用大涡模式研究对流边界层湍流

利用三维对流边界层的大涡模拟模式研究对流边界层湍流统计特征量,模拟结果与前人的同类模拟工作及实验观测结果相比较,得到了一致的结论.       

对流边界层的大涡模拟研究

本文建立了一个均匀平坦地面上对流边界层的大涡模式,模式考虑了水汽,采用了考虑浮力和固壁影响订正的一阶闭合。并用所建模式进行了由热扰动发展的对流边界层的模拟及其对地表热状况变化响应的初步探讨性模拟工作。通过模拟认为,模式较好地反映了对流边界层的主要结构。     

对流边界层中泡状结构的大涡模拟研究

本文提出“连续垂直运动区”的概念，对边界层中的对流泡状运动进行了重新定义。由此将对流边界层划分为上升泡区、下沉泡区和环境气流区，并利用大涡模拟提供的对流边界层数值模拟结果研究这种泡状运动的结构。研究结果较好地解释了一些有关泡状运动结构方面的似乎相互矛盾的观测事实，并揭示了对流泡状运动的一些新的特征。     

风切变对边界层对流影响的大涡模拟研究

利用"西北干旱区陆-气相互作用野外观测实验"加密观测期间在敦煌站的观测资料以及大涡模式,模拟了对流边界层的发展,以及示踪物从混合层向残留层传输的时空变化。模拟的对流边界层的结构及演变特征与实测结果基本一致。进一步通过有风切变和无风切变的敏感性数值试验,研究了风切变对垂直速度、位温和示踪物浓度的水平分布以及示踪物传输高度的影响。研究结果表明,在有风切变的试验中(甚至风切变仅存在于近地层中),对流边界层的增长加强,而且示踪物被传输的高度也较高。与浮力驱动的对流边界层相比,由浮力和风切变共同驱动的边界层中上升气流较弱而下沉气流较强,但前者的上升气流与下沉气流的分布在垂直方向上更为倾斜。由于夹卷作用的增强,浮力和风切变共同驱动的对流边界层较浮力驱动的对流边界层暖。在夹卷层,浮力和风切变共同驱动的边界层对流的上升气流和下沉气流都比浮力驱动的边界层对流中的强,而且垂直速度的概率密度函数分布也较对称,其位温和示踪物浓度的概率密度函数分布也比浮力驱动的边界层中的平直。对湍流动能收支的分析也表明风切变对湍流动能有重要影响,尤其对夹卷层中的湍流动能切变产生项影响较大。示踪物浓度的概率密度函数垂直分布显示,浮力驱动的边界层中示踪物浓度随高度变化较小,而浮力和风切变共同驱动的边界层中示踪物浓度随高度递减,但是示踪物传输的高度比较高。     

一个对流边界层大涡模式的建立与调试

本文介绍一个适合于对流边界层的大涡模式的建立及其调试结果。该大涡模式建立过程中注重于计算的节省，同时也强调原理与方法的简单和合理性。模式的调试表明，对于平坦均一地形的情况，模拟可以获得合理的结果。调试同时显示了模式对较低水平分辨率的适用条件，以及模式应用于模拟较大水平范围问题的可能性。     

一个直接考虑大涡、小涡作用的大气对流边界层模式

本文建立了一个大气对流边界层模式,构造此模式时,假设对流边界层中水平动量和热量在垂直方向上的湍流输送、扩散是由大涡与小涡组成的系统完成的。文中,用此模式作了一实例模拟,计算结果表明:模拟的结果与实测的结果相当一致。     

水平非均匀对流边界层热量平衡和平流输送作用的大涡模拟

采用大涡模拟所获的数据结果，分析地面热通量沿平均风方向存在 突变的条件下对流边界层的热量平衡和平流输送作用。分析表明边界层内模拟所得结果 可以很好地满足热量平衡关系。除地面热通量项以外，平流项（包括水平平流和垂直平 流）对边界层加热率的作用可达地面热通量不均匀性差值的大小，是影响边界层内热量 平衡的最重要因子，平均速度散度项对热量平衡的作用也不可忽略，但湍流通量散度项 的作用则很小。     

城市建筑动力学效应对对流边界层影响的敏感性试验

本文将大涡模拟应用于城市对流边界层(CBL)湍流结构和流场特征的研究,在大涡模式中,拖曳系数取与建筑物高度及建筑物高度标准差有关的表达式以考虑次网格建筑物对风速和湍流动能(TKE)的面积平均影响.模拟结果表明,由于城市建筑物对气流的拖曳作用,使建筑物冠层及整个CBL内风速大幅度减小,城市冠层内部风速减小尤为明显,在夹卷层内,风速有一明显的跃变.在边界层中部对流运动已经发展成为较强的热泡,城市建筑物的动力学效应使热泡的水平尺度增大,CBL内平均上升气流速度和下沉气流速度减小,同时使CBL中上升气流所占比例比平坦地面增大.城市建筑物使CBL低层热通量、动量通量、速度方差和位温方差明显增大,但对近地层高度以上的湍流量影响不大.     

对流边界层发展受覆盖逆温影响的大涡模拟研究

本文运用大涡模拟方法研究了存在覆盖逆温时对流边界层顶部的夹卷过程特征,并通过敏感性试验着重分析了在此情况之下的夹卷速度参数化问题,以及风切变的作用。模拟结果表明,初始覆盖逆温改变了夹卷层的结构特征,使得夹卷层结构参数明显增大,风切变使这一效应略有增强,以往关于夹卷速度的参数化方案不再适用。分析研究表明,在有覆盖逆温的情况下,夹卷层结构参数是控制夹卷速度的关键因子,应该将夹卷层结构参数作为变量引入夹卷速度参数化方案,该方案能够很好地预报出受覆盖逆温影响的对流边界层的发展过程。     

边界层对流对示踪物抬升和传输影响的大涡模拟研究

利用"西北干旱区陆气相互作用野外观测实验"加密观测期间敦煌站的实测资料以及大涡模式, 通过一系列改变地表热通量和风切变的敏感性数值试验, 分析了地表热通量和风切变对边界层对流的强度、形式, 以及对对流边界层结构和发展的影响。模拟结果显示风切变一定, 增大地表热通量时, 由于近地层湍流运动增强, 向上输送的热量也较多, 使对流边界层变暖增厚, 而且边界层对流的强度明显增强, 对流泡发展的高度也较高。当地表热通量一定, 增大风切变时, 由于风切变使夹卷作用增强, 将逆温层中的暖空气向下卷入混合层中, 使对流边界层增暖增厚, 但是对流泡容易破碎, 对流的强度也较弱。另外通过在模式近地层释放绝对浓度为100的被动示踪物方法, 用最小二乘法定量地分析了地表热通量和风切变分别与示踪物抬升效率和传输高度的关系。分析结果表明, 风切变小于10.5×10-3 s-1时, 增大地表热通量加强了上层动量的下传, 使示踪物的抬升效率也线性增大;地表热通量小于462.5 W m-2时, 增大风切变减弱了边界层对流的强度, 从而使示踪物的抬升效率减弱。当风切变一定时, 示踪物的平均传输高度随地表热通量增加而增大, 而地表热通量一定, 只有风切变大于临界值时, 示踪物平均传输高度才随风切变的增加而增大, 而临界风速的大小由地表热通量决定。     

扎陵湖和鄂陵湖大气边界层特征的数值模拟

利用美国科罗拉多州立大学和MRC/ASTER发展的中尺度数值模式RAMS(Regional Atmos-pheric Modeling System),设计了有、无湖对比模拟试验,以NCEP再分析资料作为初始条件和边界条件进行了120h的三重嵌套模拟试验。结果表明,模式的模拟性能良好。模拟的大气边界层特征显示,由于扎陵湖、鄂陵湖的存在,白天(夜晚)具有很好的降温(保温)作用,并表现出明显的冷(暖)湖效应;在白天(夜晚)扎陵湖、鄂陵湖有明显的湖(陆)风效应;在两湖之间的陆地低层有辐合和风切变,且鄂陵湖西岸的湖风比扎陵湖东岸的强,削弱了扎陵湖东岸的湖风;扎陵湖、鄂陵湖使得白天湖区的边界层顶低,周边陆地区域的边界层顶高,夜间则相反;扎陵湖、鄂陵湖对感热和潜热的影响有很强的日变化,白天湖面的感热小于周围陆地,夜间湖面的潜热则大于周围陆地。     

Performance of WRF Large Eddy Simulations in Modeling the Convective Boundary Layer over the Taklimakan Desert,China

The maximum height of the convective boundary layer (CBL) over the Taklimakan Desert can exceed 5000 m during summer and plays a crucial role in the regional circulation and weather. We combined the Weather Research and Forecasting Large Eddy Simulation (WRF-LES) with data from Global Positioning System (GPS) radiosondes and from eddy covariance stations to evaluate the performance of the WRF-LES in simulating the characteristics of the deep CBL over the central Taklimakan Desert. The model reproduced the evolution of the CBL processes reasonably well, but the simulations generated warmer and moister conditions than the observation as a result of the over-prediction of surface fluxes and large-scale advection. Further simulations were performed with multiple configurations and sensitivity tests. The sensitivity tests for the lateral boundary conditions (LBCs) showed that the model results are sensitive to changes in the time resolution and domain size of the specified LBCs. A larger domain size varies the distance of the area of interest from the LBCs and reduces the influence of large forecast errors near the LBCs. Comparing the model results using the original parameterization of sensible heat flux with the Noah land surface scheme and those of the sensitivity experiments showed that the desert CBL is sensitive to the sensible heat flux produced by the land surface scheme during daytime in summer. A reduction in the sensible heat flux can correct overestimates of the potential temperature profile. However, increasing the sensible heat flux significantly reduces the total time needed to increase the CBL to a relatively low altitude (< 3 km) in the middle and initial stages of the development of the CBL rather than producing a higher CBL in the later stages.     

Performance of WRF Large Eddy Simulations in Modeling the Convective Boundary Layer over the Taklimakan Desert,China

The maximum height of the convective boundary layer(CBL)over the Taklimakan Desert can exceed 5000 m during summer and plays a crucial role in the regional circulation and weather.We combined the Weather Research and Forecasting Large Eddy Simulation(WRF-LES)with data from Global Positioning System(GPS)radiosondes and from eddy covariance stations to evaluate the performance of the WRF-LES in simulating the characteristics of the deep CBL over the central Taklimakan Desert.The model reproduced the evolution of the CBL processes reasonably well,but the simulations generated warmer and moister conditions than the observation as a result of the over-prediction of surface fluxes and large-scale advection.Further simulations were performed with multiple configurations and sensitivity tests.The sensitivity tests for the lateral boundary conditions(LBCs)showed that the model results are sensitive to changes in the time resolution and domain size of the specified LBCs.A larger domain size varies the distance of the area of interest from the LBCs and reduces the influence of large forecast errors near the LBCs.Comparing the model results using the original parameterization of sensible heat flux with the Noah land surface scheme and those of the sensitivity experiments showed that the desert CBL is sensitive to the sensible heat flux produced by the land surface scheme during daytime in summer.A reduction in the sensible heat flux can correct overestimates of the potential temperature profile.However,increasing the sensible heat flux significantly reduces the total time needed to increase the CBL to a relatively low altitude(3 km)in the middle and initial stages of the development of the CBL rather than producing a higher CBL in the later stages.     

Effect of Mesoscale Land Use Change on Characteristics of Convective Boundary Layer: Semi-Idealized Large Eddy Simulations over Northwest China

Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer (CBL) has not been fully understood yet. In this study, the influence of semi-idealized strip-like patches of oases and deserts, which resemble irrigated land use in Northwest China, on the CBL characteristics, is investigated based on the Weather Research and Forecasting (WRF)-large eddy simulation (LES) driven by observed land surface data. The influences of soil water content in oases on aloft CBL flow structure, stability, turbulent kinetic energy (TKE), and vertical fluxes are carefully examined through a group of sensitivity experiments. The results show that secondary circulation (SC)/turbulent organized structures (TOS) is the strongest/weakest when soil water content in oases is close to saturation (e.g., when the oases are irrigated). With the decrease of soil water content in oases (i.e., after irrigation), SC (TOS) becomes weak (strong) in the lower and middle CBL, the flux induced by SC and TOS becomes small (large), which has a dramatic impact on point measurement of eddy covariance (EC) fluxes. The flux induced by SC and TOS has little influence on EC sensible heat flux, but great influence on EC latent heat flux. Under this circumstance, the area averaged heat flux cannot be represented by point measurement of flux by the EC method, especially just after irrigation in oases. Comparison of imbalance ratio (i.e., contribution of SC and TOS to the total flux) reveals that increased soil moisture in oases leads to a larger imbalance ratio as well as enhanced surface heterogeneity. Moreover, we found that the soil layer configuration at different depths has a negligible impact on the CBL flux properties.     

Effect of Mesoscale Land Use Change on Characteristics of Convective Boundary Layer:Semi-Idealized Large Eddy Simulations over Northwest China

Although large-scale topography and land use have been properly considered in weather and climate models, the effect of mesoscale and microscale heterogeneous land use on convective boundary layer(CBL) has not been fully understood yet. In this study, the influence of semi-idealized strip-like patches of oases and deserts, which resemble irrigated land use in Northwest China, on the CBL characteristics, is investigated based on the Weather Research and Forecasting(WRF)-large eddy simulation(LES) driven by observed land surface data. The influences of soil water content in oases on aloft CBL flow structure, stability, turbulent kinetic energy(TKE), and vertical fluxes are carefully examined through a group of sensitivity experiments. The results show that secondary circulation(SC)/turbulent organized structures(TOS) is the strongest/weakest when soil water content in oases is close to saturation(e.g.,when the oases are irrigated). With the decrease of soil water content in oases(i.e., after irrigation), SC(TOS) becomes weak(strong) in the lower and middle CBL, the flux induced by SC and TOS becomes small(large), which has a dramatic impact on point measurement of eddy covariance(EC) fluxes. The flux induced by SC and TOS has little influence on EC sensible heat flux, but great influence on EC latent heat flux. Under this circumstance, the area averaged heat flux cannot be represented by point measurement of flux by the EC method, especially just after irrigation in oases. Comparison of imbalance ratio(i.e., contribution of SC and TOS to the total flux) reveals that increased soil moisture in oases leads to a larger imbalance ratio as well as enhanced surface heterogeneity. Moreover,we found that the soil layer configuration at different depths has a negligible impact on the CBL flux properties.     

纳木错湖夏季典型大气边界层特征的数值模拟

为了进一步检验分析纳木错湖对当地地方性环流、湖气能量交换及大气边界层的影响,在本文中使用美国NCAR新版MM5V3.7非静力中尺度模式,设计了有湖面、3/4湖面、1/2湖面及无湖四组试验,以NCEP再分析资料做初、边值条件,做了48 h三重嵌套模拟试验,对比中国科学院"纳木错圈层相互作用"综合观测站实测资料,说明该模式模拟性能良好。白天在纳木错湖风与念青唐古拉山北坡谷风共同作用下,在念青唐古拉山脊处汇合,形成一强水平切变及辐合上升运动,造成了该地夏季白天复杂多变的天气;夜间由于南岸湖风与山风叠加,使得整个区域为强大南风气流控制,这也补给了水汽和热量,也为白天不稳定运动提供了能量。对大气边界层特征的模拟结果表明:由于白天(夜晚)纳木错湖的存在有很好的降温(保温)作用,该湖表现出了明显的冷(暖)湖效应;纳木错湖对感热和潜热的影响有很强的日变化,白天湖面感热、潜热均小于周围陆地,夜间湖面有强潜热通量;纳木错湖使得白天湖区边界层顶低,陆区边界层顶高,夜间相反。这样的地方性环流和大气边界层特征的配合,是该地中小尺度天气剧烈变化的重要原因。     

夏季青海湖局地环流及大气边界层特征的数值模拟

使用美国NCAR新版MM5V3.6非静力模式,采用两重嵌套方法,模拟了青海湖区域的局地环流及大气边界层特征,并且与无湖试验进行了比较。结果表明:白天由于青海湖的存在有很好的降温作用,夜晚则有保温效应,表现出明显的冷(暖)湖效应;青海湖对感热和潜热的影响有很强的日变化,白天湖面感热、潜热都小,夜间情况相反,这使得白天青海湖是冷干岛,夜间是暖湿岛;青海湖使得白天湖面边界层顶低,陆面边界层顶高,夜间相反。这样的边界层顶高度和温度、地面能量通量相配合,形成了一个很好的保护机制,对青海湖的水土保持和生态环境的维持有正效应;青海湖使得湖面上空大气下沉,陆面上空大气上升,从而产生了湖面上空大气冷干,陆面上空大气暖湿的边界层特征;青海湖边缘的陆面形成的较大的湿气柱围绕着湖面,起到了保护湖面的作用;青海湖低空白天有明显的湖面向四周的辐散气流,而夜间则为从北偏东方向来的陆风。