Numerical Study of Dual-Plume Interference in a Turbulent Boundary Layer

Direct numerical simulation is used to investigate the interference arising from the dispersion of passive scalar plumes released from a pair of point sources in a fully-developed wall-bounded shear flow. Four different lateral separations of the two sources for both near ground-level and elevated releases are considered. The downwind evolution of the correlation between the plume concentrations along the centreline between the two sources and the behaviour of the lateral profiles of the correlation at various locations downwind of the two sources are examined in detail. Differences in the exceedance probability over a high concentration level for a single plume and the total plume are highlighted and studied, and the effects of destructive and constructive interferences on the exceedance probabilities for the total plume are used to explain these differences. One significant result is that all higher-order (third-order and above) moments of the total concentration can be inferred from the application of a clipped-gamma distribution using the information embodied in only the first- and second-order concentration moments of each single plume, and in the cross-correlation coefficient of the instantaneous concentration of the two plumes.     

A Mixed-Layer Model Of The Well-Mixed Stratocumulus-Topped Boundary Layer

Recently a range of sophisticated large-eddy simulations of thecloud-topped boundary layer have been intercompared and furthercompared with observations and single column models. Here we comparethese results with perhaps the simplest model of the cloud-toppedboundary layer, namely a mixed-layer model. Results from the model aredescribed with two aims in mind. Firstly, the good results act as areminder of the success of simple models, and, secondly, we suggestthat a simple mixed-layer model could be used as a baseline for futuremodel intercomparisons.The mixed-layer model is based on two assumptions that follow previousstudies. Firstly, the liquid-water potential temperature and the total waterspecific humidity are assumed to be constant with height in the boundarylayer. Secondly, turbulence entrains air across the inversion into the boundarylayer at a rate that is assumed to be proportional to the jump in radiative flux at the cloud top and inversely proportional to the jump in buoyancy at the inversion. The constant of proportionality is called the entrainment efficiency.Results from the model for the entrainment rate and height evolutionof the boundary layer are compared with the observations and modelsconsidered in a EUCREM intercomparison study. Thepresent mixed-layer model accurately predicts the observed heightevolution of the boundary layer, but over-estimates the entrainmentrate to a similar degree as the large-eddy simulations. We show that,if the subsidence rate is reduced to the value given by observationsrather than the value used in the EUCREM intercomparison study,then the model agrees well with observed value of the entrainment rateif the entrainment efficiency is taken to be 0.6. With this value, themodel also agrees well with a further case study byBechtold et al. An entrainment efficiency of 0.6 is a little higherthan suggested by large eddy simulations, but such simulations do notcurrently resolve the entrainment events explicitly. Hence this pointdeserves further study.     

中尺度数值模拟中的边界层多尺度湍流参数化方案

该文在多尺度湍流理论的研究成果基础上, 将边界层湍流风谱与平均量的梯度相联系, 建立了边界层多尺度湍流参数化子模式, 之后放入MM5模式中进行了个例模拟研究, 并与MM5模式附带的M RF边界层参数化、Blackadar高分辨率边界层参数化的模拟结果进行比较和分析。结果表明, 多尺度湍流理论能够反映出实际大气边界层中热量垂直输送的规律, 将其用于中尺度数值预报模式的边界层物理过程参数化是可行的; 多尺度湍流参数化在地表层和边界层内各个层次上都着重考虑含能量最大的涡的作用以及水平热力不均匀性的影响, 因此在地形和下垫面比较复杂的区域, 对中尺度天气系统的模拟有进一步发展的前景。     

Turbulent Helicity in the Atmospheric Boundary Layer

We consider the assumption postulated by Deusebio and Lindborg (J Fluid Mech 755:654–671, 2014) that the helicity injected into the Ekman boundary layer undergoes a cascade, with preservation of its sign (right- or alternatively left-handedness), which is a signature of the system rotation, from large to small scales, down to the Kolmogorov microscale of turbulence. At the same time, recent direct field measurements of turbulent helicity in the steppe region of southern Russia near Tsimlyansk Reservoir show the opposite sign of helicity from that expected. A possible explanation for this phenomenon may be the joint action of different scales of atmospheric flows within the boundary layer, including the sea-breeze circulation over the test site. In this regard, we consider a superposition of the classic Ekman spiral solution and Prandtl’s jet-like slope-wind profile to describe the planetary boundary-layer wind structure. The latter solution mimics a hydrostatic shallow breeze circulation over a non-uniformly heated surface. A 180°-wide sector on the hodograph plane exists, within which the relative orientation of the Ekman and Prandtl velocity profiles favours the left rotation with height of the resulting wind velocity vector in the lowermost part of the boundary layer. This explains the negative (left-handed) helicity cascade toward small-scale turbulent motions, which agrees with the direct field measurements of turbulent helicity in Tsimlyansk. A simple turbulent relaxation model is proposed that explains the measured positive values of the relatively minor contribution to turbulent helicity from the vertical components of velocity and vorticity.     

北京稳定天气条件下城市边界层环流特征数值研究

采用科罗拉多大学和MRC／ASTER共同开发的区域大气模拟系统（RAMS）对北京地区稳定天气条件下的个例进行数值模拟,通过对数值模拟结果与观测事实的比较以及敏感性试验,分析了北京城市边界层环流特征和环流影响因子在环流发展过程中的作用。结果表明：①在山谷风环流和热岛环流相互作用下形成了北京城市边界层流场特有的局地环流。②热岛中心在决定边界层环流的辐合区位置上起相对较大的作用,边界层环流的强度和发展高度由山谷风强度和热岛强度共同决定。     

边界层参数化对“雅安天漏” 降水数值模拟的影响

选取了2001年7月3个"雅安天漏"个例,采用非静力中尺度数值模式MM5V3进行模拟,着重研究了MRF边界层参数化方案对雨量中心强度和雨区分布的影响,结果发现MRF方案的总体预报效果较好,并能很好地刻画雅安暴雨时的纬向垂直环流结构;在复杂地形条件下,特别是降水有明显日变化的地区,边界层的作用非常重要.     

天津城市边界层湍流统计特征

利用2008年2月1～28日在天津气象塔上观测的超声风、温资料和常规风、温及湿梯度资料,计算了天津城市边界层无量纲湍流速度方差、湍流温度方差、感热通量、动量通量和湍流动能。结果表明,在不稳定层结条件下,40m高度上无量纲湍流速度方差和湍流温度方差遵循莫宁-奥布克霍夫相似理论,在220m高度上只有垂直方向上的湍流速度方差...     

和预报员谈数值预报

用尽可能非专业的词语,向预报员介绍数值预报的成就、问题与应用。在简单回顾数值预报发展历史的基础上,重点分析了近十几年来数值预报的主要技术进步与有待解决的问题,对比了数值预报与主观经验预报的各自优势与不足,提出了优势互补对于提高业务预报的重要性。还分析了数值预报的技术进步对于数值预报产品释用的影响,以及预报员参与数值预报发展的可能性与必要性。     

Effects of Roll Vortices on Turbulent Fluxes in the Hurricane Boundary Layer

Boundary-layer secondary circulations or ‘roll vortices’ can have a significant influence on the turbulent exchange of momentum, sensible heat and moisture throughout the hurricane boundary layer. In this study, analyses of data from a WP-3D aircraft of the National Oceanic and Atmospheric Administration (NOAA) are presented. As part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-hurricane experiment sponsored through the Office of Naval Research and NOAA’s annual hurricane research program, flights were conducted to investigate energy exchange across the air–sea interface. We present the first in-situ aircraft-based observations of rolls in the hurricane boundary layer and investigate their influence on energy and momentum exchange. The rolls detected in Hurricane Isidore (year 2002) have a characteristic wavelength of about 900 m, in good agreement with analyses of data from a synthetic aperture radar image captured by the Canadian Space Agency’s RADARSAT satellite in the same storm. Our analyses of the airborne data suggest that roll vortices may be a significant factor modulating the air–sea momentum exchange.     

非线性平衡方程初值化方法及其在中期数值天气预报试验中的应用

本文给出了一种求解非线性平衡方程的新的有效的方法及有关的数值试验结果。和以往的求解方法相比,本方法的优点是：收敛速度快,不需要冗长的迭代计算,也不需要对初始高度场的某些记录作修改,并能节省大量的计算时间。文中利用北半球七层原始方程谱模式,使用了1982年的客观分析资料,进行中期数值天气预报试验。试验结果表明,用非线性平衡方程初值化方法制作中期数值预报比其他的如线性平衡方程初值化方程的更佳。后者因去掉了非线性项的作用,天气系统的强度预报结果偏弱且偏平滑。     

边界层参数化方案对暴雨数值模拟的影响

选取2003年7月4-5日南京暴雨个例，采用非静力中尺度模式MM5进行模拟，着重研究了不同边界层参数化方案对雨量中心强度、雨区分布的影响。结果表明：对于不同的边界层参数化方案，垂直速度场、水汽通量散度场、涡度场、水平风场的散度以及θse场都表现出不同的特征；合理边界层方案的引入对预报效果有明显的改进；结合边界层和自由大气的动力、热力结构进行了综合分析，给出了边界层作用与自由大气动力、热力结构的配置情况。说明这种配置对暴雨的形成是至关重要的。     

边界层参数化方案对不同性质降水模拟的影响

利用MM5模式的4种边界层方案(ETA方案、MRF方案、Blackadar方案、Gayno-Seman方案),对2008年9月22～27日发生在四川盆地先是对流性,后来是稳定性的暴雨过程进行了该4种不同边界层方案的数值模拟比较试验。整体而言,ETA方案对雨带的预报能力较差,但对对流性降水有一定的预报能力;MRF方案对雨带(特别是稳定性降水)的预报能力相对最强;Blackadar方案对后24h强降水最具能力,且对对流性和稳定性降水的预报能力没有太大差别;Gayno-Seman方案对后24 h强降水预报能力较差。边界层对物理量场的影响随时间增大。在预报积分的前10 h以内,各方案的涡度、相对湿度、垂直速度等物理量预报几乎无异;积分10～24 h,它们的量值间出现差异;24 h后,不同方案的预报不仅量值上有差异,甚至变化趋势都不尽相同。高度场受边界层的影响最小,受边界层方案影响最大的是U场,V场受边界层的影响呈高度和时间的分段函数,湿度场受到的影响有明显的时间滞后性特征。对于温度场而言,600 hPa似乎是温度场受边界层影响的一个拐点,边界层影响在通过600 hPa后改变了影响规律。     

Influence of Evaporating Droplets in the Turbulent Marine Atmospheric Boundary Layer

Semi-idealized model simulations are made of the nocturnal cold-air pool development in the approximately 1-km wide and 100–200-m deep Grünloch basin, Austria. The simulations show qualitatively good agreement with vertical temperature and wind profiles and surface measurements collected during a meteorological field expedition. A two-layer stable atmosphere forms in the basin, with a very strong inversion in the lowest part, below the approximate height of the lowest gap in the surrounding orography. The upper part of the stable layer is less strongly stratified and extends to the approximate height of the second-lowest gap. The basin atmosphere cools most strongly during the first few hours of the night, after which temperatures decrease only slowly. An outflow of air forms through the lowest gap in the surrounding orography. The outflow connects with a weak inflow of air through a gap on the opposite sidewall, forming a vertically and horizontally confined jet over the basin. Basin cooling shows strong sensitivity to surface-layer characteristics, highlighting the large impact of variations in vegetation and soil cover on cold-air pool development, as well as the importance of surface-layer parametrization in numerical simulations of cold-air-pool development.     

沙尘天气的数值预报

沙尘天气预报的核心问题是要理解沙尘天气形成的物理机制,并定量地描述风沙的分布、强度、输送与沉降.沙尘天气的形成是一个复杂的物理过程,涉及到大气运动与地表状况等诸多因素.作者介绍了一个以数值预报为核心、以卫星观测与地理信息系统为资料背景的沙尘天气集成预报系统,特别是其中的起沙模式.作者运用该系统预报了2002年春季的沙尘天气,并用天气观测和采样观测资料对预报效果进行了鉴定.分析表明,预报系统不仅能定性预报沙尘天气的空间分布与时间演变,而且能定量预报沙尘源地、大气中的沙尘浓度、沙尘的输送以及沉降.模式72 h的预报仍然有较高的可信度.另外讨论了存在的问题和今后的研究方向.       

Large-Eddy Simulation Of A Nocturnal Stratocumulus-Topped Marine Atmospheric Boundary Layer: An Uncertainty Analysis

A large-eddy simulation (LES) model has been used to study a nocturnalstratocumulus-topped marine atmospheric boundary layer. The main objectivesof our study have been first to investigate the statistical significance of LES-derived data products. Second, to test the sensitivity of our LES results with respect to the representation of subgrid-scale mixing and microphysical processes, and third to evaluate and to quantify the parametric uncertainty arising from the incomplete knowledge of the environmental parameters that are required to specify the initial and boundary conditions of a particular case study. Model simulations were compared with observations obtained in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). Based on these simulations the following conclusions could be drawn. Resolution(50 × 50 × 25 m³) and domain size (3.2 × 3.2 × 1.5 km³) of the LES calculations were adequate from a numerical point of view to represent the essential features of the stratocumulus-topped boundary layer. However, the ensemble runs performed in our study to investigate the statistical significance of LES-derived data products demonstrate that the area-time averaging procedure for the second-order moments produces only a low degree of statistical reliability in the model results. This illustratesthe necessity of having LES model results that are not only of adequate resolution but also of sufficiently large domain. The impact of different subgrid schemes was small, but the primary effects of drizzle were found to influence the boundary-layer structure in a climatologically significant way. The parametric uncertainty analysis revealed that the largest contribution to the variance of the LES-derived data products is due to theuncertainties in the cloud-top jump of total water mixing ratio and the net radiative forcing. The differences between the model and measurements for most of the simulated quantities were within the modelling uncertainties, but the calculated precipitation rate was found to differ significantly from that derived in the observations.     

Disentangling Turbulent Gas Diffusion from Non-diffusive Transport in the Boundary Layer

Boundary-Layer Meteorology - An analysis based on the law of linear momentum conservation demonstrates unequivocally that the mass fraction is the scalar whose gradient determines gas diffusion,...     

Turbulent Pressure Statistics in the Atmospheric Boundary Layer from Large-Eddy Simulation

We use large-eddy simulation (LES) to study the turbulent pressure field in atmospheric boundary layers with free convection, forced convection, and stable stratification. We use the Poisson equation for pressure to represent the pressure field as the sum of mean-shear, turbulence–turbulence, subfilter-scale, Coriolis, and buoyancy contributions. We isolate these contributions and study them separately. We find that in the energy-containing range in the free-convection case the turbulence–turbulence pressure dominates over the entire boundary layer. That part dominates also up to midlayer in the forced-convection case; above that the mean-shear pressure dominates. In the stable case the mean-shear pressure dominates over the entire boundary layer.We find evidence of an inertial subrange in the pressure spectrum in the free and forced-convection cases; it is dominated by the turbulence–turbulence pressure and has a three-dimensional spectral constant of about 4.0. This agrees well with quasi-Gaussian predictions but is a factor of 2 less than recent results from direct numerical simulations at moderate Reynolds numbers. Measurements of the inertial subrange pressure spectral constant at high Reynolds numbers, which might now be possible, would be most useful.     

重庆冬季大气边界层湍流扩散能力的初步研究

本文利用1989年12月16日至1990年1月14日在重庆市用三轴风速仪所测得到的三轴风速资料，分别计算了有风、静风、有雾时的大气扩散参数， 以及无雾有风条件下的湍流强度。文中对有风时、静风时、有雾时的扩散参数作了比较和研究，对无雾有风时的湍流强度作了分析。从而有助于了解重庆在有风、 静风或有雾时边界层内湍流扩散能力的大小。     

集合数值天气预报的研究进展

集合预报是目前国外广泛应用的一种新的数值预报技术,国内由于计算条件的限制,集合预报的研究应用起步较晚,目前虽已取得了一些研究成果,但还没有广泛的应用到实际的业务预报之中。集合预报的应用,在天气预报上主要是概率预报,另外在“目标观测”、资料同化等方面也有广泛应用。集合概率预报的一系列适用的验证,增加了概率预报的信度。     

利用尺度自适应大气边界层湍流参数化方案对一次陆地浓雾的数值模拟

为提高大气数值模式的模拟能力,改进大气边界层水汽、热量湍流输送计算和大雾天气的模拟效果,选用WRF三维非静力模式,采用具有局地和非局地垂直湍流尺度自适应计算能力的MYNN_SA参数化方案,对2017年12月28—29日我国华北—江淮地区的大范围浓雾过程进行了数值模拟研究,探讨从中尺度到灰区尺度分辨率范围,模式的尺度自适应大气边界层湍流参数化方案对稳定大气边界层发展、湍流输送和大雾天气模拟的影响。利用中国地面气象站观测资料和ERA5再分析数据,在接近灰区尺度的网格分辨率上,利用尺度自适应大气边界层湍流MYNN_SA参数化方案较之中尺度参数化MYNN方案,可明显改善次网格湍流输送计算,以及陆地浓雾的强度、空间分布和时间演变特征,可更精确地模拟云水混合比、逆温层和雾区的垂直结构。