Underestimation of Monostatic Sodar Measurements in Complex Terrain

Recent investigations in complex terrain have found that remote sensing instrumentation commonly finds mean wind-speed differences when compared to cup anemometery. In many cases the difference is found to be an underestimation and varies from 2 to 9% depending on topology. We describe these differences in a theoretical sense for a five-beam sodar. An investigation is conducted on a New Zealand ridge with a five-beam sodar and three computational models, consisting of a potential flow model and two computational fluid dynamical simulations, OpenFOAM and the industry standard software WindSim. All models predict the difference to within 0.1–2.5%. A comparative assessment is made and it is found that, given the computing overheads, the potential flow model provides a good compromise in the prediction of mean wind-speed difference.     

Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar

In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.     

Turbulence in Complex Terrain

Theoretical results and data are presented in order to describethe spectral modifications induced by the effects of surfaceinhomogeneity on the features of turbulent boundary-layer flows.The hill case is used by way of example, and atmospheric andlaboratory data in neutral conditions are discussed. Shear andblocking effects with different characteristics are shown to bepresent both upwind and over the hill top.     

Evolution of Cold-Air-Pooling Processes in Complex Terrain

Elucidating cold-air-pooling processes forms part of the longstanding problem of parametrizing the effects of complex terrain in larger-scale numerical models. The Weather Research and Forecasting model has been set-up and run at high resolution over an idealized alpine-valley domain with a width of order 10 km, to investigate the four-dimensional variation of key cold-air-pooling forcing mechanisms, under decoupled stable conditions. Results of the simulation indicated that the total average valley-atmosphere cooling is driven by a complex balance/interplay between radiation and dynamical effects. Three fairly distinct regimes in the evolution of cold-air-pooling processes have been identified. Starting about 1 h before sunset, there is an initial 30-min period when the downslope flows are initiated and the total average valley-atmosphere instantaneous cooling is dominated by radiative heat loss. A period of instability follows, when there is a competition between radiation and dynamical effects, lasting some 90 min. Finally, there is a gradual reduction of the contribution of radiative cooling from 75 to 37 %. The maximum cold-air-pool intensity corresponds to the time of minimum radiative cooling, within the period of instability. Although, once the flow is established, the valley atmosphere cools at broadly similar rates by radiation and dynamical effects, overall, radiation effects dominate the total average valley-atmosphere cooling. Some of the intricacies of the valley mixing have been revealed. There are places where the dynamics dominate the cooling and radiation effects are minor. Characteristics of internal gravity waves propagating away from the slopes are discussed.     

复杂地形区域之大气环境容量

复杂地形区域之风场及污染物的输送,受地形影响往往是非均匀的,这类地区大气环境容量的估算也有别于平坦地形区域。本文以粤东山区之梅州市工业居民混合区为例,介绍了复杂地形条件下大气环境容量的计算方法。主要包括:1.确定地形影响下近地面之主要流型.2.合理给出模式调整前容量的初估值,以尽量减少模式调整的工作量.3.建立非均匀风场条件下的环境容量调整模式.4.通过模式调整得出大气环境容量的准确值。     

复杂地形局地环流的数值模拟研究

采用一个三维中尺度动力学诊断模式,对重庆地区气象场进行了实例模拟,研究了复杂地形和不同下垫面型对流场的动力和热力作用,揭示了中尺度局地环流（山谷风、河陆风）的基本特征和变化规律,模拟结果与实测资料有好的一致性,表明该模式能够成功地模拟复杂地形局地环流。     

复杂地形对大理地区风场的影响研究——兼论观测、理论和数值模拟配合解答科学问题

由于观测资料不能覆盖研究所关注的高空区域,已有理论又无法解释所面对的问题,所以希望利用数值模拟来生产一套逼近真实的数据,以补充观测的欠缺。     

A Model to Estimate Global Radiation in Complex Terrain

Global radiation is an important parameter necessary for most ecological models. However, in situ data barely meets the needs of modelling mountainous ecosystems since most field stations are located in flat areas. Consequently, it is usually necessary to extrapolate radiation measurements obtained from an adjacent flat area to the complex terrain of concern. The distribution of radiation in complex terrain depends upon two factors: the local atmospheric conditions, which determine the radiation potentially available to a supposed flat surface in a given location, and the topographic effects on this possible radiation. The latter have been included in detail in most radiation models for complex terrain, but the former are often only simply treated as constant or estimated by over-simplified empirical algorithms. In this paper we propose a novel model that uses a parametric atmospheric model to calculate the potential radiation for a supposed flat surface in a given location, and then account for topographic effects. Direct radiation, diffuse radiation and reflected radiation are calculated separately in the model due to the distinctive characteristics of and the effects by topography. Based on the parametric model, this paper has investigated the relationship between radiation transmittance, clearness indices and altitude under a series of water vapour content and turbidity conditions. This combines three ratios, R _b, R _d, and R _r, defined as the direct radiation, diffuse radiation and reflected radiation received by the arbitrary surface, respectively, to their counterparts in the horizontal surface, to estimate the global radiation for any given location. The model has been validated with data from measurements in National Park Berchtesgaden, Germany, where six measurement sites with various altitudes and topographic characteristics have been deployed. The r ² of modelled and measured hourly global radiation are greater than 0.90 in all six sites, with RMSE varies from 16 to 100 W m⁻². Sensitivity analysis revealed that the model was not sensitive to change in water vapour content, which suggests the possibility to use an exponential algorithm of water vapour content when there is no in situ water vapour content information in complex terrains. The NRMSE was only reduced by 0.04, on average, in five of the six sites when water vapour content information was calculated from the in situ air temperature and relative humidity measurements.     

复杂地形下山谷风的数值模拟

利用中尺度数值模式MM5,选用NCEP全球客观分析资料(1°×1°)和国内高空地面资料为初始场,采用张驰逼近边界条件和单一冰相水汽方案,模拟了河南省西部山区的复杂地形对2006年6月19日局地环流的影响,结果表明:太行山的南麓和熊耳山的东侧都有因地形热力作用而形成的山谷风存在。     

复杂地形下山谷风的数值模拟

利用中尺度数值模式MM5,选用NCEP全球客观分析资料(1°×1°)和国内高空地面资料为初始场,采用张驰逼近边界条件和单一冰相水汽方案,模拟了河南省西部山区的复杂地形对2006年6月19日局地环流的影响,结果表明:太行山的南麓和熊耳山的东侧都有因地形热力作用而形成的山谷风存在.     

A Multiple-Cell Flat-Level Model for Atmospheric Tracer Dispersion over Complex Terrain

A multiple-cell flat-level tracer dispersion model is developed for atmospheric pollution study. The horizontal domain may be constructed with multiple-sized cells for varied resolution. The sequence of cells is arbitrary, as in unstructured grids, as long as no holes are left in the horizontal domain, which may be tailored in shape according to local orography. The vertical levels are truly flat and the level spacing may vary from level to level. The surface orography is included by removing cells from the bottom of the three-dimensional cell block. The arrangement of wind velocity and tracer concentration is similar to the Arakawa C grid. Advection and horizontal diffusion are formulated on each cell face, using tracer concentrations in the two cells that share the cell face. Pointer-orientated numerical loops are used to facilitate the arbitrary horizontal cell arrangement and orographic variation of vertical levels. A second-order upstream upper limiter advection scheme is developed for this model and numerically tested to be positive-definite and mass conserving. Vertical diffusion is solved with an implicit scheme and simplified vertical diffusivity, which is parameterised as a function of the mixing layer depth. The model is fast, compact, easy to implement and highly portable. It is suitable for studies ofmesoscale and small-scale atmospheric tracer dispersion over complex terrain, including steep slopes. The model is used to simulate traffic pollution in London, UK, and is compared with available observations.     

复杂地形背景下双偏振雷达数据质量评估方法

针对四川省复杂地形和雷达型号多样的特征,设计了一种以长时间序列统计单/双偏振天气雷达数据质量的评估方法,算法定性评估了天气雷达数据是否存在空回波、电磁干扰和地物干扰的问题,定量评估了雷达Zh(水平极化反射率因子)、Z_(dr)(差分反射率)、CC(相关系数)、Фdp(差分相移)和G_(C)(地物回波系数)的数据质量。通过评估2022年5—9月四川省12部新一代天气雷达基数据的数据质量,得到了如下结果:(1)同频电磁干扰严重影响四川省天气雷达数据质量,超过74%的宜宾和广元站数据存在电磁干扰回波;宜宾和乐山雷达站出现了较多空回波数据;超过77.91%的广元雷达站数据存在风电场地物回波干扰。(2)雷达性能正常时,地物回波系数G_(C)的均值变化不大,G_(C)系数的标准差可以有效反应出雷达数据问题,特别对空回波数据问题比较敏感。(3)四川省12部天气雷达Zh标准差总体较小,绵阳和宜宾站较优,乐山站较差。(4)四川省3部双偏振天气雷达的Z_(dr)和CC数据质量较好,?dp的数据波动较大。     

Optimization of Upstream Profiles in Modelled Flow Over Complex Terrain

This paper describes an adjoint method for data assimilation intoupstream boundary conditions of numerical modelsusing optimal control theory. Mathematical formalisms are given along with the numerical implementation of the schemein a column model of the atmospheric boundary layer. The optimized mean and turbulence profiles are used as an upstream solutionin a model of turbulent flow in complex terrain. To contrast thiswith other methods, two solutions for flow over an isolatedhill are calculated, one with an optimized upstream solution andone with a simple surface-layer formulation for the upstream solution.These two solutions are compared to observations and analytical theory. The adjoint optimization method is shown to producesolutions of flow in complex terrain that are substantively differentat the two solutions, with the optimized solution giving more accurate results.     

基于云分析的数值模拟在复杂地形中的应用试验

利用区域中尺度模式ARPS(Advanced Regional Prediction System)模拟了华北西部复杂地形条件下发生在2010年9月18-22日的降水过程,并针对云分析方法对降水和温度预报的影响开展了敏感性试验.结果表明:①模式在复杂地形下有很好的适用性.24 h和48 h模拟结果,降水的起止时间、落区以及强降水中心的位置均和实况一致.24 h模拟降水的量级和实况接近,48 h的比实况偏大;②模式能准确模拟温度的变化,插值得到的5天内6个台站温度预报的平均绝对误差只有2.69℃;③云分析能显著提高模式初值的质量,特别是其中水物质的含量,从而可显著改善48 h强降水中心降水量偏大的状况,使模拟的降水场更接近于实况.但云分析对温度模拟结果的影响不明显.     

Wind Flow Over Complex Terrain: A Historical Perspective and the Prospect for Large-Eddy Modelling

The development of our understanding of turbulent flow overcomplex terrain is described. Despite considerable advances, there are still gaps in our knowledge. Large-eddy simulation of such flows is now becoming possible. This may stimulate further advances but it is computationally very demanding. The application of this technique to flow over hills is discussed.     

Impact of the Complex Terrain in Beijing on Formation of Low-Level Jets

This study investigated how the Taihang Mountains and the Yanshan Mountains affect low-level jets (LLJs) in the Beijing area, based on conventional radiosonde observations from Nanjiao Observatory (2016–2017) and high-resolution Weather Research and Forecasting–Advanced Research WRF (WRF-ARW) model simulations. Analysis of radiosonde observations indicated that LLJs in the study area are mainly from the southwest and northwest directions,with occurrence frequency of 44.6%and 33.0%, respectively....     

A numerical investigation of mechanisms affecting drainage flows in highly Complex Terrain

Summary Numerical simulations of increasing complexity are conducted to investigate topographic controls and ambient wind effects upon drainage flows along a portion of the Colorado Front Range in the central Rocky Mountains. A series of two-dimensional simulations show the effects upon the drainage flow of changing slope gradient at the mountain-plain interface. For a given mountain slope, a decrease in the slope of the plain decelerates the mountain drainage jet as it approaches the plain and causes the jet to elevate. The integrated effects of slope and valley drainage are presented with particle trajectories for a particular drainage basin along the Front Range. A nested grid simulation of drainage flow from multiple basins along the Front Range shows that basin area is an important factor in the strength of the drainage flow and that canyon topography variations greatly affect the behavior of the drainage jet as it flows through the canyon mouth onto the plain. Strong drainage winds developed on each of four case night simulations due to the presence of only weak ambient wind below mountaintop. The weak winds represent a decoupling of the near-surface from stronger winds above mountaintop. The canyon drainage exhibited substantial temporal variability in wind speed with the inclusion of ambient winds, due to interactions between ambient and drainage winds.With 11 Figures     

Wind Climate Simulation over Complex Terrain and Wind Turbine Energy Output Estimation

Summary A statistical-dynamical downscaling procedure is applied to investigate the climatological wind field over a complex terrain area in central Germany. The model domain, 80×87 km, is dominated by flat terrain in the westerly and northerly part and encompasses the Teuteburger Wald and the Wiehengebirge areas with hills up to 330 m a.m.s.l. in the southeasterly region. The downscaling procedure combines a large-scale regionally representative wind statistic and a high-resolution numerical atmospheric mesoscale model. A cluster analysis of a 12-years time series of radiosonde data provides 143 clusters each being a combination of the geostrophic wind components and the vertical temperature gradient. These parameter sets constitute the reference state for highly-resolved steady-state wind field simulations with a non-hydrostatic model. Weighting the resulting wind fields with the corresponding cluster frequency gives climatologically representative frequency distributions of the wind speed and -direction. By combining the wind speed frequency distribution with the power curve of wind turbines the yearly energy output of 46 wind turbines inside the simulation domain was calculated and compared to the actual production. No bias or systematic trend in the deviation was found. The relative differences for the smallest turbines reach 100 percent with a decreasing tendency to larger units. Received November 13, 1998