共查询到20条相似文献，搜索用时 140 毫秒
1.
用大涡模拟方法研究地转强迫下的对流边界层流动和地表热力非均匀性影响.模拟重现了典型对流边界层的平均风廓线和动量通量垂直分布.地表热力非均匀性对区域平均风速和动量通量分布无明显影响,但边界层内的局地流动性状和湍流动量输送情况有系统性的改变.下风较热区近地面风速增强而高空流动受到阻塞,上风较冷区之上情况则正好相反.对应于平均流动场的畸变,地表较热区之上边界层大部可以出现动量向上输送的情况,较冷区成为大气动量下传的主要通道.地面应力在较热区增强、较冷区减弱的趋势明显. 相似文献
2.
高原东南缘大气近地层湍能特征与边界层动力、热力结构相关特征 总被引：3，自引：2，他引：1
基于云南省大理2008年3、5、7月GPS加密探空试验时段(14和02时)资料,结合边界层铁塔综合观测资料,采用温度梯度法、逆温强度法和涡动相关法分别计算高原东南缘对流边界层(convective boundary layer,CBL)及稳定边界层(stable boundary layer,SBL)顶高度,通过计算获取感热通量、潜热通量、湍流动能、切变项以及浮力项与大气动力、热力过程垂直相关特征综合分析,可发现湍能方程中浮力项、感热、潜热通量与NCEP再分析资料计算获取的大气视热源相关特征显著,这某种程度反映了高原东南缘近地层大气湍流动量、热量输送对低层大气视热源Q_1的重要贡献。低层视热源Q_1亦表现出与湍能方程分量类似的日变化周期,此特征反映了高原东南缘大气热源变化与下垫面水热过程及其湍流输送日变化密切相关;浮力项与湍能等项对大气低层热源与涡动特征、热力混合结构的形成有重要作用;低层大气视热源、水汽汇均与边界层高度有显著相关,综合分析结果某种程度描述了青藏高原东南缘近地层湍流动量、热量输送状况与低层大气热源,热力混合边界层结构的综合相关物理图像,初步探索了高原东南缘对流活跃区大气湍流运动与大气动力、热力过程相互作用特征。研究表明近地层湍流通量变化某种程度可反映未来局地大气视热源垂直结构变化的"强信号"特征。本文上述研究结论也可启发我们进一步关注近地层湍流通量异常变化特征及其对局地降水过程大气热源结构演变的影响问题。 相似文献
3.
陆气相互作用对中尺度对流系统影响的研究进展 总被引：1，自引：1，他引：0
文章回顾了大气对地表性质的敏感性研究,以及陆气相互作用对中尺度天气过程的影响,说明了地表性质与积云对流及对流降水之间的联系。地表性质的改变对行星边界层的热通量、水汽通量、对流有效位能产生影响,并通过湍流的垂直输送,进而影响到其上大气的性质。陆气之间存在着复杂的、非线性的相互作用。性质不均匀的下垫面造成地表向大气感热通量和潜热通量的差异,从而在近地层大气中形成温度和气压梯度,产生局地环流,在条件适合的情况下可以形成对流,并产生降水,而降水的不均匀分布,又维持了下垫面的不均匀性。土壤湿度对对流的影响受到多个因素的制约,其中天气尺度过程的影响是很显著的;由非均匀的下垫面所产生的局地环流能够触发积云对流。 相似文献
4.
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 总被引：3，自引：2，他引：3
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。 相似文献
5.
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。 相似文献
6.
边界层对流对示踪物抬升和传输影响的大涡模拟研究 总被引：3，自引：1，他引：2
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。 相似文献
7.
大气对流边界层中的涡漩结构 总被引：4，自引：1，他引：4
大气边界层中存在尺度从几百米到几十公里的大涡漩运动。它们在边界层中动量、热量、水汽等垂直输送中起重要作用。作者从边界层中对流和上部稳定层中波动相互作用的观点，发展得出大涡结构的对流波动理论。根据此理论，大涡的波谱构成主要由上、下层大气中风向、风速、层结以及两层之间的温度跃变等因素决定。本文根据卫星云图和天气资料分析了一次冷空气爆发流经暖洋面上形成云街、对流单体以及它们之间的相互演化的过程，并用对流波动理论，依据各阶段的大气条件计算出它们的波数构成，并得出了垂直速度、辐合带、界面扰动的分布，解释了云街、对流单体的形成、结构及相互转化的原因 相似文献
8.
本文采用位涡波作用密度和波作用方程,对一次南疆西部干旱区暴雨的组织化过程和机制进行了诊断研究,对影响暴雨对流系统组织化的关键物理过程进行了分析和讨论。位涡波作用密度耦合了多种影响对流云体演变的大气动热力扰动,能够良好描述对流系统的组织化过程。以此为基础,描述位涡波作用密度变化的波作用方程能够用来研究驱动对流系统组织化发展的物理因素。研究发现,波作用方程诊断得到的多个物理过程与扰动斜压性、扰动风切变和扰动涡度的发展演变有关,表明它们对对流组织化有重要作用,多条东西向的对流线发展为东北—西南向的带状对流系统过程中,包含强对流的维持和南北尺度的增大。对流线在东北向弱对流的发展增强与基本态气流对强对流区的热力输送引起扰动斜压性增强有关。影响对流线中部强对流的维持和南北向发展的关键过程包括:上升、下沉气流引起的热力输送导致对流线内扰动斜压性增强,扰动西风与扰动东风形成气旋性环流引起经向切变环流增强,及扰动经向风将扰动纬向风切变向对流中心区输送引起纬向切变增强、垂直环流增强。该研究表明,对流系统的组织化是大气多种动热力扰动演变和配合的结果,通过波作用演变方程能够比较清晰体现其中的关键过程,且波作用方程为波作用密度倾向,未来可探讨其对对流系统组织化的预报意义。 相似文献
9.
2021年8月7—8日,四川盆地中东部出现大暴雨、局地特大暴雨,是重庆2021年度社会影响最大的一次暴雨过程。采用多源观测及ERA5再分析资料,对此次大暴雨过程进行诊断分析。结果表明：大暴雨发生在低槽移入四川盆地诱发暖性西南低涡背景下,具有显著的阶段性、跳跃性和极端性特征。大暴雨先后形成于西南低涡中心东南部、西南低涡东侧和西南低涡南侧暖湿的边界层辐合线附近。各阶段大暴雨均由移动缓慢、维持时间达3～6 h的β中尺度对流系统影响形成,暖湿不稳定和弱垂直风切变为β中尺度对流系统的形成提供了有利的环境条件。涡度分析表明,西南低涡的发展主要源于低空辐合及垂直涡度输送效应,但暴雨区的正涡度发展与西南低涡并不完全相同,水平涡度倾侧效应较为显著。第一阶段暴雨区正涡度主要源于对流层中低层西南低涡中心附近显著的低空辐合、涡度垂直输送及水平涡度倾侧效应;第二阶段和第三阶段暴雨区正涡度主要源于边界层辐合及边界层以上的水平涡度倾侧效应,边界层辐合触发暖湿大气中的中尺度对流活动促进了第二阶段和第三阶段大暴雨的形成。 相似文献
10.
边界层参数化方案对南海秋季台风“莎莉嘉”(2016)模拟的影响 总被引：2，自引：2，他引：2
利用美国中尺度数值模式WRF,选取两个局地(QNSE、MYJ)闭合和两个非局地(YSU、ACM2)闭合的边界层参数化方案对台风“莎莉嘉”(2016)进行了4组模拟试验,结果表明,不同边界层方案对台风路径影响较小,但对台风强度和结构有明显的影响,就本个例研究而言,非局地闭合边界层方案明显优于局地闭合边界层方案。台风强度的差异是热力和动力共同作用的影响。局地闭合方案模拟的地表焓通量、水汽通量和动量通量更大,台风偏强;局地闭合方案模拟的边界层高度更高、边界层顶的夹卷过程更强、垂直混合更强、台风暖心结构更强,从而台风也更强。台风强度的差异和台风结构的变化密切相关。 相似文献
11.
On the Nature of the Transition Between Roll and Cellular Organization in the Convective Boundary Layer 总被引：2，自引：2，他引：0
Both observational and numerical studies of the convective boundary layer (CBL) have demonstrated that when surface heat fluxes are small and mean wind shear is strong, convective updrafts tend to organize into horizontal rolls aligned within 10–20\(^\circ \) of the geostrophic wind direction. However, under large surface heat fluxes and weak to negligible shear, convection tends to organize into open cells, similar to turbulent Rayleigh-Bénard convection. Using a suite of 14 large-eddy simulations (LES) spanning a range of \(-z_i/L\) between zero (neutral) and 1041 (highly convective), where \(z_i\) is the CBL depth and L is the Obukhov length, the transition between roll- and cellular-type convection is investigated systematically for the first time using LES. Mean vertical profiles including velocity variances and turbulent transport efficiencies, as well the “roll factor,” which characterizes the rotational symmetry of the vertical velocity field, indicate the transition occurs gradually over a range of \(-z_i/L\); however, the most significant changes in vertical profiles and CBL organization occur from near-neutral conditions up to about \(-z_i/L \approx \) 15–20. Turbulent transport efficiencies and quadrant analysis are used to characterize the turbulent transport of momentum and heat with increasing \(-z_i/L\). It is found that turbulence transports heat efficiently from weakly to highly convective conditions; however, turbulent momentum transport becomes increasingly inefficient as \(-z_i/L\) increases. 相似文献
12.
为研究黄河源区边界层湍流特征及其对物质和能量输送的影响,本文首次采用大涡模拟的方法,对比分析了黄河源区两种不同下垫面上（鄂陵湖和湖边草地）对流边界层（CBL）中精细的湍流结构特征。使用资料为2012年夏季黄河源区鄂陵湖流域野外观测实验的GPS探空资料、涡动相关观测资料。分析表明,模拟的黄河源区草地和湖上CBL的平均结构与实测结果吻合较好,但草地和湖上CBL的湍流结构特征差异较明显。模拟结果显示,草地CBL内湍能收支、湍流特征量的时空分布和湍涡结构特征均与陆地上热力驱动CBL的研究结果一致;湖上CBL顶部存在明显的对流卷特征,且夹卷层的湍流强度比草地的强,而草地近地面湍强则更大。通过改变水平分辨率的模拟试验,发现两个不同下垫面上模拟结果对模式分辨率的敏感性不同,湖面CBL的模拟要选择较高的水平分辨率（50～100 m）,以提高近湖面和夹卷层对湍流动能和湍流通量模拟的精度,也充分模拟出各种尺度的波对湍流通量的累积贡献。考虑到计算时间等影响,模拟草地边界层精细的湍流结构时建议选择网格距为100～200 m。 相似文献
13.
Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NOx transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O3 at the surface.The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NOx loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies. 相似文献
14.
A laboratory study of the turbulent velocity characteristics in the convective boundary layer 总被引：4，自引：0，他引：4
Based on the measurement of the velocity field in the convective boundary layer （CBL） in a convection water tank with the particle image velocimetry （PIV） technique, this paper studies the characteristics of the CBL turbulent velocity in a modified convection tank. The experiment results show that the velocity distribution in the mixed layer clearly possesses the characteristics of the CBL thermals, and the turbulent eddies can be seen obviously. The comparison of the vertical distribution of the turbulent velocity variables indicates that the modeling in the new tank is better than in the old one. The experiment data show that the thermal＇s motion in the entrainment zone sometimes fluctuates obviously due to the intermittence of turbulence. Analyses show that this fluctuation can influence the agreement of the measurement data with the parameterization scheme, in which the convective Richardson number is used to characterize the entrainment zone depth. The normalized square velocity wi^2/w＊^2. at the top of the mixed layer seems to be time-dependent, and has a decreasing trend during the experiments. This implies that the vertical turbulent velocity at the top of the mixed layer may not be proportional to the convective velocity （w＊）. 相似文献
15.
This case study introduces measurements of turbulent fluxes in a nocturnal boundary layer in North Germany with the new helicopter-borne turbulence measurement system HELIPOD, a detailed data analysis and examination in regard of systematic errors of the instrument, and some comparison with local similarity theory and experiments of the past, in order to confirm the occurrence of small vertical turbulent fluxes. The examined nocturnal boundary layer offered excellent conditions to analyse the quality of the measurement system. In this connection, a detailed look at a strong ground-based inversion disclosed small turbulent fluxes with a spectral maximum at ten metres wavelength or less, embedded in intermittent turbulence. For verification of these fluxes, the measurements were compared with well established results from past experiments. Local similarity theory was applied to calculate dimensionless variances of the turbulent quantities, which were found in good agreement with other observations. Since shear and stratification varied significantly on the horizontal flight legs due to global intermittency, a method was developed to determine vertical gradients on a horizontal flight pattern, by use of small fluctuations of the measurement height. With these locally determined gradients, gradient transport theory became applicable and the turbulent diffusivities for heat and momentum, the Richardson number, and the flux Richardson number were estimated within isolated strong turbulent outbursts. Within these outbursts the flux Richardson number was found between 0.1 and 0.2. The functional relationship between the gradient Richardson number and the turbulent Prandtl number agreed well with observations in past experiments and large eddy simulation. The impact of the stratification on the vertical turbulent exchange, as already described for the surface layer using Monin–Obukhov similarity, was analogously observed in the very stably stratified bulk flow when local scaling was applied. 相似文献
16.
Large-eddy simulations (LESs) are employed to investigate the turbulence characteristics in the shear-free convective boundary
layer (CBL) driven by heterogeneous surface heating. The patterns of surface heating are arranged as a chessboard with two
different surface heat fluxes in the neighbouring patches, and the heterogeneity scale Λ in four different cases is taken as 1.2, 2.5, 5.0 and 10.0 km, respectively. The results are compared with those for the
homogeneous case. The impact of the heterogeneity scale on the domain-averaged CBL characteristics, such as the profiles of
the potential temperature and the heat flux, is not significant. However, different turbulence characteristics are induced
by different heterogeneous surface heating. The greatest turbulent kinetic energy (TKE) is produced in the case with the largest
heterogeneity scale, whilst the TKE in the other heterogeneous cases is close to that for the homogeneous case. This result
indicates that the TKE is not enhanced unless the scale of the heterogeneous surface heating is large enough. The potential
temperature variance is enhanced more significantly by a larger surface heterogeneity scale. But this effect diminishes with
increasing CBL height, which implies that the turbulent eddy structures are changed during the CBL development. Analyses show
that there are two types of organized turbulent eddies: one relates to the thermal circulations induced by the heterogeneous
surface heating, whilst the other identifies with the inherent turbulent eddies (large eddies) induced by the free convection.
At the early stage of the CBL development, the dominant scale of the organized turbulent eddies is controlled by the scale
of the surface heterogeneity. With time increasing, the original pattern breaks up, and the vertical velocity eventually displays
horizontal structures similar to those for the homogeneous heating case. It is found that after this transition, the values
of λ/z
i
(λ is the dominant horizontal scale of the turbulent eddies, z
i
is the boundary-layer height) ≈1.6, which is just the aspect ratio of large eddies in the CBL. 相似文献
17.
利用大涡模式模拟了对流边界层结构演变以及深对流触发过程。通过改变鲍恩比的敏感性试验研究不同大气初始状况下湿润和干旱下垫面湍流特征及其对深对流触发过程的影响。结果表明:干旱下垫面的混合层干而暖,厚度较大;湿润下垫面相反。由于地表感热通量对热力湍流形成的作用更大,干旱下垫面上湍流混合和夹卷作用更强,使得水汽和相当位温在边界层内分布更均一,而在边界层顶有较大的负扰动;干旱下垫面上对流强度较湿润下垫面大,但均表现为泡状对流,水平方向上呈网状结构。不同下垫面上深对流的发生与大气初始状况有关,当初始时刻1—3 km的逆温强度较弱时（0.15 K/（100 m））,边界层内湍流迅速发展,深对流首先在干旱下垫面发生,但因对流有效位能较小,云层厚度小于湿润下垫面。当1—3 km的逆温强度增加到0.55 K/（100 m）时,云层形成时间较晚,云层厚度明显减小,仅当边界层顶的比湿较大时,有深对流发生,但仍首先发生在干旱下垫面,考虑贯穿对流在边界层顶引起的较强冷却作用,云层厚度大于湿润下垫面。 相似文献
18.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(1):143-157
Effects of wind on quasi-steady, shallow convection in the Martian boundary layer are studied using a large-eddy simulation
model. Convection in the model is generated by the radiative flux divergence and the strength of the surface heat flux, which
do not vary in time. The resulting convective boundary layer exhibits transient, irregular, horizontal cellular structures,
transported by wind, and a lack of well-pronounced regular horizontal rolls, observed for analogous conditions on Earth. The
dimensionless statistics of turbulence are generally similar to those generated in the windless conditions, and depend on
the ratio F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer. The simulations show that
variations of the radiative heating influence the temperature statistics, while their effects on the wind velocity are relatively
small. The horizontal velocity variances do not show a strong dependence on parameter F, in contrast with the vertical velocity variances, which are strongly dependent on F. 相似文献
19.
Large-Eddy Simulations of Surface Heterogeneity Effects on the Convective Boundary Layer During the LITFASS-2003 Experiment 总被引：3，自引：3，他引：0
We investigate the impact of observed surface heterogeneities during the LITFASS-2003 experiment on the convective boundary layer (CBL). Large-eddy simulations (LES), driven by observed near-surface sensible and latent heat fluxes, were performed for the diurnal cycle and compare well with observations. As in former studies of idealized one- and two-dimensional heterogeneities, secondary circulations developed that are superimposed on the turbulent field and that partly take over the vertical transport of heat and moisture. The secondary circulation patterns vary between local and roll-like structures, depending on the background wind conditions. For higher background wind speeds, the flow feels an effective surface heat-flux pattern that derives from the original pattern by streamwise averaging. This effective pattern generates a roll-like secondary circulation with roll axes along the mean boundary-layer wind direction. Mainly the upstream surface conditions control the secondary circulation pattern, where the fetch increases with increasing background wind speed. Unlike the entrainment flux that appears to be slightly decreased compared to the homogeneously-heated CBL, the vertical flux of sensible heat appears not to be modified in the mixed layer, while the vertical flux of latent heat shows different responses to secondary circulations. The study illustrates that sufficient time averaging and ensemble averaging is required to separate the heterogeneity-induced signals from the raw LES turbulence data. This might be an important reason why experiments over heterogeneous terrain in the past did not give any clear evidence of heterogeneity-induced effects. 相似文献
20.
Thermal stability changes the properties of the turbulent atmospheric boundary layer, and in turn affects the behaviour of wind-turbine wakes. To better understand the effects of thermal stability on the wind-turbine wake structure, wind-tunnel experiments were carried out with a simulated convective boundary layer (CBL) and a neutral boundary layer. The CBL was generated by cooling the airflow to 12–15 °C and heating up the test section floor to 73–75 °C. The freestream wind speed was set at about 2.5 m s?1, resulting in a bulk Richardson number of ?0.13. The wake of a horizontal-axis 3-blade wind-turbine model, whose height was within the lowest one third of the boundary layer, was studied using stereoscopic particle image velocimetry (S-PIV) and triple-wire (x-wire/cold-wire) anemometry. Data acquired with the S-PIV were analyzed to characterize the highly three-dimensional turbulent flow in the near wake (0.2–3.2 rotor diameters) as well as to visualize the shedding of tip vortices. Profiles of the mean flow, turbulence intensity, and turbulent momentum and heat fluxes were measured with the triple-wire anemometer at downwind locations from 2–20 rotor diameters in the centre plane of the wake. In comparison with the wake of the same wind turbine in a neutral boundary layer, a smaller velocity deficit (about 15 % at the wake centre) is observed in the CBL, where an enhanced radial momentum transport leads to a more rapid momentum recovery, particularly in the lower part of the wake. The velocity deficit at the wake centre decays following a power law regardless of the thermal stability. While the peak turbulence intensity (and the maximum added turbulence) occurs at the top-tip height at a downwind distance of about three rotor diameters in both cases, the magnitude is about 20 % higher in the CBL than in the neutral boundary layer. Correspondingly, the turbulent heat flux is also enhanced by approximately 25 % in the lower part of the wake, compared to that in the undisturbed CBL inflow. This study represents the first controlled wind-tunnel experiment to study the effects of the CBL on wind-turbine wakes. The results on decreased velocity deficit and increased turbulence in wind-turbine wakes associated with atmospheric thermal stability are important to be taken into account in the design of wind farms, in order to reduce the impact of wakes on power output and fatigue loads on downwind wind turbines. 相似文献