四川地形扰动对降水分布影响

引入一维加权平均的谱分析方法定量研究四川地形强迫对该区域降水分布的影响。结果表明:纬向地形和冬季降水谱峰锁相于同一波长（475.8 km）,呈共振关系,地形与其他季节降水呈漂移关系,这与经向和纬向上环流变动有关,即冬季纬向环流占主导,纬向地形触发的大气波动对冬季降水策动作用大;夏季降水是各种不同尺度系统相互作用的结果,地形是重要因素之一。经向和纬向地形特征尺度分别为296.8 km和475.8 km,反映了地形强迫的中尺度特征,且纬向地形谱峰比经向大1个数量级,纬向强迫更明显。夏季降水谱峰比冬季大2个数量级,降水系统纬向特征尺度比冬季小约150 km,说明夏季在纬向地形强迫下,降水系统尺度减小的同时其强度大大增加,这在一定程度上可以解释中尺度对流性降水在夏季偏多。四川夏季最大降水位于雅安地区,其地形扰动比四川整体扰动更明显,故产生的降水也更大。夏季降水和经向地形锁相于同一波长（37.1 km）,经向地形对雅安夏季强降水起关键作用。     

四川地形谱特征及中尺度模式水平网格分辨率选取

四川地形复杂多样,暴雨频发,常诱发山洪、泥石流等灾害。在利用中尺度模式对复杂地形区域的暴雨进行研究时,模式水平分辨率的选取缺乏定量依据。为了揭示四川地形的复杂特征和给中尺度模式水平网格分辨率的选取提供定量依据,利用二维离散余弦变换,对四川地形高度场和暴雨分布场进行谱分解,根据暴雨分布特征分区讨论了四川盆地地形特征,同时利用地形谱方差和数值试验定量讨论了数值模式水平分辨率的选取问题,得到主要结论有:(1)二维离散余弦变换能较好地表现出研究区域各向异性的复杂特征;(2)雅安地区和四川盆地西北部的地形谱与降水谱有较好的同相关系,盆地东北部和盆地中部的地形谱与降水谱在波长较大处出现反相关系;(3)针对某个区域的地形特征,可以通过计算模式能分辨的地形方差与总地形方差的比值来确定合适的中尺度模式水平网格分辨率。     

阳江复杂地形对特大暴雨影响机理的数值研究

采用中尺度天气预报模式WRF (Weather Research Forecast)对广东省阳江地区2019年5月26日00时—28日00时暴雨事件进行数值模拟,并通过一系列的地形敏感试验,讨论地形对暴雨发生和发展的影响。结果表明:地形对暴雨的影响显著,地形的高度影响暴雨的强度和位置;与未作任何改变的控制试验相比,地形降低试验和细网格地形高度取平均试验无山脉的阻挡,阳江地区无爬流和绕流运动及相对涡度减小,南风将低层水汽和能量带到更北的位置,暴雨中心随之北抬;增高地形试验,由于山脉阻挡,更多水汽和能量堆积,以及地形的阻挡产生绕流和爬流运动,绕流有利于局地涡旋生成,爬流运动会增加垂直运动速度,正涡度中心增强,低层的水汽辐合上升凝结,造成更大暴雨。      

Horizontal variability and boundary-layer modeling

Micrometeorologists have traditionally set aside consideration of horizontal variability and have studied boundary-layer structure with horizontal homogeneity. The numerical forecasting of boundary-layer structures, over normally varying terrain and including normal disturbances such as fronts, requires selection of an appropriate horizontal scale.A simple analysis of steady-state balance between horizontal advection and vertical diffusion provides estimates of the vertical scale (or depth) of surface-induced features. The scale height is a function of the horizontal scale of the variations. Models neglecting important terrain scales of length below ~ 1000 km can predict down to levels of ~ 0.5 to 1 km while those that neglect important terrain scales below ~ 100 km can predict down to ~ 0.2 to 0.6 km. Below these levels, any predicted features will be dominated by the vertical diffusion so that they are solutions of a one-dimensional boundary-value problem.The boundary-induced advection effects dominate free atmosphere advection effects in the lowest few hundred meters as well. This means that if mesoscale advections are resolved and terrain influences are strong, the predictions in the layer ~ 0.2 to 0.8 km can provide mesoscale detail without mesoscale initial conditions above the surface, because the surface forcing will dominate the solution.     

Comparison of Measured and Numerically Simulated Turbulence Statistics in a Convective Boundary Layer Over Complex Terrain

The Weather Research and Forecasting (WRF) model can be used to simulate atmospheric processes ranging from quasi-global to tens of m in scale. Here we employ large-eddy simulation (LES) using the WRF model, with the LES-domain nested within a mesoscale WRF model domain with grid spacing decreasing from 12.15 km (mesoscale) to 0.03 km (LES). We simulate real-world conditions in the convective planetary boundary layer over an area of complex terrain. The WRF-LES model results are evaluated against observations collected during the US Department of Energy-supported Columbia Basin Wind Energy Study. Comparison of the first- and second-order moments, turbulence spectrum, and probability density function of wind speed shows good agreement between the simulations and observations. One key result is to demonstrate that a systematic methodology needs to be applied to select the grid spacing and refinement ratio used between domains, to avoid having a grid resolution that falls in the grey zone and to minimize artefacts in the WRF-LES model solutions. Furthermore, the WRF-LES model variables show large variability in space and time caused by the complex topography in the LES domain. Analyses of WRF-LES model results show that the flow structures, such as roll vortices and convective cells, vary depending on both the location and time of day as well as the distance from the inflow boundaries.     

1997年7月19日影响北京地区的暴雨个例分析

针对在实际业务预报中经常遇到的北京地区降水的两种特殊性(明显天气系统移来时，北京地区降水明显比周围偏多或偏少)，选取一个实例利用中尺度非静力模式(MM5)对降水比周围偏多情况进行了高分辨率数值模拟和敏感性试验。结果表明，该模式比较成功地模拟了此次天气过程及其相关的中尺度系统的发生发展，凝结潜热对天气过程的发生发展有重要作用，而地形在天气过程中也起着主要作用，低层潜在不稳定能量的储备和输送是暴雨发生不可缺少的条件。     

The stably stratified boundary layer over the great plains

Airplane measurements of the stably stratified boundary layer obtained during the Severe Environmental Storms and Mesoscale Experiment (SESAME) over rolling terrain in south-central Oklahoma indicate that considerable horizontal variability exists in the flow on scales of several kilometers. Much of this wave-like structure appears to be tied to the terrain. The criteria for existence of stationary gravity waves indicate that these waves can exist under the observed conditions. The spectrum of terrain variations also supports the existence of these waves. Observed spectra of the vertical velocity have two peaks: one at wavelengths of several kilometers, which is due to waves and the other at wavelengths of about 100 m, which is due to turbulence. The variance at several kilometers wavelength increases somewhat with height at least up to about 800 m, but the variance contributed by turbulence decreases rapidly with height.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

地形对华南飑线升尺度影响机制的数值模拟研究

本文利用NCEP/NCAR提供的1°×1°的再分析资料,应用WRF4.0中尺度数值模式对2016年4月13日华南地区的一次飑线升尺度过程进行模拟,并设计一系列的敏感性试验,详细研究了南岭对飑线升尺度增长的影响以及可能的机制。结果表明:WRF模式较好的模拟了本次飑线过山前后的变化以及其降水的分布。强对流在过山后比过山前发展要强烈,水平的尺度增长快。但不同高度的地形敏感性试验表明,适宜的地形高度对于风暴的发展更有利。地形影响了飑线的尺度和组织,地形过高会使得广东北部的对流分散。地形可以通过改变水平流场、水汽场、垂直运动以及低层的垂直风切变等来间接影响飑线中的对流单体的分布和对流单体的强度。无地形阻挡时,有利于急流的北进,水汽输送更为有利。但是,一定的地形高度对低层的垂直运动是有利的。地形较高,则会利于高层的垂直运动,低层更多的可能以绕流为主。当地形超过一定高度时,低层的辐合场也相应的减弱。     

Observations of complex-terrain flows using acoustic sounders: Experiments,topography, and winds

Acoustic sounders have now been used extensively in a series of noctural drainage flow experiments carried out by the U.S. Department of Energy's Atmospheric Studies in Complex Terrain (ASCOT) program. Doppler acoustic sounders, located in three different valleys during the sequence of experiments, reveal drainage-wind profiles that depend strongly on the ambient meteorological conditions and the elevation of each observing site relative to surrounding terrain. In elevated sites that drain easily, Doppler-sounder derived wind profiles show a simply-structured flow; in lower lying areas, subject to topographic constriction and cold-air pooling, and where Archimedean forces are comparable to those due to synoptic and mesoscale pressure gradients, the wind profiles show considerable vertical and temporal variation. In particular, in the Geysers area of northern California, the seabreeze and the depth of the Pacific Coast marine inversion affect not only the initiation of drainage winds but also their subsequent evolution.     

Investigation of the planetary boundary layer height variations over complex terrain

The effects of sea-breeze interactions with synoptic forcing on the PBL height over complex terrain are investigated through the use of a 3-D mesoscale numerical model. Two of the results are as follows. First, steep PBL height gradients—order of 1500 m over a grid interval of 10 km — are associated with the sea-breeze front and are enhanced by the topography. Second, a significant horizontal shift in the maximum PBL height relative to the mountains, is induced by a corresponding displacement of the thermal ridge due to the mountains, in the presence of large scale flow.     

北京地区降水的特殊性及其预报方法

实际业务预报中，北京地区降水经常发生两种特殊情况，即明显系统移来时北京地区降水明显比周围偏多或偏少。对此，选取两个实例，利用中尺度非静力模式(MM5)进行了高分辨率数值模拟和敏感性试验。结果显示，该模式较成功地模拟出了这两次天气过程及其相关的中尺度系统的发生发展，还显示，凝结潜热对迎风坡天气过程的发生发展产生了重要作用，地表特征对背风坡天气过程产生了重要影响，而地形在两次天气过程中都起着主要作用，低层潜在不稳定能量的储备和输送是暴雨发生不可缺少的条件。     

Effects of land-surface heterogeneity on numerical simulations of mesoscale atmospheric boundary layer processes

Landscape heterogeneity that causes surface flux variability plays a very important role in triggering mesoscale atmospheric circulations and convective weather processes. In most mesoscale numerical models, however, subgrid-scale heterogeneity is somewhat smoothed or not adequately accounted for, leading to artificial changes in heterogeneity patterns (e.g., patterns of land cover, land use, terrain, and soil types and soil moisture). At the domain-wide scale, the combination of losses in subgrid-scale heterogeneity from many adjacent grids may artificially produce larger-scale, more homogeneous landscapes. Therefore, increased grid spacing in models may result in increased losses in landscape heterogeneity. Using the Weather Research and Forecasting model in this paper, we design a number of experiments to examine the effects of such artificial changes in heterogeneity patterns on numerical simulations of surface flux exchanges, near-surface meteorological fields, atmospheric planetary boundary layer (PBL) processes, mesoscale circulations, and mesoscale fluxes. Our results indicate that the increased heterogeneity losses in the model lead to substantial, nonlinear changes in temporal evaluations and spatial patterns of PBL dynamic and thermodynamic processes. The decreased heterogeneity favor developments of more organized mesoscale circulations, leading to enhanced mesoscale fluxes and, in turn, the vertical transport of heat and moisture. This effect is more pronounced in the areas with greater surface heterogeneity. Since more homogeneous land-surface characteristics are created in regional models with greater surface grid scales, these artificial mesoscale fluxes may have significant impacts on simulations of larger-scale atmospheric processes.     

Comparison of the DRAIS and EURAD model simulations of air pollution in a mesoscale area

Summary In this paper the results of simulations of air pollution carried out with the mesoscale model system KAMM/DRAIS are presented. They are compared with results of the European scale model EURAD which have been provided by the EURAD-Group, Cologne. With this comparison it is intended to analyse to what extent better resolution of topography and emission data used by the mesoscale model effects the model results. The simulations have been carried out for July 15, 1986, a typical summer day. The model domain contains south-west Germany and part of Alsace with a resolution of 5 km. The emissions for this resolution have been derived by a combination of the coarse EURAD emission data with the data of the TULLA experiment which are available on a much finer grid. The initial and boundary conditions for the species concentrations are determined from the results of the EURAD model. This coupling introduces the long range transport of pollutants into the mesoscale simulation.The meteorological and concentration data of the EURAD model are compared with the corresponding DRAIS model results. The mesoscale flow field is characterized by the channeling along the Upper Rhine Valley, which is not resolved in the EURAD model. The concentration distributions of both models are similar during midday, because of the strong vertical mixing. In the night and especially, in the morning and evening hours the spatial distribution is much better represented by the DRAIS model results. The better resolution of the emissions and the topography in the DRAIS model compared with the EURAD model (80 km grid size) becomes really noticeable. The difference of the ozone concentrations between cities and the, surrounding areas and between the Rhine Valley and the limiting mountains are in the order of 30 ppb as compared to a few ppb in the EURAD simulation. In the morning NO concentrations of about 200 ppb are simulated in the area between Heilbronn and Stuttgart. The EURAD model provides only about 5 ppb. Comparisons with measurements show that the DRAIS simulations are more realistic than the EURAD model results. The features mentioned are also found in an evaluation of the concentration variations in areas corresponding to a grid cell of the EURAD model. Two completely different areas are selected to demonstrate the possible range of the concentration variation. In the area around the City of Stuttgart the ozone concentration in the morning and the evening varies between zero ppb and 50 ppb, approximately. The mean value is nearly the same in both simulations.List of Abbreviations DRAIS Dreidimensionales Regionales Ausbreitungsund Immissions-Simulationsmodell - EMEP European co-operative program for Monitoring and Evaluation of the long-range transmission of air Pollutants - EUMAC European Modelling of Atmospheric Constituents - EURAD European Acid Deposition Model - EUROTRAC EUROpean experiment on TRAnsport and transformation of environmentally relevant trace Constituents in the troposphere over Europe - JDR Joint Dry Case - KAMM Karlsruher Atmosphärisches Mesoskaliges Modell - MM4 Mesoscale Model 4 - NCAR National Center for Atmospheric Research - RADM Regional Acid Deposition Model - TADAP Transport and Deposition of Acidifying Pollutants - TULLA Transport und Umwandlung von Luftschadstoffen im Lande Baden-Württemberg und aus Anrainerstaaten; in English: Transport and Transformation of Air Pollutants in the State of Baden-Württemberg and from neighbouring countries With 17 Figures     

低层偏东气流对贵州梵净山东侧强降水的作用

利用常规观测资料、地面加密自动站资料、雷达探测资料与NCEP 1°×1°再分析资料等,对低层偏东气流影响下贵州铜仁梵净山东侧4次强降水天气过程进行了分析,重点探讨了在低层偏东气流与地形共同作用下的强降水形成机制,并归纳低层偏东气流影响下的梵净山东侧强降水概念模型。结果表明:(1)高空槽、低层切变线、地面中尺度辐合线是影响梵净山东侧强降水的主要天气系统;(2)低层浅薄偏东气流对梵净山东侧强降水起着关键作用,当低空气流u分量随高度减小时,地形迎风坡气流辐合上升,而气流v分量随高度增加时,地形迎风坡会产生与山脉垂直的水平涡管,在地形抬升作用下涡管向上凸起形成两个涡管环流圈,涡度垂直分量使山脚附近上升气流加强而有利于山脚产生强降水;(3)梵净山东侧强降水区的形成存在三种机制,即迎风坡山脚多次触发对流形成雨量叠加效应、地面中尺度辐合线自身触发组织对流、回波沿地面中尺度辐合线东移形成“列车效应”,三种机制产生的降水带与地面中尺度辐合线走向一致。     

修正WRF次网格地形方案及其对风速模拟的影响

复杂地形区域风场模拟的准确率一直是风能研究领域的难点和重点。WRF模式是目前风能评估领域应用最广泛的天气数值模式之一,但该模式在复杂地形区域存在对平原、山谷风速高估且对山顶风速低估的系统性误差,并有研究建立次网格地形方案以订正误差。而次网格地形方案在不同水平分辨率下常出现错误的修正结果,该文基于高精度地形高程数据分析了方案失效的主要原因,发现其方程组中判断山体形态特征的阈值-20在过低和过高水平分辨率下均失去参考性。针对这一原因,将方案中影响关键参数C_t的地形高度算子与模式水平分辨率进行拟合,形成地形高度算子与水平分辨率相依赖的线性关系,获得不同分辨率下更适合的山体形态阈值。通过与自动气象站10 m风速对比分析了修正前后WRF对低层风速的模拟效果,结果显示:修正后的次网格地形方案能够分别在较低和较高分辨率下,部分矫正原方案错误的订正结果,使低层风速模拟更接近实况。修正后的次网格地形方案可为复杂地形区域开展高分辨率风场模拟提供参考。     

垂直变量配置对静力适应过程的影响

为了研究垂直变量配置对静力适应过程的影响,本文从描写静力适应过程的方程组出发,分别在将所有变量置于整层上的非跳点N网格;将垂直速度和温度放置在整层,水平速度、气压和密度等变量放置在半层的Charney Phillips跳点网格(CP网格);将水平速度、气压和温度放置在整层,将垂直速度和密度放置在半层的Lorenz跳点网格(L网格);将密度变量放置在整层的Charney Phillips跳点网格(CP_N网格);将密度放置在整层的Lorenz跳点网格(L_N网格)上进行离散,垂直格距分1 km、0.5 km、0.2 km和0.01 km,研究了在这5种网格上产生的频率和垂直群速的相对误差。结果表明:(1)L_N网格和CP网格是完全等效的两种网格。(2)不论垂直格距为多少,CP网格和L网格的误差都是最小,N网格次之,CP_N网格的误差最大。(3)随着垂直格距的减少,在这几种网格上产生的误差都在减小。对于CP网格、L网格和N网格,在水平长波和垂直短波处产生的误差较大。而CP_N网格对水平波长变化不敏感,垂直波长越短,误差越大。(4)当垂直格距为0.01 km时,这几种网格都对水平波长的变化不敏感了,仅对垂直波长敏感。(5)CP网格、L_N网格和L网格在描写静力适应过程和斜压地转适应过程都是误差最小的垂直变量配置方案,因此在非静力完全可压缩深层大气数值预报模式中应优先选择这3种方案。     

Large-Eddy Simulations of Atmospheric Flows Over Complex Terrain Using the Immersed-Boundary Method in the Weather Research and Forecasting Model

Atmospheric flow over complex terrain, particularly recirculation flows, greatly influences wind-turbine siting, forest-fire behaviour, and trace-gas and pollutant dispersion. However, there is a large uncertainty in the simulation of flow over complex topography, which is attributable to the type of turbulence model, the subgrid-scale (SGS) turbulence parametrization, terrain-following coordinates, and numerical errors in finite-difference methods. Here, we upgrade the large-eddy simulation module within the Weather Research and Forecasting model by incorporating the immersed-boundary method into the module to improve simulations of the flow and recirculation over complex terrain. Simulations over the Bolund Hill indicate improved mean absolute speed-up errors with respect to previous studies, as well an improved simulation of the recirculation zone behind the escarpment of the hill. With regard to the SGS parametrization, the Lagrangian-averaged scale-dependent Smagorinsky model performs better than the classic Smagorinsky model in reproducing both velocity and turbulent kinetic energy. A finer grid resolution also improves the strength of the recirculation in flow simulations, with a higher horizontal grid resolution improving simulations just behind the escarpment, and a higher vertical grid resolution improving results on the lee side of the hill. Our modelling approach has broad applications for the simulation of atmospheric flows over complex topography.     

A numerical study of second-order turbulent moments in the stably stratified nocturnal boundary layer

The structures and the vertical profiles of turbulent variance and covariance of the stably stratified boundary layer (SBL) are simulated with a second-order closure turbulence model. The results confirm that the vertical profiles of the dimensionless turbulence variance and covariance can be well represented by the form F = A(1 - Z / h)x. Here h is the height of SBL. and both exponent a and coefficient A are the functions of terrain, baroclinicity, radiation cooling and the state of temporal development of SBL. Comparing with Minnesota and Cabauw experiment data, we have analysed the value of a and expounded the main reasons that great difference in a exists among different literatures.     

Role of convective parameterization in simulations of heavy precipitation systems at grey-zone resolutions — case studies

We have investigated the role of convective parameterization in simulations of heavy precipitation systems at grey-zone (2–10 km) resolutions using an approach similar to that used in “observing system simulation experiment”. Simulations with a 1-km grid serve as benchmark simulations. The impacts of convective parameterization at greyzone resolutions (i.e., 3, 6, and 9 km) are then investigated. This study considers two heavy precipitation systems including one associated with a mesoscale cyclone generated over the Shandong Peninsula on 24–25 July 1991, and the other associated with a cloud cluster occurred on 15–16 July 2009. The present study indicates that convective parameterization does not affect much the simulations of the two heavy precipitation systems with 3-km grid size. However, it significantly affects simulations for grid sizes of 6 and 9 km. Simulations with the Kain-Fritsch scheme produce deficiencies such as relatively small heavy rainfall area, smaller maximum precipitation rate, wider area of weak precipitation, etc. Simulations without convective parameterization have also some negative effects such as the overprediction of area-averaged precipitation rate and others. A modified trigger function in the Kain-Fritsch scheme is found to improve the simulations of the heavy precipitation systems over the Korean Peninsula by reducing excessive trigger of convection, especially for simulations with 6- and 9- km grids.