植物冠层动量交换特征的实验研究

使用湍流梯度测试资料，对植物冠层动量交换特征进行了详细研究，结果表明:森林冠层内惯性副区能谱曲线仍可用幂指数描述，但斜率比-2/3更负；森林冠层内湍流尺度有变小的趋势；森林上层的耗散系数比下层大；由植被吸收引起动量及动量通量随冠层深度增加而明显减小；冠层下层的动量通量和耗散系数分别与上层的量有好的正相关；森林冠层内耗散系数和动量通量随大气稳定度有明显变化。     

植物冠层内物质交换特征的实验研究

使用湍流和SO2通量梯度测试资料，对植被冠层内物质交换特征进行了详细研究，揭示了一些有意义的结果: 植物的生物过程对物质的吸收作用非常明显； 在冠层内物质通量是随冠层深度加大而明显减小；对高的植被，上层沉积速度(Vgu)大于下层 (Vgd)； 冠层内的沉积速度(Vg) 表现出明显的日变化，冠层内物质的生物吸收作用与太阳总辐射量有直接联系；冠层的Vg与速度尺度V＊和平均风成正变关系；森林的Vg比麦地小；新的Vg理论公式能更好地预测重庆森林和麦地的结果。     

多层城市冠层模式的建立及数值试验研究

为在城市气象数值模拟中更好地体现由城市发展引起的下垫面土地利用改变及人为活动对大气过程的影响,建立了基于建筑物三维分布的多层城市冠层模式,冠层内动力方程组考虑了建筑物冠层拖曳力的作用及雷诺应力的影响,通过引入建筑物宽度、间距以及垂直分布密度指数等建筑物形态特征参数,以更好地体现城市复杂地表对大气温度、湿度及动量方程的影响.同时,该模式分屋顶、4个侧壤及地面分别考虑辐射及能量平衡求解表面温度,计算各表面与大气的通量交换,并考虑辐射阴影效应、冠层内部各个面之间的可视因子、以及与冠层内建筑物密度指数、可视因子等相关的多重反射辐射导致的辐射截陷作用.模式的离线检验结果表明:(1)冠层模式计算风廓线与风洞实验测量数据吻合良好;(2)离线冠层模式能够模拟实际小区的风速、温度垂直廓线,并能够较好地体现小区内气温日变化.冠层模式与区域边界层模式耦合检验结果表明:(1)耦合模拟的近地面(2 m处)气温及地表温度的结果明显优于传统的水泥平板方案,尤其是在夜间,水泥平板方案与实测气温最大偏差4 K左右,耦合模拟方案为1-2 K;(2)耦合模拟方案考虑了建筑物对冠层之上的拖曳力影响以及建筑物形态结构对雷诺应力的影响,风速(10 m处)计算结果与观测值相差约在1 m/s,水泥平板方案偏差3 m/s左右.     

棉花冠层微气象特征研究

利用棉花花铃期作物冠层和农田土壤小气候观测资料分析研究了棉花群体内温度、湿度和风速的时空变化规律,探讨了叶片水势、空气饱和差和冠层净辐射之间的关系,对土壤温度和土壤热通量的变化也进行了客观分析。     

一种单层城市冠层模式的建立及数值试验研究

本文在引进先进的城市地表能量平衡方案 (Town Energy Balance, 简称TEB) 的基础上建立了一个单层城市冠层模式, 并对南京市典型居民区1 km2范围内的局地尺度地表能量平衡各分量进行离线模拟, 将模拟结果与同期观测值作了比对, 发现: TEB方案对城市地表能量平衡各分量的模拟效果良好, 而该方案的模拟性能受建筑物表面材料反照率取值的影响较大。在离线研究的基础上, 本文又将TEB方案成功耦合到南京大学区域边界层模式 (NJU－RBLM) 中, 作为该模式的地表能量平衡参数化方案之一, 分别选取该边界层模式中原有的地表能量平衡参数化方案SVAT (Soil－Vegetation－Atmosphere－Transfer model) 和新引入的TEB方案对冬夏两季不同个例进行模拟, 以常规近地面气温观测资料和Landsat卫星观测的地表反照率资料对模拟结果进行比较, 结果表明: TEB方案对原大气边界层模式的模拟效果有明显改善, 对近地面热力场的改善效果尤为明显, 可以很好地模拟出城市冠层中的“陷阱效应”。     

城市冠层模式在GRAPES模式中的应用

在GRAPES中尺度模式中耦合了城市冠层模式(Urban Canopy Model,简写为UCM),并选择珠江三角洲(简称"珠三角")地区2011年7月10日夜间强降水过程和2012年8月19日的高温个例,考察了GRAPES中尺度模式与城市冠层模式耦合在珠三角地区的模拟效果,分别设置了两组试验,在GRAPES模式中耦合和未耦合城市冠层模式(UCM和noUCM)。结果发现,UCM对强降水中心和降水落区的模拟都较noUCM有明显的改善,从降水的四个评估指数(命中率、反命中率、威胁指数及偏差)也可以看出,UCM的结果较noUCM有了较明显的提高,尤其是命中率和威胁指数;对2 m高度的温度和地表热通量而言,两组试验的日间温度和热通量差异较大,中心城区UCM模拟的温度较noUCM高2~4℃,最大差值达5~6℃,热通量差异基本在150 W/m~2,而在夜间,两组试验模拟的温度和热通量差异减小,温度差值基本在0.5~1.0℃之内,热通量差异一般在10 W/m~2左右。两组试验对降水模拟差异的主要原因是UCM模拟出了降水前期地面强的辐合区及相应的温度高值区,对流层低层较大范围的垂直上升运动和辐合上升,对流层中层的暖湿中心及地面较大的CAPE值(达2 000~3 000 J/kg),而noUCM的模拟偏弱或是未能模拟出来。从另外高温个例观测与模式模拟的对比结果来看,UCM模拟的近地面气温、风速、感热通量、向下短波辐射通量和向上长波辐射通量与观测的误差减小,但感热通量与向下短波辐射通量与观测存在一定差距。     

城市冠层过程的研究与进展

介绍了城市冠层过程研究的概况和历史,概括了城市冠层的特征及其作用于局地气候和中尺度系统的主要方式,总结了城市冠层参数化方案或模式的研究进展.比较了几种参数化方法和模式的特点,提出目前存在的一些问题.认为：现有的参数化方案和模式尚未能完整体现城市下垫面特征、准确反映人类活动影响,二者的发展有赖于对城市下垫面建筑特征更合理、细致地描述,对人为热量、水汽影响更准确的估计与刻划.与局地因素密切相关的城市冠层模式还需要更多地在模式中引入局地差异影响.同时指出,要提高耦合城市过程的中尺度模式预报水平,需要相应改进包括云、降水、次网格地形及边界层动力学等的参数化.     

城市冠层中湍流运动的统计特征

对１９９７ 年夏天和冬天北京湍流运动的各种统计特征量进行了初步的统计分析。结果表明,城市冠层中湍流运动的各种统计特征量与平坦下垫面条件下边界层湍流运动的相比, 有不同的地方也有相似的地方; 无论白天还是夜晚, 垂直方向的湍流强度和湍流脉动风速标准差均小于水平方向的, 水平方向的相应湍流特征量则总是接近相等; 城市冠层中湍流脉动强度和标准差几乎均大于平坦下垫面边界层的; 平均风速ｕ≥１ ｍ ／ｓ 时的湍流统计特征量与ｕ＜ １ ｍ ／ｓ 时的有所不同; 城市冠层的阻力系数较大, 可达００６２５,Ｐａｎｏｆｓｋｙ 等提出的公式σｗ ／ｕ＊ ＝ １３ （１－ ３ｚ／Ｌ）１／３在城市冠层中并不适用。     

盘锦湿地芦苇群落冠层内辐射分布特征

基于盘锦湿地芦苇群落冠层内不同层次的微气象要素与生物学特性观测,探讨芦苇群落冠层内总辐射分布与叶面积指数(LAI)的关系。结果表明:芦苇群落内太阳辐射的垂直分布在生长前期(5月)和后期(9月)呈S型曲线,生长盛期(6月和7月)呈指数曲线变化。冠层内不同层次的太阳辐射透射率随叶面积指数的增加而减少,消光系数(k)存在明显的季节变化,表现为从萌芽期到成熟期逐渐减小趋势(最小值k=0.436)。     

海表动量和热量通量的数值模拟

本文分析了用IAP两层大气环流模式模拟的海表动量和热量通量,并将其同Han等和Esbensen等的气候资料比较.模拟的热量通量与观测估计值有类似的水平分布和季节变化,但在中低纬地区有偏多的热量由海洋向大气输送,尤其1月北半球中纬大气需要从海洋获得过分多的热量;模拟的海表动量通量和气候估计值也类似,但模拟的北半球冬季中高纬西风动量通量中心位置偏东,赤道中西太平洋和大西洋的东风动量通量偏弱,南半球环绕南极的西风带模拟得过分弱(尤其在7月份).本文还检验了基本变量的日变化和日际变化对计算海表动量和湍流扩散热量通量的影响,结果表明用月平均,日平均和每小时的基本量计算的动量和湍流扩散热量通量依次增大,尤其在中高纬地区更明显.     

城市边界层动量和保守物通量的特征

利用2005年1-5月北京325 m气象塔47 m高度的湍流脉动资料(风速、温度、水汽和CO2),对城市边界层冠层内的湍流运动统计特征(相似关系、高阶矩、通量和谱等)进行了分析。其中,谱分析的结果表明,城市冠层内稳定度对湍流谱的影响比较小,而水平风速的影响比较大。因此,速度和温度的相似关系在夜间稳定条件下也成立。但是,由于水汽和CO2还受其他因素的影响,相似关系并不适用。更高阶矩的研究表明它们的陡峭度与偏斜度之间存在平方关系。而水汽和CO2之间也存在差异,它们的通量日变化特征明显不同,CO2通量的日变化更能体现人类活动的影响。同时,感热通量、潜热通量和CO2通量存在季节变化,尤其是潜热通量季节差异很大。     

Momentum Absorption in Rough-Wall Boundary Layers with Sparse Roughness Elements in Random and Clustered Distributions

A wind-tunnel experiment has been used to investigate momentum absorption by rough surfaces with sparse random and clustered distributions of roughness elements. An unusual (though longstanding) method was used to measure the boundary-layer depth δ and friction velocity u _* and thence to infer the functional relationship z ₀/h = f(λ) between the normalised roughness length z ₀/ h and the roughness density λ (where z ₀ is the roughness length and h the mean height of the roughness elements). The method for finding u _* is based on fitting the velocity defect in the outer layer to a functional form for the dimensionless velocity-defect profile in a canonical zero-pressure-gradient boundary layer. For the conditions investigated here, involving boundary layers over sparse roughness with strong local heterogeneity, this velocity-defect-law method is found to be more robust than several alternative methods for finding u _* (uw covariance, momentum integral and slope of the logarithmic velocity profile).The experimental results show that, (1) there is general agreement in the relationship z ₀/h = f(λ) between the present experiment with random arrays and other wind-tunnel experiments with regular arrays; (2) the main effect of clustering is to increase the scatter in the z ₀/h = f(λ) relationship, through increased local horizontal heterogeneity; (3) this scatter obscures any trend in the z ₀/h = f(λ) relationship in response to clustering; and (4) the agreement between the body of wind-tunnel data (taken as a whole) and field data is good, though with scatter for which it is likely that a major contribution stems from local horizontal heterogeneity in the field.     

基于风廓线雷达的广东登陆台风边界层高度特征研究

针对8个登陆广东省的热带气旋,利用经过数据质量控制的风廓线雷达连续、高时空分辨率的风场观测数据,对热带气旋边界层特征进行了分析。研究结果表明:热带气旋边界层中切向风速大值区垂直范围越大、风速越强、持续时间越久,则热带气旋强度越大、登陆后强度维持时间越久。眼区外入流层厚度越大,入流层气流越强,热带气旋登陆后强度维持时间则越久。风廓线雷达信噪比垂直梯度对大气湍流信息有一定的指示作用,对于入流层高度在2000 m以下的热带气旋,其入流层顶所在高度与信噪比梯度最大值所在高度相近,对于入流层较为深厚的热带气旋,用信噪比垂直梯度确定的边界层高度虽接近入流层顶高,但仍有一定差距。不同特点的热带气旋其边界层高度并不相同,对于登陆后强度迅速减弱的热带气旋边界层高度在500~1000 m;登陆后强度持续时间短的热带气旋,其边界层高度约1000~2000 m;登陆后强度持续时间长的热带气旋,其边界层高度在2000 m之上,最高可达5000~7000 m。这些结果加深了对登陆台风边界层高度演变特征的认识。     

Multifractal Characteristics of Intermittent Turbulence in the Urban Canopy Layer

The multifractality of energy and thermal dissipation of fully developed intermittent turbulence is investigated in the urban canopy layer under unstable conditions by the singularity spectrum for the fractal dimensions of sets of singularities characterizing multifractals. In order to obtain high-order moment properties of smallscale turbulent dissipation in the inertial range, an ultrasonic anemometer with a high sampling frequency of 100 Hz was used. The authors found that the turbulent signal could be singular everywhere. Moreover, the singular exponents of energy and thermal dissipation rates are most frequently encountered at around 0.2, which is significantly smaller than the singular exponents for a wind tunnel at a moderate Reynolds number. The evidence indicates a higher intermittency of turbulence in the urban canopy layer at a high Reynolds number, which is demonstrated by the data with high temporal resolution. Furthermore, the temperature field is more intermittent than the velocity field. In addition, a large amount of samples could be used for verification of the results.     

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.     

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 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 stratiˉcation is a?ected to diffierent extent at diffierent 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.     

Numerical Study of the Response of an Atmospheric Surface Layer to a Spatially Nonuniform Plant Canopy

High-accuracy large-eddy simulations of neutral atmospheric surface-layer flow over a gapped plant canopy strip have been performed. Subgrid-scale (SGS) motions are parameterized by the Sagaut mixed length SGS model, with a modification to compute the SGS characteristic length self-adaptively. Shaw’s plant canopy model, taking the vertical variation of leaf area density into account, is applied to study the response of the atmospheric surface layer to the gapped dense forest strip. Differences in the region far away from the gap and in the middle of the gap are investigated, according to the instantaneous velocity magnitude, the zero-plane displacement, the potential temperature and the streamlines. The large-scale vortex structure, in the form of a roll vortex, is revealed in the region far away from the gap. The nonuniform spatial distribution of plants appears to cause the formation of the coherent structure. The roll vortex starts in the wake of the canopy, and results in strong fluctuations throughout the entire canopy region. Wind sweeps and ejections in the plant canopy are also attributed to the large vortex structure.