应用城市冠层模式研究建筑物形态对城市边界层的影响

文中将城市冠层模式耦合到南京大学城市尺度边界层模式中,通过模拟对比发现,耦合模式对城市地区气温模拟结果更接近于观测值,尤其是对城市地区夜间气温模拟的改进.运用改进耦合模式通过多个敏感性试验的模拟,从城市面积扩张、建筑物高度增加、建筑物分布密度变化等角度研究城市建筑物三维几何形态变化对城市边界层及城市气象环境的影响,试验结果表明:(1)城市面积扩张使得城市下垫面的热通量增大,热力湍流活动增强,动量通量输送增强,城市湍能增大,湍流扩散系数变大,城市气温升高,且对不同时刻城市区域大气层结稳定度均有不同程度的影响.(2)建筑物高度增加增大了城市下垫面的粗糙度和零平面位移.同时也增大了城市街渠高宽比.城市建筑物越高,白天城市地区地表热通量越小,城市上空大气温度越低,平均风速减小,湍能减小;夜间由于高大建筑物释放储热比低矮建筑物要多,其热力湍流相对活跃,地表热通量增大,使得城市区域气温较高.(3)建筑物密度增大,会减小城市下垫面的粗糙度同时增强街渠对辐射的影响.建筑物密度增大在白天会减小地表热通量和动量通量,使城市气温降低,平均风速增大,城市湍流活动能力减弱;夜间城市释放较多储热使得气温较高.     

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea(SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic(warm-core anticyclonic) eddies tend to cool(warm) the overlying atmosphere and cause surface winds to decelerate(accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce(enhance) the vertical momentum transport over the cold(warm) eddy, resulting in the decrease(increase) of sea surface wind. Diagnoses of the model's momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.     

Study on Effects of Building Morphology on Urban Boundary Layer Using an Urban Canopy Model

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 turbulence, 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.     

A Numerical Study of the Urban Intensity Effect on Fog Evolution in the Beijing-Tianjin-Hebei Region

The influence of urban intensity on fog evolution in the Beijing-Tianjin-Hebei (BTH) region (China) is investigated numerically with the the Weather Research and Forecasting (WRF) model coupled with the urban canopy parameterization-building energy model (UCP- BEM) urban physics scheme. The experiments were designed with a focus on the influence of different urban intensities, which are represented by a different fractional coverage of natural land, buildings, and energy consumption inside buildings in an urban environment. The results of this study indicate that urban areas notably influence fog evolution when natural land is reduced to a small fraction (e.g., less than 10%). Developed land changes fog evolution through urban effects. Higher urban intensity (HUI) generally results in warmer temperatures and lower wind speeds throughout the day, while inhibiting morning specific humidity loss and afternoon specific humidity gain because of the HUI effect on surface heat flux, surface roughness, and surface moisture flux. HUI leads to later and weaker liquid water content formation, with a higher liquid water content base, primarily due to its effect on near surface temperatures. This finding implies that HUI may inhibit the conditions for fog formation. In addition, urban areas with equal natural and developed land coverage seem to greatly enhance the upward surface moisture flux, which is attributed to the combination of a relatively large potential evaporation on developed land and an ample moisture supply from natural land. As a result, the specific humidity increases in the afternoon.     

Variations in momentum,mass and heat fluxes due to changes in the sea surface temperature of Hudson Bay

Abstract

The atmospheric model of Danard et al. (1983) is used to investigate the changes in heat, mass and momentum fluxes at the air‐sea interface in Hudson Bay when the seasonal sea surface temperature is varied. Comparisons of model predictions with data from a meteorological buoy located 400 km offshore showed that the model predicted the variations in wind speed and air temperature fairly well but underestimated their magnitudes. In addition it provided offshore heat and mass fluxes for which no direct observations were available.

The most important parameter determining air‐sea fluxes is the temperature difference between air and water. This determines the stability and the degree of vertical convection of the air. In the spring the colder water stabilizes the air, which depresses vertical convection. This reduces wind stress and evaporation while increasing the heat flux into the water. During the fall, the opposite occurs. The sea surface temperature is thus buffered against man‐made changes. When the temperature is decreased, for example, as the result of hydroelectric development in surrounding watersheds, the heat flux into the water increases while the wind stress decreases. Both effects increase the sea surface temperature, opposing the initial decrease. A one‐degree depression of sea surface temperature in summer is slowly offset by increased heating and no noticeable change in temperature remains at the end of the fall.     

Numerical study of mesoscale cellular convection

Mesoscale cellular convections over the East China Sea during cold air outbreaks are simulated with a high-resolution numerical model. The model incorporates important physical processes involved in shallow convection, such as the exchange of heat and moisture between water and air; condensation; evaporation; and vertical turbulent transfer of heat, moisture, and momentum.The results show that open cells develop with aspect ratios as large as 14. The structure of the convection is examined in detail. The organized mesoscale circulation is responsible for breaking up the initial stratus cloud deck and enhancing turbulence in the upward-moving area (especially inside cumulus clouds). However, it is found that the heat flux contributed by MCC's themselves is much smaller than the eddy heat flux.     

Mesoscale spatial and temporal variability of meteorological observations from an array of buoys in JASIN-1978

Observations of wind speed and direction, air and sea temperatures and solar radiation were obtained from an array of buoys in JASIN-1978 conducted in the area northwest of Scotland in the summer of 1978. The observations were analyzed to show spatial and temporal variability in the mesoscale fields. Spectra of wind speed and air and sea temperatures were computed to illustrate the distribution of variance over periods ranging from 3.5 min to 40 days. When plotted on log-log graphs, the spectral estimates generally decreased with slopes between –3/2 and –2 with increasing frequency. Spectra of air and sea temperatures had a peak at the diurnal period but not the wind speed spectrum. When plotted in variance-preserving form, the spectrum of wind speed was consistent with a spectral gap and was qualitatively similar to other observations of low-frequency spectra. On the basis of auto- and cross-correlation analyses, it appeared that mesoscale eddies propagated through the array of buoys with the mean wind speed except during times of frontal passages. The cross-correlation between wind speed and air temperature showed evidence of horizontal roll vortices or some other forms of organized convection.     

Evolution Mechanism of a Severe Squall Line Triggered by the Coupling of a Sea Breeze Front and a Gust Front

In the present study, a severe squall line (SL) was analyzed by using intensive observational surface data and radar monitoring products. In this process, mesoscale convergence lines, such as the sea breeze front (SBF), gust front and dry line, served as the main triggering and strengthening factors. The transition from convection triggering to the formation of the initial shape was mainly affected by the convergence line of the SBF, which combined with thermal convection to form the main parts of the SL. In the later stage, the convergence line of the gust front merged with other convergence lines to form a series of strong convective cells. The SBF had good indicative significance in terms of severe convective weather warnings. The suitable conditions of heat, water vapor and vertical wind shear on the Shandong Peninsula were beneficial to the maintenance of the SL. Before SL occurrence, tropopause folding strengthened, which consequently enhanced the baroclinic property in the middle and upper troposphere. The high sensible heat flux at the surface easily produced a positive potential vorticity anomaly in the low layer, resulting in convective instability, which was conducive to the maintenance of these processes. In the system, when precipitation particles passed through the unsaturated air layer, they underwent strong evaporation, melting or sublimation, and the cooling effect formed negative buoyancy, which accelerated the sinking of the air and promoted the sustained development of the surface gale. Together with the development of low- level mesocyclones, the air pressure decreased rapidly, which was conducive to gale initiation.     

MM5模式中城市冠层参数化方案的设计及其数值试验

文中在综合国外一些较先进的中尺度模式城市作用参数化方案的基础上 ,从城市下垫面结构对城市边界层大气作用的物理机制及实际应用两方面出发 ,对城市下垫面结构和人为活动等因素对边界层结构的影响及中尺度模式中城市化作用的合理体现等问题进行了较全面的考虑 ,改进和设计出能够较全面、细致地描述城市结构对大气边界层动力、热力结构的影响 ,且适合中尺度模式结构特点的城市冠层参数化方案 (UCP) ,并实现了其与MM5模式的耦合。进行了耦合后的UCP方案及采用原城市作用方案的MM5模式对BECAPEX试验期间北京地区气象条件多重嵌套细尺度进行了模拟试验 ,并与观测结果对比 ,结果表明 :相比于MM 5模式中原有表示城市作用的参数化方案来讲 ,设计的UCP方案在很大程度上提高了MM 5模式对城市边界层热力和动力结构的模拟能力。     

盛夏我国东部地区热量平衡的个例计算

本文选取1959年7月23—26日4天正当高空副热带高压控制我国大陆时,对我国东部地区计算了能量方程中各项数值,以及各边界上的能量输送。计算表明,这个地区为能源区城,向外输送热量和水汽。至于在东、西、南、北四个边界上输送的情况有很大的差异,南界以输入为主;东界低层为输出,高层为输入。这种输送是由这个地区低空为大陆热低压,高空为副热带高压的环流特点所决定的。在热量输送中,扰动输送比平均输送为重要。在能量平衡中,支出项主要为平流输送和辐射冷却,而收入项主要是由下垫面向上输送感热和水汽潜热,大气本身的热量和水汽含量的变化在能量平衡中作用较小。湍流感热输送约为蒸发潜热输送的1.84倍。     

成都精细下垫面信息对城市气象影响的模拟试验

为了提高成都市精细化天气预报水平,使用成都地区精细下垫面土地利用资料,在WRF中耦合了单层城市冠层模式,对2008年7月6 日晴空背景下的成都城市气象特征进行了模拟,并和使用旧土地利用资料、slab模式的模拟结果进行了对比分析.模拟结果表明城区因为不透水下垫面的增加,使得地表蒸发和地表水汽通量显著减小,潜热通量减小,感...     

A Vegetated Urban Canopy Model for Meteorological and Environmental Modelling

An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ _l ) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model, and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling.     

Spatial variability of chilling temperature in Turkey and its effect on human comfort

Summary Air temperature, absolute humidity and wind speed are the most important meteorological parameters that affect human thermal comfort. Because of heat loss, the human body feels air temperatures different to actual temperatures. Wind speed is the most practical element for consideration in terms of human comfort. In winter, due to the strong wind speeds, the sensible temperature is generally colder than the air temperature. This uncomfortable condition can cause problems related to tourism, heating and cooling. In this study, the spatial and temporal distributions of cooling temperatures and Wind Chill Index (WCI) are analyzed for Turkey, and their effect on the human body is considered. In this paper, monthly cooling temperatures between October and March in the years 1929 to 1990 are calculated by using measured temperature and wind speed at 79 stations in Turkey. The influence of wind chill is especially observed in the regions of the Aegean, west and middle Black Sea and east and central Anatolia. The wind chill in these regions has an uncomfortable effect on the human body. Usually, the WCI value is higher in western, northern and central Anatolia than in other regions.     

Pine forest microclimate simulation using different diffusivities

Proper understanding of, e.g., evaporation from a forest requires an understanding of its microclimate. A well established, steady-state model was used to simulate microclimate and evaporation of a sparse pine forest in central Sweden. Model input included profiles of turbulent diffusivity, boundary-layer resistance, stomatal resistance, wind speed, net and global radiation and needle area density. Momentum balance, energy balance and exponentially decreasing diffusivities were used to study the sensitivity of the evaporation rates and of the temperature and humidity profiles. Model output proved to be unreliable when measured temperature and humidity at the bottom of the stand were used instead of a measured ground heat flux as the lower boundary condition. Energy balance diffusivity was usually larger than momentum balance diffusivity at the canopy top but decreased rapidly to a minimum at approximately the height where the momentum balance diffusivity had its maximum. Energy balance diffusivity commonly showed a secondary maximum below the height of the maximum needle area density. Profiles of Richardson number showed that thermal effects became important just below the canopy top. Bluff-body effects distinguished the energy balance from the momentum balance diffusivity and both were subject to shelter effects. Total evaporation was not very sensitive to the choice of diffusivity when soil heat flux was given as the lower boundary condition.     

Influence of Thermal Effects on the Wind Field Within the Urban Environment

Micrometeorological conditions in the vicinity of urban buildings strongly influence the requirements that are imposed on building heating and cooling. The goal of the present study, carried out within the Advance Tools for Rational Energy Use towards Sustainability (ATREUS) European research network, is the evaluation of the wind field around buildings with walls heated by solar radiation. Two computational fluid dynamics (CFD) codes were validated against extensive wind-tunnel observations to assess the influence of thermal effects on model performance. The code selected from this validation was used to simulate the wind and temperature fields for a summer day in a specific region of the city of Lisbon. For this study, the meteorological data produced by a non-hydrostatic mesoscale atmospheric model (MM5) were used as boundary conditions for a CFD code, which was further applied to analyze the effects of local roughness elements and thermodynamic conditions on the air flow around buildings. The CFD modelling can also provide the inflow parameters for a Heating, Ventilation and Air Conditioning (HVAC) system, used to evaluate the building energy budgets and to predict performance of the air-conditioning system. The main finding of the present three-dimensional analyses is that thermal forcing associated with the heating of buildings can significantly modify local properties of the air flow.     

鼎新戈壁下垫面近地层小气候及地表能量平衡特征季节变化分析

利用在甘肃酒泉金塔地区进行的 “绿洲系统能量与水分循环过程观测与数值研究” 野外试验中的鼎新戈壁下垫面近地层2年的微气象观测资料, 分析了鼎新戈壁下垫面近地层小气候和能量平衡特征的季节变化规律。结果表明: 鼎新戈壁地区四季的平均风速在2～6 m/s之间, 没有明显的地方性风, 局地环流和局地相互作用可忽略; 冬季8 m向上比湿梯度垂直减小, 其他季节接近等比湿分布。7月, 典型晴天天气下, 能量平衡各量表现出标准的日循环形态, 白天感热通量峰值达到574 W/m2, 地热流量峰值为101 W/m2, 潜热很小, 在夜间和正午时刻存在负水汽输送现象; 然而降水天气下, 地气之间能量传输却以潜热蒸发为主, 感热很小, 地热流量为负值; 无论晴天和阴天, 地表和浅层 (5 cm和10 cm) 土壤温度都表现为准正弦曲线变化; 与敦煌戈壁相同, 土壤温度的活动层基本在10 cm范围以内, 40 cm以下地温已不存在日变化特征; 降水天气下, 地表和浅层土壤温度有随时间减小的趋势。能量平衡各分量均具有明显的季节变化规律, 感热通量季节平均的峰值在250～400 W/m2之间, 在夏季最大, 冬季最小; 潜热通量季节平均的最大值只有15 W/m2; 地气之间能量传输全年都以感热输送为主; 云和降水使夏季能量平衡各分量与晴天相比减少25％左右, 这种影响程度比极端干旱的敦煌荒漠戈壁地区稍大, 而小于半干旱的陇中黄土高原地区。另外, 在这一地区干旱情况很严重, 夏季的Bowen比极值达到62.4, 季节平均值也高达16.5。     

Surface mesoscale features associated with leading convective line-trailing stratiform squall lines over the Gangetic West Bengal

In this paper an attempt is made to identify the mesoscale features in surface pressure pattern, if any, associated with thunderstorm over the Gangetic West Bengal region in India. The study was conducted over Kharagpur and the adjoining area in the Gangetic West Bengal, frequently affected by thunderstorms during the pre-monsoon seasons of April–May. Observations recorded at 50 m instrumented micro-meteorological tower and upper air sounding at Kharagpur under nationally coordinated Severe Thunderstorm Observations and Regional Modeling (STORM) Programme are used to study the variation in surface pressure, wind speed and direction, temperature and relative humidity associated with the squall lines with trailing stratiform precipitation region. In the surface pressure variation, pre-squall mesolow, mesohigh and wake low are identified with the passage of the squall line at Kharagpur. It is observed that in the squall line with trailing stratiform precipitation shield, the mesohigh is associated with convective line and wake low exists at the rear of the storms. The position of the mesohigh is typically found in the vicinity of the heavy rain directly beneath the downdraft. The mesohigh seems to be initiated by the cooling due to evaporation of precipitation in the downdraft and intensified due to the non-hydrostatic effect because of the rainfall directly beneath the downdraft. It is also observed that the passage of trailing edges of the stratiform precipitation coincided with the wake low. Upper air sounding shows mid-tropospheric cooling and lower tropospheric warming. It may be possible due to the dominance of evaporative cooling in the mid-levels and dynamically forced descending motion leading to adiabatic warming in the low levels which may lead to the formation of the wake low.     

Combining a Detailed Building Energy Model with a Physically-Based Urban Canopy Model

A scheme that couples a detailed building energy model, EnergyPlus, and an urban canopy model, the Town Energy Balance (TEB), is presented. Both models are well accepted and evaluated within their individual scientific communities. The coupled scheme proposes a more realistic representation of buildings and heating, ventilation and air-conditioning (HVAC) systems, which allows a broader analysis of the two-way interactions between the energy performance of buildings and the urban climate around the buildings. The scheme can be used to evaluate the building energy models that are being developed within the urban climate community. In this study, the coupled scheme is evaluated using measurements conducted over the dense urban centre of Toulouse, France. The comparison includes electricity and natural gas energy consumption of buildings, building façade temperatures, and urban canyon air temperatures. The coupled scheme is then used to analyze the effect of different building and HVAC system configurations on building energy consumption, waste heat released from HVAC systems, and outdoor air temperatures for the case study of Toulouse. Three different energy efficiency strategies are analyzed: shading devices, economizers, and heat recovery.     

DIURNAL VARIATION OF SST IN RELATION TO SEASON AND WEATHER PHENOMENA IN THE BOHAI REGION

This study investigated the relationships between sea surface temperature(SST) and weather phenomena in different seasons in the Bohai region(China). Five categories of weather phenomena were screened(i.e., fine, cloudy,foggy, rainy and windy conditions) and their relationships with the difference between air temperature and SST observed at Oil Platform A during 2003-2010 were analyzed statistically. The effects of the difference between air temperature and SST in different weather phenomena were examined using the flux method of the atmospheric boundary layer and a formula for the difference between air temperature and SST. The results revealed diurnal variation of the difference between air temperature and SST of-1.0 to +1.0 ℃, i.e., air temperature above the sea surface is subtracted from the SST in corresponding weather phenomena in different seasons in the Bohai region. Moreover,according to the formula for the difference between air temperature and SST, wind and shortwave radiation are the most important factors in terms of the effects of SST on weather processes. In conclusion, the effects of SST on weather phenomena are manifest via the exchange of momentum and energy from sea to air. When the air temperature above the sea surface is lower than the SST, the SST helps develop mesoscale convection systems within the synoptic system through moisture and sensible heat fluxes. When the air temperature above the sea surface is greater than the SST,synoptic systems transfer energy into the sea through heat flux, which affects SST variation. Moreover, a mesoscale convection system will weaken if the synoptic system passes over a colder underlying surface.     

Effects of Sea Spray Evaporation and Dissipative Heating on Intensity and Structure of Tropical Cyclone

To examine effects of sea spray evaporation and dissipative heating on structure and intensity of a real tropical cyclone,the sea spray flux parameterization scheme was incorporated into the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model(MM5).Sensitivity tests were performed with varying the spray source function intensities and with and without dissipation heating.The numerical results indicate that sea spray evaporation increases the interfacial sensible heat flux,which is increased by 16% for the moderate spray and 47% for the heavy spray,but has little effect on the interfacial latent heat flux.The net effect of sea spray evaporation is to decrease the total sensible heat flux and to increase the total latent heat flux.The total enthalpy flux is increased by 1% and 12% with moderate and strong spray amounts,respectively.Consistent with these results,the intensity of the tropical cyclone is increased by 5% and 16% in maximum 10-m wind speed,respectively,due to sea spray evaporation.Sea spray evaporation and dissipative heating modify the tropical cyclone structure in important but complex ways.The effect of sea spray on the near-surface temperature and moisture depends on the spray amounts and its location within the tropical cyclone.Within the high-wind region of a tropical cyclone,the lower atmosphere becomes cooler and moister due to the evaporation of sea spray.However,the dissipative heating offsets the cooling due to sea spray evaporation,which makes the lower atmosphere warmer.