreement with the observation, especially in the night time, than that simulated by the traditional slab model. The coupled model is used to study the effects of building morphology on urban boundary layer and meteorological environment by changing urban area, building height, and building density. It is found that when the urban area is expanded, the urban boundary layer heat flux, thermal turbu- lence, and the turbulent momentum flux and kinetic energy all increase or enhance, causing the surface air temperature to rise up. The stability of urban atmospheric stratification is affected to different extent at different times of the day. When the building height goes up, the aerodynamic roughness height, zero plane displacement height of urban area, and ratio of building height to street width all increase. Therefore, the increase in building height results in the decrease of the surface heat flux, urban surface temperature, mean wind speed, and turbulent kinetic energy in daytime. While at night, as more heat storage is released by higher buildings, thermal turbulence is more active and surface heat flux increases, leading to a higher urban temperature. As the building density increases, the aerodynamic roughness height of urban area decreases, and the effect of urban canopy on radiation strengthens. The increase of building density results in the decrease in urban surface heat flux, momentum flux, and air temperature, the increase in mean wind speed, and the weakening of turbulence in the daytime. While at night, the urban temperature increases due to the release of more heat storage.展开更多
Based on the successful simulation of a typical winter urban boundary layer(UBL) process over Beijing area during the Beijing City Air Pollution Experiment (BECAPEX) in 2001by the use of MM5 coupled with urban canopy ...Based on the successful simulation of a typical winter urban boundary layer(UBL) process over Beijing area during the Beijing City Air Pollution Experiment (BECAPEX) in 2001by the use of MM5 coupled with urban canopy parameterization, a series of simulation experiments areperformed to investigate the effects of urban influence, surrounding terrain, and different extentof urbanization on urban boundary layer structures over Beijing area. The results of factorseparation experiments of urban influence indicate that the total effect of urban influence, whichis the synthetic effect of urban infrastructure on thermal and dynamic structures of atmosphere, isresponsible for the formation of main UBL features over Beijing area. Meanwhile, the relativeimportance of thermal and mechanical factors of urban infrastructure and interaction between thermaland mechanical factors for the formation and evolution of UBL over the Beijing area are alsoexplored. The results show that, during nighttime, mechanical factors are responsible for maincharacteristics of nocturnal urban boundary layer such as elevated inversion layer over downtownarea, smaller wind speed and stronger turbulent kinetic energy (TKE) and its behavior with peak atthe top of canopy layer, whereas in the daytime, thermal factors play dominant role in the structureof UBL, such as the intensity of mixed layer and temperature in the lower atmosphere in urban area.The interaction between mechanical and thermal factors plays an important role in the formation andevolution of UBL, but its specific characteristics of mechanisms are complex. The results ofsurrounding terrain experiment show that terrain surrounding Beijing area not only determines thecharacteristic of prevailing airflow over Beijing area, but also has obvious effect on thermalstructure of UBL, such as the distribution of elevated inversion and urban heat island, and makesthem with special localization feature. The results of different extent urbanization experiment showthat with the increase in the density and height of buildings in Beijing area, wind speed woulddecrease and TKE increase. Meanwhile, the bottom of nocturnal elevated inversion would increase indowntown area, and the intensity of urban heat island would strengthen, and even probably is obviousin the daytime.展开更多
A two-layer model based on the linearized time-independent atmospheric dynamical equations is proposed in this paper. The analytical solutions of the vertical, the horizontal motions and the potential temperature fiel...A two-layer model based on the linearized time-independent atmospheric dynamical equations is proposed in this paper. The analytical solutions of the vertical, the horizontal motions and the potential temperature field induced by the anthropogenic source of urban surface heating are obtained, therefore the heat island circulation existing in unstable boundary layer is verified theoretically. From the analytical solutions, some conclusions can be drawn. (1) The vertical motion induced by urban heat island consists of two parts, namely, the cross-hill wave and the lee wave; (2) The cross-hill wave only exists in the unstable boundary layer, and varies with height according to exponential function law; (3) The vertical motion induced by heat island reaches the maximum at the top of the unstable boundary layer; (4) The wave generated by heat island not only propogates to the downwind district but also travels to the upwind area; (5) γ≠0 is not the necessary condition of the lee wave generation.展开更多
An urban boundary layer model (UBLM) is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate...An urban boundary layer model (UBLM) is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate the wind fields over three typical urban blocks over the Beijing area with different height-towidth ratios. For comparisons, the wind fields over the same blocks are simulated by an urban sub-domain scale model resolving the buildings explicitly. The wind fields simulated from the two different methods are in good agreement. Then, two-dimensional building morphological characteristics and urban canopy parameters for Beijing are derived from detailed building height data. Finally, experiements are conducted to investigate the effect of buildings on the wind field in Beijing using the improved UBLM.展开更多
In this work, it is investigated the Urban Heat Island (UHI) using conservative thermodynamic variables observed by surface weather stations on the Metropolitan Area of Porto (Oporto) in Portugal, under adiabatic cond...In this work, it is investigated the Urban Heat Island (UHI) using conservative thermodynamic variables observed by surface weather stations on the Metropolitan Area of Porto (Oporto) in Portugal, under adiabatic conditions at the surface. These conditions are usually present and associated with the development of a mixture layer into the diurnal Convective Boundary Layer (CBL), which residual layer in the late afternoon defines the initial state for the development of the nocturnal UHI. Both the spatial structure and temporal variation of potential temperature and specific humidity were considered, along the hours and days of the year, from a statistical point of view, resulting in hourly climatology. Details of the hourly evolution of the meteorological variables on the Oporto surface are presented and discussed. Results show a seasonal variation of the potential temperature up to 17°C throughout the year, which is associated with horizontal thermal gradients that can control and trigger mesoscale circulations such as sea-land, urban and valley-mountain breezes.展开更多
Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes(RANS)approaches are used to examine the urban boundary layer(UBL)development above Kowloon Peninsula,Hong Kong Special Administrative Region(HKSA...Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes(RANS)approaches are used to examine the urban boundary layer(UBL)development above Kowloon Peninsula,Hong Kong Special Administrative Region(HKSAR).The detailed urban morphology is resolved by computational fluid dynamics(CFD)and is fabricated by 3D-printing(reduced scale)for wind tunnel experiments.Different from the majority existing results based on idealized,homogeneous urban geometries,it was found that the wind and turbulence in the UBL over downtown Kowloon are characterized by the wake behind several high-rise buildings.In particular,local maxima of turbulence kinetic energy(TKE)and shear stress are found at the roof level of those high-rise buildings.In the downstream region where the flows are already adjusted to the urban surfaces,the urban roughness sublayer(URSL)can be further divided into two layers based on the structures of the mixing length/m,effective drag Dx and dispersive stress.In the lower URSL(z<100 m),lm is rather uniform,and the Reynolds stress and dispersive stress are comparable.In the upper URSL(100 m z s 300 m),on the contrary,lm is peaked at the mid-height and the magnitude of dispersive stress is smaller than that of the Reynolds stress(<30%).The effective drag Dx is negligible in the upper URSL.展开更多
基金The research was supported by the National Natural Science Foundation of China under Grant Nos.40333027 and 40075004.
文摘Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind vel
基金sponsored by the National Basic Research Program of China (2010CB428501 and 2011CB952002)National Natural Science Foundation of China (Grant No. 41005008)
文摘A strong urban heat island (UHI) appeared in a hot weather episode in Suzhou City during the period from 25 July to 1 August 2007. This paper analyzes the urban heat island characteristics of Suzhou City under this hot weather episode. Both meteorological station observations and MODIS satellite observations show a strong urban heat island in this area. The maximum UHI intensity in this hot weather episode is 2.2℃, which is much greater than the summer average of 1.0℃ in this year and the 37–year (from 1970 to 2006) average of 0.35℃. The Weather Research and Forecasting (WRF) model simulation results demonstrate that the rapid urbanization processes in this area will enhance the UHI in intensity, horizontal distribution, and vertical extension. The UHI spatial distribution expands as the urban size increases. The vertical extension of UHI in the afternoon increases about 50 m higher under the year 2006 urban land cover than that under the 1986 urban land cover. The conversion from rural land use to urban land type also strengthens the local lake-land breeze circulations in this area and modifies the vertical wind speed field.
基金Supportly by the National Natural Science Foundation of China under Grant No. 40333027
文摘An urban canopy model is incorporated into the Nanjing University Regional Boundary Layer Model. Temperature simulated by the urban canopy model is in better agreement with the observation, especially in the night time, than that simulated by the traditional slab model. The coupled model is used to study the effects of building morphology on urban boundary layer and meteorological environment by changing urban area, building height, and building density. It is found that when the urban area is expanded, the urban boundary layer heat flux, thermal turbu- lence, and the turbulent momentum flux and kinetic energy all increase or enhance, causing the surface air temperature to rise up. The stability of urban atmospheric stratification is affected to different extent at different times of the day. When the building height goes up, the aerodynamic roughness height, zero plane displacement height of urban area, and ratio of building height to street width all increase. Therefore, the increase in building height results in the decrease of the surface heat flux, urban surface temperature, mean wind speed, and turbulent kinetic energy in daytime. While at night, as more heat storage is released by higher buildings, thermal turbulence is more active and surface heat flux increases, leading to a higher urban temperature. As the building density increases, the aerodynamic roughness height of urban area decreases, and the effect of urban canopy on radiation strengthens. The increase of building density results in the decrease in urban surface heat flux, momentum flux, and air temperature, the increase in mean wind speed, and the weakening of turbulence in the daytime. While at night, the urban temperature increases due to the release of more heat storage.
基金Supported by the Beijing Urban Environment Project (973 Project).
文摘Based on the successful simulation of a typical winter urban boundary layer(UBL) process over Beijing area during the Beijing City Air Pollution Experiment (BECAPEX) in 2001by the use of MM5 coupled with urban canopy parameterization, a series of simulation experiments areperformed to investigate the effects of urban influence, surrounding terrain, and different extentof urbanization on urban boundary layer structures over Beijing area. The results of factorseparation experiments of urban influence indicate that the total effect of urban influence, whichis the synthetic effect of urban infrastructure on thermal and dynamic structures of atmosphere, isresponsible for the formation of main UBL features over Beijing area. Meanwhile, the relativeimportance of thermal and mechanical factors of urban infrastructure and interaction between thermaland mechanical factors for the formation and evolution of UBL over the Beijing area are alsoexplored. The results show that, during nighttime, mechanical factors are responsible for maincharacteristics of nocturnal urban boundary layer such as elevated inversion layer over downtownarea, smaller wind speed and stronger turbulent kinetic energy (TKE) and its behavior with peak atthe top of canopy layer, whereas in the daytime, thermal factors play dominant role in the structureof UBL, such as the intensity of mixed layer and temperature in the lower atmosphere in urban area.The interaction between mechanical and thermal factors plays an important role in the formation andevolution of UBL, but its specific characteristics of mechanisms are complex. The results ofsurrounding terrain experiment show that terrain surrounding Beijing area not only determines thecharacteristic of prevailing airflow over Beijing area, but also has obvious effect on thermalstructure of UBL, such as the distribution of elevated inversion and urban heat island, and makesthem with special localization feature. The results of different extent urbanization experiment showthat with the increase in the density and height of buildings in Beijing area, wind speed woulddecrease and TKE increase. Meanwhile, the bottom of nocturnal elevated inversion would increase indowntown area, and the intensity of urban heat island would strengthen, and even probably is obviousin the daytime.
文摘A two-layer model based on the linearized time-independent atmospheric dynamical equations is proposed in this paper. The analytical solutions of the vertical, the horizontal motions and the potential temperature field induced by the anthropogenic source of urban surface heating are obtained, therefore the heat island circulation existing in unstable boundary layer is verified theoretically. From the analytical solutions, some conclusions can be drawn. (1) The vertical motion induced by urban heat island consists of two parts, namely, the cross-hill wave and the lee wave; (2) The cross-hill wave only exists in the unstable boundary layer, and varies with height according to exponential function law; (3) The vertical motion induced by heat island reaches the maximum at the top of the unstable boundary layer; (4) The wave generated by heat island not only propogates to the downwind district but also travels to the upwind area; (5) γ≠0 is not the necessary condition of the lee wave generation.
基金funded by National Nat-ural Science Foundation of China(Grants Nos.40505002,40652001,and 40775015)Beijing Natural Science Foun-dation(Grant No.8051002)+1 种基金Beijing New Star Project of Science and Technology(Grant No.2005A03)the Ministry of Science and Technology of China(Grant Nos.2008BAC37B04,2006BAJ02A01,and GYHY200906035)
文摘An urban boundary layer model (UBLM) is improved by incorporating the effect of buildings with a sectional drag coefficient and a height-distributed canopy drag length scale. The improved UBLM is applied to simulate the wind fields over three typical urban blocks over the Beijing area with different height-towidth ratios. For comparisons, the wind fields over the same blocks are simulated by an urban sub-domain scale model resolving the buildings explicitly. The wind fields simulated from the two different methods are in good agreement. Then, two-dimensional building morphological characteristics and urban canopy parameters for Beijing are derived from detailed building height data. Finally, experiements are conducted to investigate the effect of buildings on the wind field in Beijing using the improved UBLM.
文摘In this work, it is investigated the Urban Heat Island (UHI) using conservative thermodynamic variables observed by surface weather stations on the Metropolitan Area of Porto (Oporto) in Portugal, under adiabatic conditions at the surface. These conditions are usually present and associated with the development of a mixture layer into the diurnal Convective Boundary Layer (CBL), which residual layer in the late afternoon defines the initial state for the development of the nocturnal UHI. Both the spatial structure and temporal variation of potential temperature and specific humidity were considered, along the hours and days of the year, from a statistical point of view, resulting in hourly climatology. Details of the hourly evolution of the meteorological variables on the Oporto surface are presented and discussed. Results show a seasonal variation of the potential temperature up to 17°C throughout the year, which is associated with horizontal thermal gradients that can control and trigger mesoscale circulations such as sea-land, urban and valley-mountain breezes.
基金supported by the RGC Theme-based Research Scheme(TRS)T24-504/17-Nthe RGC Collaborative Research Fund(CRF)C706418G+1 种基金as well as the National Natural Science Foundation of ChinaMacao Science and Technology Development Joint Fund(NSFC-FDCT,No.41861164027).
文摘Wind tunnel experiment and steady-state Reynolds-averaged Navier-Stokes(RANS)approaches are used to examine the urban boundary layer(UBL)development above Kowloon Peninsula,Hong Kong Special Administrative Region(HKSAR).The detailed urban morphology is resolved by computational fluid dynamics(CFD)and is fabricated by 3D-printing(reduced scale)for wind tunnel experiments.Different from the majority existing results based on idealized,homogeneous urban geometries,it was found that the wind and turbulence in the UBL over downtown Kowloon are characterized by the wake behind several high-rise buildings.In particular,local maxima of turbulence kinetic energy(TKE)and shear stress are found at the roof level of those high-rise buildings.In the downstream region where the flows are already adjusted to the urban surfaces,the urban roughness sublayer(URSL)can be further divided into two layers based on the structures of the mixing length/m,effective drag Dx and dispersive stress.In the lower URSL(z<100 m),lm is rather uniform,and the Reynolds stress and dispersive stress are comparable.In the upper URSL(100 m z s 300 m),on the contrary,lm is peaked at the mid-height and the magnitude of dispersive stress is smaller than that of the Reynolds stress(<30%).The effective drag Dx is negligible in the upper URSL.