稳定行星边界层相似性函数A,B,C的实验研究

本文使用位于粗糙和不均匀地形上北京320米气象塔夜间边界层风速、风向和温度资料,用行星边界层u分量第一个极值高度作为行星边界层高度尺度,用边界层内的平均风速和平均位温作为速度和位温特征尺度,分析得到在稳定条件下相似性函数A_m、B_m和C_m以及行星边界层高度的参数化表达式。 分析表明,可以将广义相似性理论推广到粗糙而不均匀地形上的大气边界层中。     

城市建筑物动力冠层方案的引入及应用研究

文中将建筑物动力冠层方案引入到城市边界层精细模式中,该方案描叙了城市地Ⅸ建筑物对气流的拖曳作用以及建筑物形态对城市地区湍流活动动量输送的影响.采用建筑物拖曳法和建筑物动力冠层方案两种不同建筑物动力学处理方法对北京地区东南部进行模拟,通过城市地区水平风速的模拟结果与自动气象站实测资料对比发现模拟结果与实测相当吻合,建筑物动力冠层方案的引入能够更好地实施对城市地区的水平风速分布的数值模拟.分别采用建筑物拖曳法与建筑物动力冠层方案对一个实际小规模城市进行模拟试验,分析表明引入建筑物动力冠层方案可以模拟出小规模城市地区水平风速偏低的现象;还可以模拟出城市地区建筑物动力作用对湍流活动的影响;对湍流动能模拟结果表明比较符合实际分布情形.采用建筑物动力冠层方案对小规模城市地区建筑物高度变化对城市地区的流场及湍流活动的影响进行模拟分析,结果表明:城市地区建筑物高度增加,风速变小,但是高大建筑物底部风速略高于低矮建筑物底部;城市地区大气湍能增加,高大建筑物底部湍能较小,冠层高度内湍能随高度增加而增加;同时城市地Ⅸ的垂直扩散系数也随建筑物高度增加而增加;城市地区污染物排放高度处污染物浓度较低,下游乡村地区地面污染物浓度较低,但高空污染物浓度却较高.     

建筑群对城市冠层动力、热力效应的数值试验研究

利用中尺度模式MM5,分别耦合传统的边界层参数化方案和城市冠层参数化方案,模拟了2001年冬季北京冠层大气的动力、热力特征,并和观测资料进行了对比分析.分析结果表明,城市冠层参数化方案更能细致描述建筑群对城市冠层大气的动力、热力作用.由于建筑群的动力、热力作用,使北京城区的温度升高,风速减小,湍流动能增强,形成了明显的增温区、阻风区和湍能增强区,中心均位于城区高大建筑群附近,并向郊区递减.     

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.     

WRF模式对福建沿海风电场风速预测的效果分析

在WRF模式中选取不同的边界层、近地面层以及陆面过程参数化方案,设计了4种不同物理过程参数化方案,组合模拟福建沿海某测风塔站2010年1月1—11日和7月1—11日的逐时风速,将数值模拟结果和同期测风塔观测数据进行对比,以寻求最佳参数化方案。经分析比较,采用MYJ边界层方案,Monin-Obukhov近地面层方案以及Noah陆面过程方案的方案2模拟效果最好。使用该方案对2010年1月和7月的风速进行模拟,按不同风速级别分别对数值模拟结果进行对比分析,结果表明:方案2对6~15m/s风速模拟的平均相对误差在20%左右,能够满足风电预测的精度需求;而对0~6m/s风速模拟的误差相对较大,这可能是由于模式地形分辨率不够精细以及风塔所处海陆交界处的特殊位置,使低风速容易受地面扰动以及海陆气流影响所致。     

WRF耦合城市冠层模式对珠三角城市群天气模拟影响的评估

基于是否耦合有城市冠层模式的3个对比试验(无城市冠层模式:W-NUR;单层冠层模式:W-UCM;多层冠层模式:W-BEP),应用WRF模式模拟了珠江三角洲(简称珠三角)地区发生于2011年6月21日午后的一次强降水过程,发现W-UCM、W-BEP试验模拟结果均优于W-NUR试验,但降水落区与实际观测相比仍有差异。为此应用前期10天(2011年6月11—20日)的模拟结果,通过对珠三角城市群6个站点基本气象要素模拟效果的评估,重点考察了W-NUR及W-UCM试验结果的差异,并对模拟降水存在差异形成的原因进行分析。评估结果表明:无论是W-NUR还是W-UCM,模拟的10 m高度风速普遍偏强,其中W-NUR模拟的平均风速与观测对比偏强1.61 m/s,W-UCM偏强1.58 m/s;W-NUR模拟的2 m高度温度及温度露点差均较观测偏低,温度平均偏差为-1.28 ℃,温度露点差平均偏差达-1.39 ℃,而W-UCM模拟的温度及温度露点差较观测偏高,温度平均偏差略高0.14 ℃,温度露点差平均偏差1.12 ℃。平均偏差及均方根误差分析反映出,温度模拟的误差最小,其次为温度露点差,风速误差最大,而且模拟温度和温度露点差与观测相关性更好,相关系数分别达到了0.60和0.50以上,通过0.001显著性水平检验,而风速的相关性则相对较弱。总体来看,尽管耦合冠层模式后WRF对地面气象要素的预报有所改善,但模拟的10 m高度风速仍然偏强。就21日强降水过程的模拟来说,由于偏北风偏强造成了切变线南压,可能是模拟降水落区偏南的一个原因。      

南京复杂下垫面条件下的三维城市热环境模拟

运用WRF模式,选取考虑城市冠层结构(UCM算例)及不考虑城市冠层(NOUCM算例)两种城市下垫面参数化方案,对南京2010年夏季晴天小风典型天气条件下的城市热环境以及不同下垫面的边界层特征进行了模拟研究。结果表明:1)UCM方案模拟结果与实际情况较为吻合。其中2 m气温的模拟有较大的改进,模拟结果明显高于NOUCM方案,与观测更为吻合,同时更好地模拟出了冠层建筑物对于近地层风速的拖曳,10 m风速的模拟有非常明显地提高。2)UCM方案较好地模拟出了城市的三维热岛分布。由于建筑物地表对辐射的截留,白天14时(北京时间,下同)热岛较强,地面2 m高度处热岛范围较大,热岛面积大约为120 km2,强度为2℃。同时建筑物的存在使得城市湍流动能更大,向上的垂直扩散增加,距地面20 m时,依然能看出明显的热岛效应,热岛强度为1.5℃。距地面55 m处,UCM模拟所得的热岛范围缩小,热岛强度为1.1℃。UCM模拟所得的白天地表热量的扩散影响可达143 m,02:00 2 m处热岛最强为2℃,热岛影响也可达70 m以上。3)不同下垫面呈现出了不同的边界层特征,城市冠层结构对周边下垫面边界层结构存在程度不等的影响,14:00城市区域的湍流混合更强,城市边界层高度升高100 m左右,02:00,城市冠层结构的存在,导致近郊庄稼下垫面及紫金山植被下垫面的稳定逆温层结明显减弱。     

二阶闭合模式在污染气象学中的某些应用

本文采用二阶闭合的湍流边界层模式,进行一系列数值试验以模拟边界层中连续线源的扩散状况。试验表明:无论在稳定的或不稳定的边界层中,高源的扩散能力都低于低源;在稳定层中,粗糙地表上的大气扩散能力高于光滑表面;在相同风速和地表净辐射情况下,粗糙表面上的大气扩散能力反而低于光滑表面;对流边界层中存在反梯度输送,因而K理论的应用受到限制。试验还表明,修正的Kazanski-Monin参数可能比Monin-Obukhov长度更能反映大气的扩散能力。     

关于边界层阻力定律在非定常均匀条件下的推广

用自由大气实际风速取代地转风,得到了非定常均匀条件下阻力定律和边界层内参数的一些性质及相似性函数Ａ、Ｂ的特性。该方法和结果可应用于大气模式中的边界层参数化以及与相似性函数有关的一些研究。     

边界层方案对华北低层O3垂直分布模拟的影响

利用WRF-Chem模式,采用3种边界层参数化方案 (YSU, MYJ和ACM2),针对1个晴空、静稳日 (2013年8月26日20:00—27日20:00(北京时)) 进行模拟,着重分析不同边界层参数化方案对夜间残留层形成及日出前后O₃浓度垂直分布形式的模拟效果,并与固城站地面及垂直同步观测资料进行对比。结果表明:3种边界层参数化方案均能够模拟出温度及风速的区域分布形式以及风温垂直结构的变化特征;相比之下,MYJ方案模拟的夜间边界层高度较YSU方案和ACM2方案明显偏高,该对比结果可能是导致近地面污染物浓度模拟差异的重要原因;在夜间稳定层结至日出后稳定状态打破的边界层结构演变过程中,采用YSU方案和ACM2方案模拟的温度和风速垂直扩线形式与观测结果更为接近;同样采用非局地闭合的YSU方案和同时考虑局地和非局地闭合的ACM2方案,对于边界层高度内O₃浓度垂直分布形式的模拟效果具有明显优势。     

Footprints and Fetches for Fluxes over Forest Canopies with Varying Structure and Density

A stochastic trajectory model was used to estimate scalar fluxfootprints in neutral stabilityfor canopies of varying leaf area distributions andleaf area indices. An analytical second-order closure model wasused to predict mean wind speed, second moments and the dissipationrate of turbulent kinetic energy within a forest canopy.The influence of source vertical profile on the flux footprint wasexamined. The fetch is longer for surface sourcesthan for sources at higher levels in the canopy. In order tomeasure all the flux components, and thus the total flux, with adesired accuracy, sources were located at the forest floor in thefootprint function estimation. The footprint functions werecalculated for five observation levels above the canopy top. Itwas found that at low observation heights both canopy density andcanopy structure affect the fetch. The higher abovethe canopy top the flux is measured, the more pronounced is the effectof the canopy structure. The forest fetch for flux measurements isstrongly dependent on the required accuracy: The 90% flux fetchis greater by a factor of two or more compared to the 75% fetch. Theupwind distance contributing 75% of flux is as large as 45 timesthe difference between canopy height and the observation heightabove the canopy top, being even larger for low observationlevels.     

北京地区一次空气重污染过程的气象条件模拟参数化敏感性试验

气象预报是影响大气重污染预报精度的关键所在。针对2016年12月16～21日北京市一次重污染过程,开展了中尺度气象模式WRF的参数化方案配置敏感性试验。对微物理过程、长波辐射过程、短波辐射过程、陆面过程、边界层过程、近地面过程以及积云对流参数化过程进行组合优选,共设计51组参数化方案组合,分析不同模拟方案下北京市8个气象站点温度、相对湿度、10 m风速的模拟精度及其敏感性。试验结果表明:温度模拟对长波过程参数化方案最为敏感,集合离散度达2.4～7.4°C,再次是短波过程参数化方案;相对湿度模拟也对长波过程参数化方案最敏感,再次是陆面过程;风速模拟对不同过程参数化方案的敏感性程度差异不大。通过模拟结果与观测的统计对比,优选出模拟误差最小的方案组合为Lin微物理方案、RRTMG长波方案、RRTMG短波方案、Tiedtke积云对流方案、Noah陆面方案、MYNN 3rd边界层方案和MYNN近地面方案,并将其与集合平均、基准方案进行对比。对于集合平均来说,其温度模拟与观测相关系数为0.69,高于基准方案,其模拟偏差与均方根误差比基准方案低25%和11%;集合平均的相对湿度和风速模拟相比基准方案变化较小。与集合平均相比,优选方案能同时改进温度、相对湿度和风速模拟,使温度模拟偏差和均方根误差比基准方案下降35%和17%,使相对湿度模拟偏差和均方根误差下降43%和13%,使风速模拟偏差和均方根误差下降33%和24%。以上结果表明,参数化方案的敏感性试验和优选能显著减小重污染期间气象要素的模拟误差,重污染预报改进需重点关注参数化方案模拟上的不确定性。本研究也发现MYNN3rd边界层方案在这次重污染过程的气象要素模拟上具有良好性能,可为未来重污染预报改进提供参考。     

Interpolating methods for solar radiation in Uganda

Summary Existing literature lacks information on evaluation of interpolation methods for the estimation of solar radiation in the East African region. It follows that this study investigates the performance of five interpolations in Uganda which include: Nearest Point, Moving Average, Moving Surface, Trend Surface and Ordinary Kriging. Results have shown that the Moving Average linear decrease is the most appropriate for interpolation of solar radiation in Uganda and subsequent drawing of solar maps. The corresponding normalized mean bias error and root mean square error is 0.035 and 0.078, respectively. The worst performing is the Nearest Point interpolation with normalized mean bias error and root mean square error of 0.084 and 0.149, respectively.     

Parameterisation of the Planetary Boundary Layer for Diagnostic Wind Models

The planetary boundary-layer (PBL) parameterization is a key issue for the definition of initial wind flow fields in diagnostic models. However, PBL theories usually treat separately stable, neutral, and convective stability conditions, so that their implementation in diagnostic wind models is not straightforward. In the present paper, an attempt is made to adopt a comprehensive PBL parameterisation, covering stable/neutral and unstable atmospheric conditions, which appears suitable to diagnostic models. This parameterisation is implemented into our diagnostic mass-consistent code. A validation of the consistency between the implemented PBL parameterisations has been checked through an analysis of the sensitivity of the vertical wind profiles to atmospheric stability.     

风能输入参数对珠江口波浪模拟的影响

珠江口是粤港澳大湾区的核心区域,台风、风暴潮和巨浪等海洋灾害频发,对沿岸人民的生命财产安全构成严重威胁。准确的高分辨率波浪模拟/预报对区域经济建设和防灾减灾具有重要意义。波浪预报/模拟的质量很大程度上取决于风能输入的误差。本研究基于WAVEWATCH Ⅲ（WW3）的南海-珠江口双重嵌套精细化海浪模式,探讨不同的风场产品与风能输入参数方案组合对珠江口波浪动力过程模拟的影响,确定最优的风场和参数方案组合。ERA5风场更适合珠江口海域的风浪模拟,其模式结果略优于采用CFSR风场的模式结果。ERA5风场+T500方案的组合对珠江口波高变化过程的模拟效果最好,ERA5风场+T471方案的组合次之,ERA5风场+ST6方案再次之。CFSR风场与T471f参数方案最为适配,其结果稍差于ERA5风场+ST6参数方案。T500方案调整高风速下的风能输入和涌浪对风能输入的反馈作用,并考虑水深引起的波浪破碎效应,更适合水深限制的珠江口浅水区域。另外,WW3模式开关ST4的参数方案的表现优于开关ST6的参数方案。      

Stability Dependence of Canopy Flows over a Flat Larch Forest

The dependence on atmospheric stability of flow characteristics adjacent to a very rough surface was investigated in a larch forest in Japan. Micrometeorological measurements of three-dimensional wind velocity and air temperature were taken at two heights above the forest, namely 1.7 and 1.2 times the mean canopy height h. Under near-neutral and stable conditions, the observed turbulence statistics suggest that the flow was likely to be that of the atmospheric surface layer (ASL) at 1.7h, and of the roughness sublayer (RSL) at 1.2h. However, in turbulence spectra, canopy-induced large coherent motions appeared clearly at both heights. Even under strongly stable conditions, the large-scale motions were retained at 1.2h, whereas they were overwhelmed by small-scale motions at 1.7h. This phenomenon was probably due to the enhanced contribution of the ASL turbulence associated with nocturnal decay of the RSL depth, because the small-scale motions appeared at frequencies close to the peak frequencies of well-known ASL spectra. This result supports the relatively recent concept that canopy flow is a superimposition of coherent motions and the ASL turbulence. The large-scale motions were retained in temperature spectra over a wider region of stability compared to streamwise wind spectra, suggesting that a canopy effect extended higher up for temperature than wind. The streamwise spacing of dominant eddies according to the plane mixing-layer analogy was only valid in a narrow range at near neutral, and it was stabilised at nearly half its value under stable conditions.     

A parameterization for modelling the meteorological effects of tall forests — A case study of a large clearing

A parameterization scheme has been developed to describe the effects of a tall forest on the mean structure of the atmospheric boundary layer (ABL). The main advantage of the scheme is that dynamical and thermodynamical effects of a forest surface can be simulated satisfactorily using only a coarse-grid resolution within numerical models. Thereby, the canopy layer is parameterized as a quasi-subgrid phenomenon. This makes it possible to study meteorological phenomena within the ABL in a very economical way (with respect to computational time) whereby, nevertheless, more detailed information concerning the forest surface is taken into account than could be done using the same grid resolution and quite simple assumptions describing the canopy, e.g., the effective roughness.The applicability in numerical models is shown by using a slightly modified two-dimensional version of the mesoscale model FITNAH. For comparison, simulations with a high numerical grid resolution within the canopy have been carried out.Model results reproduce the known meteorological phenomena in forested areas, e.g., a stable thermal stratification near the surface during the day, and at night, a neutral — or slightly unstable condition — and, in general, reduced windspeed within the canopy layer.Diurnal variations and spatial distributions of temperature and humidity are found to be similar for both cases. Also, a thermally-induced local circulation system in the vicinity of a large clearing has been simulated satisfactorily.A comparison of the calculated results verifies that the parameterization scheme is quite suitable for simulating the effects of plant canopies on the distributions of meteorological variables in the ABL.     

An Analytical Model for Mean Wind Profiles in Sparse Canopies

Existing analytical models for mean wind profiles within canopies are applicable only in dense canopy scenarios, where all momentum is absorbed by canopy elements and, hence, the effect of the ground on turbulent mixing is not important. Here, we propose a new analytical model that can simulate mean wind profiles within sparse canopies under neutral conditions. The model adopts a linearized canopy-drag parametrization and a first-order turbulence closure scheme taking into account the effects of both the ground and canopy elements on turbulent mixing. The resulting wind profile within a sparser canopy appears to be more like a logarithmic form, with the no-slip condition at the ground being satisfied. The analytical solution converges exactly to the standard surface-layer logarithmic wind profile in the case of zero canopy density (i.e., no-canopy scenario) and tends to be an exponential wind profile for a dense canopy; this feature is unique compared with existing analytical models for canopy wind profiles. Results from the new model are in good agreement with those from laboratory experiments and numerical simulations.     

探空和飞机观测资料联合同化对台风“苏迪罗”（2015）数值模拟的影响研究

基于中尺度数值天气预报模式WRF和WRFDA三维变分同化系统,针对2015年台风“苏迪罗”二次登陆过程,利用NCEP ADP提供的无线电探空、飞机报观测资料,同时同化两种资料,并进行36 h模拟预报,从台风移动路径、强度、降水及模式初始场改进等方面分析了同化模拟效果。结果表明,同时同化两种资料能够有效改善台风移动路径、中心附近最低气压的模拟。对浙闽降水关键区24 h降水的TS评分结果表明,该同化方案对台风降水的预报评分有一定程度的改进,尤其是中雨和特大暴雨的改进最明显。对于模式初始场的温度、相对湿度、纬向风、经向风在各高度上的均方根误差,除湿度外均有不同程度的减小。并且同化对于台风结构的模拟也有所调整。      

Mean Flow and Turbulence Characteristics in an Urban Roughness Sublayer

In this study, a detailed model of an urban landscape has been re-constructed inthe wind tunnel and the flow structure inside and above the urban canopy has beeninvestigated. Vertical profiles of all three velocity components have been measuredwith a Laser-Doppler velocimeter, and an extensive analysis of the measured meanflow and turbulence profiles carried out. With respect to the flow structure inside thecanopy, two types of velocity profiles can be distinguished. Within street canyons,the mean wind velocities are almost zero or negative below roof level, while closeto intersections or open squares, significantly higher mean velocities are observed.In the latter case, the turbulent velocities inside the canopy also tend to be higherthan at street-canyon locations. For both types, turbulence kinetic energy and shearstress profiles show pronounced maxima in the flow region immediately above rooflevel.Based on the experimental data, a shear-stress parameterization is proposed, inwhich the velocity scale, u_s, and length scale, z_s, are based on the level and magnitude of the shear stress peak value. In order to account for a flow region inside the canopy with negligible momentum transport, a shear stress displacement height, d_s, is introduced. The proposed scaling and parameterization perform well for the measured profiles and shear-stress data published in the literature.The length scales derived from the shear-stress parameterization also allowdetermination of appropriate scales for the mean wind profile. The roughnesslength, z₀, and displacement height, d₀, can both be described as fractions of the distance, z_s - d_s, between the level of the shear-stress peak and the shear-stress displacement height. This result can be interpreted in such a way that the flow only feels the zone of depth z_s - d_s as the roughness layer. With respect to the lower part of the canopy (z < d_s) the flow behaves as a skimming flow. Correlations between the length scales z_s and d_s and morphometric parameters are discussed.The mean wind profiles above the urban structure follow a logarithmic windlaw. A combination of morphometric estimation methods for d₀ and z₀ with wind velocity measurements at a reference height, which allow calculation of the shear-stress velocity, u_*, appears to be the most reliable and easiest procedure to determine mean wind profile parameters. Inside the roughnesssublayer, a local scaling approach results in good agreement between measuredand predicted mean wind profiles.